The Garden Professors™

Plant Disease Primer Part 5: Malicious Misfits

Over the last several months, I’ve covered plant disease basics and discussed plant diseases caused by fungi, bacteria, and viruses. In this fifth and final installment, I’m going to talk about diseases caused by anything but those three different agents. There are a few diseases caused by pathogens that fall outside of those well-known classifications. This list is by no means exhaustive, but it is a good start to show you just what is out there. Where applicable, I’ll be discussing signs and symptoms of the disease, potential control or prevention efforts, and dive a little deeper into describing the actual causal agent.

First and foremost, some of the diseases I’m sharing might have already been covered in one of my previous installments. The reason for this is that for simplicity’s sake we often lump diseases caused by these “different” pathogens in with a causal agent that they’re similar or related to or evolved from. It makes describing these things simpler to the general public. But in this chapter, it is my hope to describe and explore just how these things are different as a lesson in how marvelous, interesting, and varied the world is around us.

Phantom Phytoplasmas

Phytoplasmas are single-celled organisms often lumped in with bacteria, seeing as they are actually descended from bacteria. However, phytoplasmas have lost the cell wall that gives bacteria their shape. Phytoplasmas can therefore change shape in response to their environment and to fit in their surroundings better than bacteria which retain shapes like spherical (cocci), rod (bacilli), and spiral (spirilla).

Since they don’t have the protection of a cell wall, phytoplasmas cannot live outside of a host organism and are considered obligate symbiotes. In the case of a plant parasite, it is either the phloem cells of a plant or the gut of an insect vector. Bacteria, on the other hand, are free-living and can exist in the environment and can move between hosts without the aid of a vector insect.

The best known phytoplasma plant diseases are the yellows, with Aster Yellows being the best known. Yellows diseases get their names because plants or plant parts often turn yellow. They become stunted and can develop mis-formed or misshapen parts. Witches brooming, where many twigs, branches, or flowers develop from one point (which can look like a broom) is common. In aster yellows, misshapen and discolored flowers are common. It affects over 300 species, but coneflower, asters, zinnia, and marigolds are common sufferers. In Ash Yellows, ash trees develop unusual leaf growths and witches’ brooms throughout the tree.

Unusual floral growth as a result of aster yellows Source: Douglas/Sarpy Extension – Nebraska

Phytoplasmas require an insect vector, and in the case of yellows, like Aster Yellows, the culprit is a tiny leaf hopper. For Ash Yellows, spittlebugs may also be carriers in addition to leaf hoppers. Since there isn’t a treatment, infection by yellows phytoplasmas can be permanently effective or fatal. The stunting and yellowing will eventually cause a decline in tree health. For herbaceous perennials suffering from Aster Yellows there is no way to revert back to normal blooms. The only way to reduce the likelihood of spread to other plants is removal of whole plants and it can often be too-little-too-late as leafhoppers spread quickly from plant to plant and infection in other plants often occurs prior to noticeable symptoms in nearby origin plants.

Ash tree exhibiting overall decline and witches brooming from Ash Yellows. Source: Missouri Botanical Garden

Vicious Viroids

In part 4 of this series, we discussed plant viruses and how devastating they are to plants. Viroids are also very destructive disease-causing agents and, like viruses, cannot be cured. Viroids are non-living agents, just like viruses, and are even simpler. Where viruses are genetic material (DNA or RNA) surrounded by a protein coat, viroids are just simple strands of pathogenic RNA without a coat. These circular, single-stranded RNA molecules do not encode any proteins and rely on the host cells for replication.

Viroid symptoms include stunting, misshapen growth, leaf abnormalities, and reduced yield. It is possible for some plants to be asymptomatic while being infected and providing a source of viroids to infect other nearby plants.

Viroids can spread to plants in similar ways to viruses, through transmission on dirty tools, propagation from infected plant materials, on seeds, through touch. There is some evidence that viroids can spread through insect vectors like aphids, which is not common for viral diseases. Prevention relies on good sanitation like cleaning tools, planting disease-free and certified cuttings and seeds, and quarantine of new plants that could be infected.

Common viroid diseases include:

Potato Spindle Tuber Disease (PSTVd)

Common Symptoms: stunted growth, deformed potato tubers, yield reduction

Deformed potato tubers with PSTVd. Source: Wikimedia Commons

Chrysanthemum Stunt Viroid (CSVd)

Common Symptoms: stunting, spotted leaves, poor rooting, flower color change, disruption of photoperiod response for flower initiation. It is one of the biggest threats to the chrysanthemum production industry.

Effects of CSVd on florist chrysanthemum. Source: invasive.org

Oafish Oomycetes

While many still lump oomycetes in with fungi, many scientists consider them to be a distinct group because they have a number of differences. One common name for this group is “water mold” because of their preference for wet environments and their mold-like appearance. While they do absorb nutrients and produce mycelia like fungi, there are differences in their composition, genetics, and reproduction. The biggest difference is the production of oospores, which are thick-walled reproductive spores that can rest, or hibernate, for extended periods of time in unfavorable conditions and “germinate” when conditions are favorable for the organism to grow and reproduce. Another difference is the composition of its cell walls. Fungi cell walls are composed of chitin, which is the same chemical that gives hardness to the exoskeleton of insects. Oomycete cell walls are composed of cellulose and beta glucans (polysaccharides that make soluble fibers).

While late blight of potatoes Phytophthora infestans is often called a fungus (and I discussed it in the fungus installment), it is technically an oomycete. The same with downy mildews of various species (Plasmopara) and blights caused by Pythium spp.. One of the more devastating oomycete diseases is Sudden Oak Death Phytophthora ramorum, which as it sounds, is responsible for the sudden death of plants. But many are now calling it Ramorum blight because it effects way more plants than just oak (Rhododendron, Viburnum, Camellia, Azalea, blueberries, Douglas fir, lilacs, and mountain laurel to name a few). Early symptoms of Sudden Oak Death include foliage dieback, leaf discoloration and water-soaked lesions, shoot and twig dieback. As the disease progresses, trees develop cankers that ooze or bleed dark colored sap and eventually decline and die. There is no treatment or cure for sudden oak death and prevention relies on good sanitation and abiding by quarantine regulations.

Oozing oak canker from Ramorum Blight/Sudden Oak Death. Source: Ohio State University Extension

Water soaked lesion on *Rhododendron* caused by Ramorum Blight. Source: UMD Extension

Wrapping it up

Just like diseases caused by the familiar fungi, bacteria, and viruses, these plant diseases can be devastating and difficult to prevent or manage. As always, an ounce of prevention is worth a pound of cure (which is really true when there isn’t a cure). The best way to deal with these diseases, as with any disease, it to practice integrated pest management with good sanitation, procuring plants from trusted sources, and being vigilant for signs of disease so that infected plants can be removed quickly to reduce the chance of spread.

Knowing some of the common diseases and their signs and symptoms is key in early detection and decision making. Hopefully, some of the info I’ve shared in this series can help you keep an eye out for diseases. As always, when in doubt contact your local extension office for help with diagnosis, verification, and to discuss possible treatment or prevention options.

Sources

Underrated Beneficial Arthropods Part 1: Pollinators

The world of beneficial arthropods (insects and their relatives) far exceeds some of the common critters that we often associate with this category. Many of them perform vital functions in our own yards, gardens, and ecosystems all over the world. A very small sliver of all arthropods are considered pests of any kind though there are certainly some pretty devastating pests in this category. Most of these other organisms are either providing benefits or maintaining important ecosystem functions. They are often overlooked, as some of the more charismatic ones (like butterflies, bees, mantids, and lady beetles) take most of the spotlight. These other not-so-glamorous beneficial arthropods are just as important as the more famous ones, and often perform many tasks that go unseen and underappreciated.

I wanted to talk about each group of common beneficial arthropods (Pollinators, Natural Enemies, and Nutrient Cyclers), but as I was writing, I admittedly got a bit carried away. So in order to prevent this blog post from being extremely long, I decided to split this into 3 parts. Stay tuned for the next installments in my spring and summer GP blog posts. In the meantime let’s dig into some under-appreciated pollinators.

Underrated Pollinators

Goldenrod Soldier Beetle ( *Chauliognathus pensylvanicus*) feeding on pollen. Photo: Abiya Saeed

Ah yes, pollinators! Many people consider this their favorite category of beneficials (because very few things are as striking or charismatic as a bee or butterfly sipping nectar from a flower). This is also one of the first groups that come to mind when people think about beneficials in general. The classic image of a monarch butterfly or a honey bee on a flower is often associated as the representative image of this group. That being said, honey bees are just one example of the over 20,000 bee species that are found worldwide. But this group far exceeds butterflies and bees- and some of the less charismatic critters often get an unfair reputation, or just a lack of awareness about what they do. For a variety of reasons, including their anatomy, efficiency, abundance, and direct economic impact, bees are considered the most important pollinators. But many other animals are also great pollinators, some of which are the sole pollinators of certain plant species. In fact most arthropods that visit flowers have the potential to move pollen around, making them possible pollinators. But since I don’t have time to write a whole book’s worth of information into this blog post, I will focus on a few of the larger groups of these less famous pollinators. If you are interested in doing a deeper dive into the world of beneficial arthropods, I will include some resources at the end.

Flies

Flies (order: Diptera) are a huge group of insects, with over 110,000 described species in 150 different families. This group spans a wide variety of very well-known groups like house flies [Muscoidea] and mosquitoes [Culicidae], to not so well-known groups like stalk-eyed flies [Diopsidae] and long-legged flies [Dolichopodidae]. They encompass nearly all biomes and have a broad range of functional groups including predators, parasites, decomposers, and pollinators. In fact, some studies consider flies to be the second most important flower visitors after bees.

Green Bottle Fly (*Lucilia* sp.) on a Prickly Pear (*Opuntia* sp.) Photo: Abiya Saeed

According to a literature review by Cook et al. (2020): flies from 86 different families have been reported to visit over 1100 plant species. These flower-visiting fly species also include some that have important potential for crop pollination and have been recorded to visit many horticultural crops. This includes commonly known pollinating flies, e.g., bee flies [Bombyliidae], hoverflies [Syrphidae], and flower flies [Anthomyiidae]. But some flies we don’t usually associate with this role such as blow flies [Calliphoridae], flesh flies [Sarcophagidae], and horse flies [Tabanidae], and some that many may never have heard of such as nose flies [Rhiniidae] and march flies [Bibionidae], are included. Some species are even considered to have potential as managed pollinators, a role that we most commonly associate with honey bees and some other bee species.

Fun fact: only female mosquitoes need a blood meal in order to reproduce, whereas male mosquitoes feed on nectar, making mosquitoes pollinators! In fact, mosquitoes have been studied as pollinators of orchids, like the Blunt-leaf Orchid, Platanthera obtusata, among other plants.

Moths

We all know butterflies and moths (order: Lepidoptera) are well-documented flower visitors and important pollinators. Despite this, butterflies often get most of the limelight and attention from the general public while many moths often end up being overlooked. Even though moths make up nearly 90% of the over 160,000 described Lepidoptera species, there is a disproportionate amount of research that has historically been conducted on them when compared with butterflies. It has also been demonstrated that moths are the most important nocturnal pollinators, which is fairly intuitive when you think about their nocturnal foraging biology. A study conducted in Sussex by Anderson et al. (2023) demonstrated that moths had higher pollen deposition rates on bramble species indicating that they are more efficient pollinators of brambles than their diurnal counterparts. This has implications for the importance of moths in other plant groups as well, as new research continues to be conducted.

Clearwing Moth (*Hemaris* sp.) Photo: **Steven Katovich, Bugwood.org**

Fun fact: My favorite story to tell about plant-pollinator interactions is of Darwin’s Star Orchid (Angraecum sesquipedale). Charles Darwin was sent a sample of this striking orchid from Madagascar in 1862. Upon examination he found that the nectar tubes were 30 cm (~12 inches) deep! Based on this, Darwin hypothesized that it would take something that has a really long tongue to be able to access that nectar but nobody believed there could be such an insect and he was ridiculed by other scientists. In 1867 Alfred Russel Wallace examined the orchid and predicted there must be a moth in Madagascar that can reach this nectar in order to pollinate the plant. But no moth had ever been discovered which had a proboscis (a coiled and elongated mouthpart of butterflies and moths that is used to suck up nectar) that long. It wasn’t until decades later in 1903 a moth meeting these specifications was discovered. Aptly named in honor of the scientist who predicted its existence, Wallace’s sphinx moth (Xanthopan praedicta) also known as the ‘predicted moth’ has the longest proboscis (sometimes referred to as a tongue, though it is not quite a tongue) of any insect. This just demonstrates just how amazing plant and insect interactions and coevolutionary relationships truly are!

The Star Orchid alongside the ‘Predicted Moth’! Photographed by Robert Clark for Evolution

Wasps

Wasps are in the order Hymenoptera, shared with bees and ants. They often have a bad reputation due to a few particularly aggressive social wasp species that most of us have likely had an unfortunate interaction with. That being said, the wasp group is extremely large, diverse, and species-rich. With over 103,000 described species in the category (and scientist estimates stating that the actual number could be in the millions), wasps span a lot of crucial categories of beneficials including parasitoids, predators, and pollinators.

Many wasp species resemble bees and it can be easy to confuse them for each other when they are visiting flowers. The major differences between the two are the thread-like waist that wasps have, and their less-hairy sometimes shiny, overall appearance. In addition most wasp species are primarily carnivorous, feeding on insects and other sources of meat for their protein needs, making them a great resource for deterring common garden pests (stay tuned for more on that in the next part of the Underrated Beneficials series). Even though most of these wasps are carnivorous, they supplement their diet with sugars which they often get from nectar or honeydew produced by sap-sucking insects, e.g., aphids, and occasionally fruit.

White-Striped Black Mason Wasps ( *Pseudodynerus quadisectus* ) mating on a Goldenrod (*Solidago* sp.) Photo: Abiya Saeed

There are also some species of vegetarian wasps. A common example of these are the 300 species of pollen wasps (Masarinae) which, like bees, are nectar and pollen feeders (and many of which are important pollinators of certain plant species, such as the Water Leaf, Hydrophyllaceae). Due to the fact that they have fewer hairs, wasps aren’t as efficient at pollination as bees, however, they can still be very important pollinators. Like bees, some wasps are generalist pollinators, visiting a wide-array of flowering plants, while others are specialists where a group of wasps relies on a group of flowering plants and vice versa. In these cases the pollination of those plants are reliant on these wasps.

Studies have shown that some generalist wasp species are better than some generalist bees at pollinating specific flowers. A 2018 study by Thomson examining the pollinators of the California Bee Plant (Scrophularia californica) showed that the western yellowjacket (Vespula pensylvanica) was a more effective pollinator in terms of pollen deposition when compared with honey bees and bumble bees. Some species of African pineapple lilies (Eucomis autumnalis and Eucomis comosa) and African milkweed (Pachycarpus grandifloras) are primarily pollinated by spider-hunting wasps (Pompilidae) in the genus Hemipepsis. Additionally over 100 species of orchids are reliant on wasps for pollination some of which use sexual mimicry to attract male wasps to flowers! And I would be remiss if I didn’t mention fig wasps (family: Agaonidae), who have been coevolving with their host plant for tens of millions of years. The fig (Ficus sp.) ‘fruit’ is actually an inflorescence (an enlarged stem with lots of little flowers inside). In order to pollinate those flowers, the female fig wasp squeezes into a small opening and moves around, laying her eggs in the ovaries of these flowers thereby spreading pollen from the fig that she was born in. The male offspring will remain in the fig while the new batch of females will emerge and look for a new fig in which to lay their eggs (see resources for more on this fascinating mutualism).

For more information on wasps as pollinators, check out the awesome article by Hooks and Espíndola, linked in the resources!

Fun fact: Sexual mimicry is used by some flowers to attract their pollinators. In these situations, the flowers use a combination of visual and chemical cues including mimicking the scent of specific female wasps and bees to attract males. An example of this can be seen in the wasp family Thynnidae, where male winged-wasps are searching for wingless females to mate with. When they stumble across the warty hammer orchid (Drakaea livida) they confuse it with a female thynnid wasp, because of the similar shape and scent, and try to mate with it. This process results in the pollen being deposited on the abdomen of the male wasp. As he goes to the next orchid in order to mate, the pollen is deposited on the new flower, resulting in pollination.

Beetles

Beetles (order: Coleoptera) are considered to be the largest insect order with over 350,000 described species, which makes up 25% of all known animal species on Earth! Like some of the previously mentioned orders, they include a large diversity of functional groups, including pollinators. Due to the incredible size of this order, they are considered to be the largest and most diverse group of pollinators with an estimated 77,000 flower-visiting species. In fact, based on pollen-covered specimens preserved in amber from 100 million years ago which is 30 million years earlier than the first records of bee pollinators, beetles are considered to be the first recorded insect pollinators! Even now they are considered to be vital pollinators of some of the most primitive flowering plant groups that still exist today, such as Magnolias.

Flower Longhorn Beetle (*Analeptura lineola*) on a Multiflora Rose (*Rosa multiflora*). Photo: **Ansel Oommen, Bugwood.org**

Although some beetles are specialists of certain plant groups ,especially those that are descendants of some of the earliest flowering plant groups including water lilies and magnolias, most are generalist pollinators and will visit a wide array of flowering plants. Some scientists even estimate that flower-visiting beetle species will visit 90% of all 350,000 flowering plant species. Beetle pollination is also essential for certain agricultural crops including Paw Paw (Asimina sp.) and the Atemoya (Annona x Atemoya).

For more information on the fascinating world of beetle pollination, check out the awesome article by Hooks and Espíndola, linked in the resources!

Fun fact: The process of cross-pollination that depends on beetles is referred to as ‘cantharophily’.

Resources

Cook et al. (2020). The Role of Flies as Pollinators of Horticultural Crops: An Australian Case Study with Worldwide Relevance. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7349676/

Anderson et al. (2023). Marvellous moths! Pollen deposition rate of bramble (Rubus futicosus L. agg.) is greater at night than day. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0281810

Hooks and Espíndola. (2017). Wasps, surprisingly cool pollinators. https://blog.umd.edu/agronomynews/2020/08/31/wasps-surprisingly-cool-pollinators/

The story of the fig and its wasp. https://www.esa.org/esablog/2011/05/20/the-story-of-the-fig-and-its-wasp/

Hooks and Espíndola. (2017). Beetles and Pollination.
https://blog.umd.edu/agronomynews/2020/06/29/beetles-and-pollination/

People and Plants

In this late fall edition of People and Plants we’ll take a look at an early American female botanist, Martha Daniell Logan.

Martha Logan’s signature. Courtesy of The South Carolina Historical Society.

She was born in 1704 in St. Thomas Parish, South Carolina, the second child of Robert Daniell and his second wife Martha Wainwright. After her father died in 1718 she inherited his land along the Wando River. In 1719, Martha married George Logan, Jr. and they lived on the Wando River, ten miles from Charleston, where both the Daniell and the Logan families owned extensive property. Over the next sixteen years, she gave birth to eight children, six surviving to adulthood. In 1750 the family moved to a plantation near Charleston. Needing to enhance the family income she advertised her services as a teacher but her attention gradually shifted to horticulture. She began her botanical career collecting in the woods near her home.

The title page of the 1757 South Carolina Almanack which contained Martha Logan’s “Gardener’s Kalendar.” Image courtesy of the South Carolina Historical Society, Charleston, S.C.

Martha soon gained the reputation of a skilled gardener and maintained a well-known garden “on the Green, near Trott’s Point in Charles Town.” Gardening became her focus and occupation and she embarked on a career as a “purveyor of botanical goods,” selling seeds and plants from her home.
In addition to native plants, she dealt in imported specimens. Gardening, especially landscaping with rare plants, had become a favored pastime among wealthy locals and Martha was quick to capitalize on this. An advertisement published in the Gazette on November 12, 1753, announced the availability of “a parcel of very good seeds, flower roots, and fruit stones of several kinds” that were “just imported from London.”

Page of the 1757 South Carolina Almanack print of Martha Logan’s “Gardener’s Kalendar.” Image courtesy of the South Carolina Historical Society, Charleston, S.C.

She exchanged seeds, roots, and plants, like gardeners do, with other botanical enthusiasts including the naturalist John Bartram. His visit in 1760 initiated a three year correspondence and trade of specimens. They swapped lists of available plants and used silk bags to send seeds to each other. They also exchanged lists of plants that each desired from the other’s geographical area. Logan enthusiastically sent Bartram plants from Carolina which “may be New to you” and “be an adision [addition] to yr Collection.” In return, she asked him to send bulbs and double-flowering plants that her London contacts had failed to procure or took too long to send. She shipped and received tubs of cuttings and roots on ships traveling between Charleston and Philadelphia, where Bartram lived. Bartram praised her in a letter to a London friend and wrote, “Mrs. Logan’s garden is her delight and she has a fine one.”

With the popularity of urban gardening on the rise Martha realized that many people needed help and guidance with their horticultural endeavors. In 1752 her first advice column titled “Gardners Kalander [sic], done by a Lady of this Province, and esteemed a very good one.” appeared in the South Carolina Almanack. Her first publication was so successful she continued to publish her calendar, updating and enlarging it each year.

Martha continued her business, what we nowadays would call a garden center, for the rest of her life. She even wrote a treatise on gardening at the age of seventy. In 1809 the early Charleston historian David Ramsay described her as “a great florist, and uncommonly fond of a garden,” and claimed she “reduced the knowledge she had acquired by long experience, and observation, to a regular system which . . . to this day regulates the practice of gardens in and around Charleston.”

Martha died in Charleston on June 28, 1779, and was buried in St. Philip’s Churchyard. She is considered one of the founding gardeners of South Carolina.

Read back through the pages of her 1757 Gardener’s Kalendar shared above. In your opinion, how much of it is still applicable? One bit of her advice that is always appreciated: “What was neglected last month may be successfully done in this.”

5th National Climate Assessment and an Update on the Plant Hardiness Zone Map

This month has been an exciting one for climatologists around the United States with the November 14 release of the Fifth National Climate Assessment (NCA5), a massive project that is undertaken every four years to capture our current understanding of climate change based on recent research. I was a chapter author for the Southeast and spent the last two years working with over 700 authors around the United States to gather and document how the climate is changing and how it is affecting all of us. This week I will explain how NCA5 was put together, what it says about climate, and what gardeners can do to help reduce the future impacts of global warming and other climate changes. But this month was also exciting because USDA just released an updated Plant Hardiness Zone map, just a few weeks after my post in October about how the 2012 map was outdated. I guess they were listening (just kidding!). I will discuss that briefly at the end of this post, too.

Frost on the grass/moss, Timo Newton-Syms, Commons Wikimedia

What is the National Climate Assessment?

The National Climate Assessment (NCA) is a report mandated by Congress to compile the latest scientific findings on how climate is changing so that we can respond to reduce its future impacts. It is published every four years, and the last one (the 4^th NCA) was released on the day after Thanksgiving in 2017. While the underlying message has not changed, each assessment focuses on the newest scientific research that has been published since the last assessment was done. The document is divided into chapters so that the authors of each chapter could concentrate on that topic.

NEWS STREAM VI – REFLECTIONS by Taina Litwak as part of the NCA5 art competition (see all entries along with artist statements at https://nca2023.globalchange.gov/art-climate/).

NCA5 starts with a review of the general scientific principles of how the climate is changing. That is followed by seventeen chapters focused on national topics such as agriculture, water, energy, and transportation as well as specific groups that are being especially affected by climate change such as indigenous peoples. Following the national topics, chapters address changes that are happening in ten different regions of the country . These address how we need to reduce future greenhouse gas emissions that are driving the warming of the earth as well as how we can adapt to the changes that are already happening now and may get worse in the future.

How was NCA5 produced?

There is a long process involved in producing a national climate assessment. Teams of scientists from an array of disciplines were chosen as authors for each chapter to write the initial text of the document. To keep the authors on task and within tight word limits, there were lead chapter authors and technical advisors who moderated group meetings where the key messages for our chapter were identified. Initial figures to include in each chapter were drafted by a graphics team or requested from scientific journals. After the first draft was complete it was first reviewed by federal agencies to make sure that their concerns were addressed and then by the public, who provided many additional comments. All of these comments were provided to the chapter authors so they could refine their text and figures for the next draft. In all, the document when through six different reviews and all comments were addressed.

Frost on a borago officinalis flower, Stanzilla, Commons Wikimedia

Where can I read NCA5 and learn more about what it says?

The NCA contains a vast amount of information in its 32 chapters, five appendices, and special topics, so it is hard to summarize. I encourage you to explore the document online to see what it says about your region and special topics of interest like agriculture, land, and ecosystems. A good starting place is the introductory website https://www.globalchange.gov/our-work/fifth-national-climate-assessment, which explains how the report was written and provides links to read the report, attend a webinar on an individual chapter, and see where the figures came from. I also encourage you to explore the excellent interactive atlas developed in conjunction with the report. Many other resources such as podcasts are available, too.

What are some steps that gardeners can take to respond to climate change?

There are two approaches that gardeners (and all of us) need to take to respond to the challenges of a changing climate. We are already dealing with the consequences of trends towards warmer temperatures and more extreme swings in the water cycle such as increases in floods and droughts. Gardeners are adapting to these changes in climate by planting different plants that are better suited to the warmer climate and changing how they manage their gardens using rain gardens, drip irrigation, and other techniques. Adaptation is a key approach that gardeners will continue to need to follow as the climate continues to get warmer and more variable.

In addition all of us, including gardeners, have a responsibility to cut the emissions of additional fossil fuels which are driving most of the warming (mitigation is reducing the inputs to prevent future harm). This will reduce the impacts that our world will have to navigate in the future. Even a small decrease in the emission of greenhouse gases now can prevent the worst outcomes. A prime target for gardeners is the elimination of gasoline-powered equipment like blowers, mowers, and trimmers. These small tools have highly inefficient engines that emit a lot of greenhouse gases as well as air pollutants (and a lot of noise, too). Switching to electric tools and vehicles, composting, adding solar power to houses and businesses, and conserving energy and water (which often uses energy to purify it) through carefully chosen plantings as well as through other methods can also help reduce future warming.

Frosted flower buds, Tony Hisgett, Commons Wikimedia.

What about the new USDA Plant Hardiness Zone map?

I was surprised last week that the USDA had just produced an update to the 2012 map that I discussed last month. In that post, I noted that the 2012 map was already outdated due to the increasing temperatures we have seen in the 21^st century. The new 2023 map uses data from 1991-2020, the current 30-year normal period, to identify the current plant hardiness zones for the United States. You can see the new map and zoom to your city at https://planthardiness.ars.usda.gov/. By comparing it to the 2012 map, you will see that more than half the country has increased by half a zone, which correlates to about a 5-degree F increase in the average lowest minimum temperature a location experiences each year. I asked USDA for a map that showed the changes of zone and was provided one by Chris Daly of the PRISM group that put together the 2023 map (below). Areas in tan experienced a half-zone change since the 2012 map. (There are a few areas in the Mountain West where the zones got colder, as shown in green, but these are mostly linked to new datasets that were available for the analysis rather than any changes to the local climate there.)

Science has made it clear that the earth’s climate is changing and that most of the warming we are experiencing is due to burning of fossil fuels. We must learn to adapt to these changes and make sure that all groups can be protected from the worst impacts of the more extreme weather we are likely to experience. But we can also make changes now to reduce those future impacts, and I know gardeners will be part of that solution.

November PDX leaves, Loren Kerns, Commons Wikimedia

Plant Disease Primer- Part 4: Going Viral

Previously in this series I started with some plant disease basics and then covered some common fungal diseases and then bacterial diseases. Now let’s turn our attention to viruses. Just like with fungi and bacteria before, in this installment I’m going to talk about some of the most common viral plant diseases with some suggestions for treatment and prevention. This by no means will be an exhaustive list of diseases (there are so many!), but I hope to cover some of the most common ones that trouble gardeners.

Unlike fungi and bacteria and just like human viruses like the common cold, there typically aren’t treatments that you can use to “cure” or treat a viral infection. Therefore prevention is the only way to limit viral disease spread in plants. Also unlike fungi and bacteria, viruses are not living organisms. They don’t have cellular “machinery” and are typically a snippet of genetic material (DNA or RNA) encased in a protein coat or similar structure. Since they aren’t living, they don’t reproduce outside of a host organism and don’t typically have the ability to spread themselves around the environment, instead relying on hosts to carry them. For plant viral diseases this usually involves manual movement on humans, tools, or possibly animals or inside of a secondary host organism like an insect’s digestive tract. Aphids are a common vector, as they consume and secrete infected sap. Viruses can often spread through infected seed or vegetative propagules like seed potatoes or cuttings. And since they aren’t living entities and are microscopic there are no signs (visible presence of causal agent) of disease, only symptoms.

Mosaic Viruses

I’m lumping mosaic viruses together because there are lots and lots of them, each affecting a different range of host plants but with similar symptoms. Many viruses affect a specific species or genus of plants while others have a broad host range. Common symptoms include: yellowing, mottling, mosaic patterns on leaves, curling, stunted growth, reduced fruit quality and size, and necrosis.

Tobacco mosaic virus (TMV) is the most persistent and infectious virus and has a very wide host range, including tobacco and other members of the Solanaceae family like tomatoes, potatoes, and peppers, other vegetables, and ornamental plants. The host range is estimated to be up to 350 species.

Tobacco Mosaic Virus (TMV) on Tobacco, Source: UK Extension

TMV is spread through sap and infected seeds. Virus transmission through sap can be a result of physical contact (brushing against plants when moving through a field/garden), dirty tools, aphid feeding, or even from tobacco use. TMV is so pervasive and persistent that many nursery and greenhouse businesses have strict tobacco policies for employees and for employees who do use tobacco there is usually a hand sanitation requirement.

Tomato Mosaic Virus (ToMV) on Tomato, Source: UF IFAS

Other mosaic viruses include Tomato Mosaic Virus (ToMV), Cucumber Mosaic Virus (CMV), Zucchini Yellow Mosaic Virus (ZYMV), Cauliflower Mosaic Virus (CaMV), Squash Mosaic Virus (SqMV), Bean Common Mosaic Virus (BCMV), and Rose Mosaic Virus (RMV).

Zucchini Yellow Mosaic Virus (ZYMV), Source: Wikimedia Commons

Tomato Yellow Leaf Curl Virus (TYLCV)

Common symptoms: Yellowing and curling of leaves, stunted growth
Host Plants: Tomatoes, Peppers
Insect Vectors: Whitefly

Tomato Ringspot Virus (ToRSV)

Common symptoms: yellow rings on leaves, mottling, distortion, mosaic, rings on fruits, necrosis
Host Plants: Tomatoes, other Solanaceous crops, wide range of others
Vector: Nematodes

Potato Virus Y (PVY)

Common symptoms: Leaf discoloration, mosaic patterns, tuber deformation
Host Plants: Potatoes, Tomatoes, Peppers
Insect vector: aphids

Hosta Virus X (HVX)

Common Symptoms: irregular yellow or light green streaks, mottling, leaf distortion, feathering pattern (looks like colors painted on by brush), stunting, and reduced vigor
Host Plants: Hosta
Insect Vectors: None/unknown

Rose Rosette Disease (RRD)

Common symptoms: Rapid growth, witches brooming, excessive red coloration (in terminal bracts), deformed leaves, excessive thorns
Host Plants: Roses
Insect Vectors: Eriophyid mites

Plum Pox Virus (PPV)

Symptoms: Leaf distortion, fruit deformities
Host Plants: Plum, peach, apricot
Insect Vectors: Aphids

Control and Prevention

Unfortunately, since there is no treatment for viruses and plants don’t have immune responses that eliminate them like humans and animals do, the only “control” for viruses in the garden is by removal of infected plants. There is no way to “cure” an infected plant, but removal from the landscape or garden can reduce the viral load available to vectors in the garden and can help slow or eliminate the spread to other plants. Since symptoms may not appear right away, it is possible that viruses can spread to multiple plants before detection.

Since viral infection typically means a death-sentence for the plant, prevention through Integrated Pest Management is of utmost importance. Here are some common and effective IMP practices that can help reduce the spread of viruses in the garden.

Purchase certified disease-free seeds or plant cuttings. Unfortunately, viruses can spread easily through untested seeds and cuttings so take caution in sharing at places like plant and seed swaps.
Practice good sanitation: remove and destroy infected leaves and plants ASAP
Clean tools regularly: viruses spread through sap transfer, so cleaning and disinfecting tools is a must. Sometimes in high-value or susceptible plants, disinfection should be done between using tools on individual plants, especially pruners. Use a dilute 10% bleach solution, rubbing alcohol, or horticultural sanitizer for best results.
Quarantine new plants: If a plant appears suspicious, keep it potted in an out of the way place until you can determine possible infection. This is especially important for plants from discount retailers, plant swaps, etc but can hold true for plants from any source.
Purchase disease-resistant cultivars when possible.
Control vector insects, especially aphids.
Wash hands before gardening, ESPECIALLY if you are a tobacco user.
Do not use tobacco products while gardening.

Wrapping it up

There are lots of bacterial diseases that can damage or kill plants in our gardens or landscapes. Prevention is key, as treatments only help slow the spread of disease. In the next (and final) installment, we’ll talk about diseases that are caused by things that aren’t fungi, bacteria, or viruses.

Ok–I know something is wrong, but what is it?

Facebook and other social media attempt to help us solve problems. This group and others seek to inform gardeners and solve problems they are having growing plants. Looking at queries and posted responses there is so much information missing, leading to wrong and misleading comments in many of these discussions. I think it is a good idea to reexamine the diagnostic process and how gardeners can solve their own diagnostic questions.

I know there is something wrong with this *Ficus* but what is it? To diagnose this tree disorder many steps need to be taken to understand the problem

Diagnosis is always the precursor to solving a plant problem. In the world of plant pathology, palliative care (treating symptoms) is often ineffective if the cause of the disorder is unknown. It is amazing how on social media so many cures, fixes, MacGyvers, or treatments are suggested even before a diagnosis is made. The diagnostic process has many components so its good to be familiar with some of the steps in this process.

Identify the plant

All plants have published names and are based on herbarium specimens. The published names of plants are all scientific binomial names. The first name is the genus and second the specific epithet or species.

Host identification comes as the first step in diagnosis. It sounds simple or silly, but knowing the host name is the first step in diagnosis. Find the scientific name of the plant and then specific disorders of that taxa can be sought out in a web search. Common names are misleading and it is critical to associate disorders with the exact plant you have a problem with. If you are diagnosing remotely (as I am often forced to do), knowing the location is the next question as many disorders are regional. For instance we don’t have black knot of plum in southern California while in southern Ontario, Canada and New York state that is a big problem.

Look at the whole organism

So many gardeners only focus on where they see symptoms. A leaf, shoot, or branch with something that does not look right is a good place to start looking, but always consider the entire plant. It is important to see the entire plant and what the distribution of above ground symptoms is. Don’t forget the “whole organism” includes its root system which is often neglected in diagnosis.

Examine the entire plant including its roots for symptoms

Look at all the components of the plant

Symptoms which are plant responses to a disease or disorder often occur on leaves. The problem, however, may be in the roots. Root rots may go undetected until almost the entire root system is decayed; only then do symptoms start to appear on the distal or far portion of the plant. These rapidly or slowly spread until the entire plant is affected. Whenever there is uniform symptomology of the foliage, always check the root system. Symptoms on only a single branch of a perennial may be localized to that branch, so follow the symptomatic branch back to its attachment point to locate any damage or disease along its stem.

Examining stems in the ficus picture above shows clear canker symptoms typical of *Botryosphaeria* canker in *Ficus*. The yellowing leaves are a symptom, but not the cause of the disorder.

Look closely

It often helps to use a hand lends to closely observe insects, insect products like webbing, eggs, pupal cases, or frass, or just to validate that there are no insects or their products present. Many many fungi form fungal fruiting bodies in dead stem portions and these look like tiny grains of pepper under a hand lens. A closer view is often helpful in deciding if a problem is localized or system in the plant.

A hand lens can supply 15-25x magnification

Look for symptoms and signs

Symptoms are plant responses to attack from pathogens, insects or abiotic causes such as herbicides, toxic salts, high and low temperatures etc. Symptoms alert the gardener that there is something wrong but may or may not point the way to the cause of the problem. It is also important to look for signs which are parts of the biology causing the problem. Fungal growths, spores, fruiting bodies insects adults and immatures stages of insects and the products they produce and leave behind are all signs. Signs give more direct evidence of the cause of a problem.

Look around

You may not be the only gardener with a plant problem. Look to see if other plants in your garden are similarly affected. If only a single taxa is affected it could possibly be a disease or insect problem. If many different kinds of plants are affected it may be from a non-biological cause–an abiotic disease or environmental disorder. Solving these diagnoses often requires lab work and specific soil or plant sampling

Distortion of new growth is a symptom. it has many causes but the fact that it is occurring on multiple taxa in a single site suggests herbicide toxicity. In this case the culprit is an herbicide called Polaris and the active ingredient is imazapyr.

Seek confirmation

Once you have collected all the the information (symptoms and signs) over the entire plant (including if necessary root symptoms), it is time to put the information to work. Searching on your own, on the internet, is daunting because there is so much misleading information. If you have the scientific name, you can put that in a search engine along with the symptoms and tentative ID of insects or pathogens and then look at all the images that match what you have. Click on the image and check the source of the file. If from an .edu or educational source, it is likely a higher quality of information. Read these first.

Taking samples to a lab or University Extension office is of limited value because they can’t see the entire plant. It’s best to take samples to an expert when you have a good hunch what is going on and you want to confirm it (you should include images of the whole plant if you can). So much money is spent sending random leaf or twig samples to labs and they end up sending information that is misleading or just wrong as far as the diagnosis goes. Thousands of fungi grow on plant surfaces and labs will isolate these, some are pathogens but may not be on your specific plant as pathogens can be quite specific to plants they have a disease relationship with. The lab report comes back with a finding Alternaria spp. This is indeed a pathogen of tomato and many other plants but it is also a very common saprophyte often found growing on dead plant tissues. So lab findings are helpful when they confirm your own suspicions, but often unhelpful when random plant tissues are sent by a gardener that has no idea what is happening. This is true of any lab, university or private. The more information the lab has, the more helpful they can be. And all labs everywhere would prefer to have the entire organism for diagnosis.

Diagnosis is hard. The best diagnosticians are correct (solve the diagnostic problem) about 2/3 of the time. Sometimes diagnosis of a problem can take years. Some diagnoses are never solved. But for most common plant problems you can find answers by intelligently searching the internet and with some help from the “ologists” of University and private diagnostic firms.

This disorder of *Lantana camara* took over ten years to diagnose. Samples sent to the state agriculture lab were studied for virus and fungal pests. No results came of it. The disorder was finally resolved when flies of the genus *Liriomyza* spp. were reared from leaves. Lantana Blotch Miner is widely distributed in Southern California only on *L. camara*.

How accurate is the USDA Plant Hardiness Zone map?

UPDATE: As of 11/15/2023, the USDA has published an updated Plant Hardiness Zone map that covers the 1991-2020 period, which includes a lot of the warmest years on record for the US. This map shows more detail than the old map and generally increases the zones in most areas by maybe a half-category. It also now includes Canada and Mexico. You can see it and read about it at USDA Plant Hardiness Zone Map | USDA Plant Hardiness Zone Map.

One of the first questions a gardener should ask when they are considering adding new plants to their garden is whether the plants can survive and thrive in the weather and climate conditions in their yard. One of the most useful tools for this is the USDA Plant Hardiness Zone designation. It provides a quick snapshot of the coldest weather the location is likely to experience, a key factor for how well the plants will survive in that area.

Purple aster, Patty O’Hearn Kickham, Commons Wikimedia.

What are plant hardiness zones?

Plant hardiness zones are based on the average annual minimum winter temperature at a location. For simplicity the zones are based on 10-degree Fahrenheit ranges. Each zone is further subdivided into “a” and “b” categories for the colder and warmer halves of the range. You can see the temperature ranges listed on the USDA Plant Hardiness Zone Map website, which also includes a link to an interactive map that will help you determine what zone your location is in. My home in Athens GA is listed as being in zone 8a, which has an average annual minimum temperature range of 10-15 degrees F. Linda provided good descriptions of how to use the zones in this blog in 2019 in A Gardener’s Primer to Cold Hardiness, Part 1 and Part 2.

How accurate is the USDA Plant Hardiness Zone map?

The latest official version of the map was published in 2012 and showed that most areas had experienced a half-zone change to a warmer zone from the previous map because of rising temperatures. There has been no new map since that time but as temperatures have continued to rise it seems pretty clear to me that the current map is outdated. And in fact, even back in 2012 shortly after it was published, Bert Clegg posted an article in this blog showing that the 2012 map was likely already outdated when it was published because it was based on a 30-year average in an era when temperatures are rising and minimum temperatures are rising much faster than maximum temperatures due to increases in humidity and urbanization.

This graph is created from the NCEI Climate at a Glance tool and can be customized to any location in the US if you want to play with numbers for your location.

We need to be a little bit careful with this comparison because the average minimum temperature is not the same thing as the average annual minimum temperature. The average minimum temperature is the average of all the daily minimums in a specified time period, while the average annual minimum temperature is the average of the single lowest daily temperature that occurred each year. You can have a fairly warm winter which still experiences an extreme cold outbreak that has a very low minimum temperature on one or two days. In fact, December 2022 had exactly that situation with the fiercely cold outbreak that occurred right around Christmas across a lot of the eastern United States. The extremely cold air was barely seen in the winter average temperature at all since February 2023 was extremely warm for most of the month and washed out the impact of the extreme cold since it occurred over just a few days in the average. But it certainly caused a lot of damage to plants that were exposed to the frigid air on those few icy days! If the 2012 map was outdated when it was published, it is surely more out of date now after an additional decade with some of the warmest years on record.

How will the plant hardiness zones change in the future?

As global warming continues, the average annual minimum winter temperature is expected to continue to rise. This will result in a northward movement of plant hardiness zones over time. For example, areas that are currently in Zone 6 may become Zone 7 or 8. The rate of change will depend on how fast the earth warms and that depends on how much and how quickly humans respond to minimize greenhouse warming. It would not surprise me if our hardiness zones in most parts of the United States now are at least a half-zone warmer than what is shown on the 2012 map and it could be even greater in some locations. Not all areas of the country (and the world, for that matter) are warming at the same rate, and areas closer to the poles tend to be warming more quickly because of the loss of snow and ice in winter, especially in the Northern Hemisphere.

Fall Foliage, Portland Japanese Garden, Daderot, Commons Wikimedia.

How will the shift in plant hardiness zones affect gardens?

This shift will have a significant impact on gardening and agriculture. Plants that are not adapted to warmer temperatures may struggle to survive. For example, some fruit trees that are currently grown in Zone 6 may not be able to produce in Zone 7 because they require a certain amount of cold weather to set a good flush of blossoms that form the fruit. Warmer winter temperatures will increase the chance of insect pests and diseases surviving over the colder months, leading to more problems in the next growing season. The last spring frost is likely to come earlier and the first fall frost later in the year. This might make some gardeners happy, since they can get out and start planting earlier, but has implications for pollination since the pollinators may not be able to adapt to the changes in the timing of flowering. That would result in less fertile crops and potentially lower yields of vegetables and other crops.

Gardeners and growers will need to adapt to the changing climate by selecting plants that are suited to warmer temperatures. You may already be doing this by choosing varieties and species for your gardens that are listed as being suitable for a warmer Plant Hardiness Zone than the 2012 map suggests. Gardeners may also need to change their planting practices, such as planting earlier in the spring or providing more shade for plants. In addition, changes in precipitation (which are not included in the Plant Hardiness Zones) also affect what kind of plants you need to put in your garden since drought is likely to increase in warmer conditions at the same time that individual storm events may drop more rain than in previous years.

Of course, this does not negate the effects of local climate variation across your plot of land. Variations in shade, soil, and drainage will continue to affect variations in the microclimate across your garden, as I discussed in my first blog post, The Weather Where You Are. However, the local variations will occur on top of the changes to the overall plant hardiness in your region and global temperature increases are likely to cause much bigger changes to your local climate in the long term.

National Arboretum in October, DC Gardens, Commons Wikimedia.

How many plants are native to urban areas?

***Does this look like a deciduous forest ecosystem?***

The emotionally-charged native plant debate only seems to be growing. Well-meaning but misinformed decision-makers continue to institute native plant policies with pressure from special interest groups. Most recently, North Carolina’s General Assembly weighed in on the side of emotional appeal rather than research-based information in mandating “that native trees, shrubs, and other vegetation are [to be] used for landscaping at state parks, historic sites, and roadways.”

***Roadways seem a less than ideal place for attracting wildlife***

Don’t get me wrong – I love native plants and recommend the use of well-suited native plants in gardens and landscapes. I’m co-author of a book that helps gardeners in the Pacific Northwest choose native species that are likely to thrive in their gardens. But the belief that native plants are superior to introduced species in urban and other unnatural areas is just a knee-jerk reaction to the very real environmental and ecological problems we face. It gives believers a false sense of accomplishment in that they can reverse significant threats such as climate change, wildlife extinction, and pollinator decline simply by using native plants rather than introduced species.

Supporters for this native-only policy list the same tired (and false) reasons that native plants are superior to introduced plants. Here are some of those reasons cited in the North Carolina decision, along with my commentary:

“There are many environmental benefits to native plants, and they are much more likely to thrive in our weather and soils” (North Carolina Department of Natural and Cultural Resources Secretary D. Reid Wilson)

The concept of nativity is subjective and many scientists argue that such a subjective division makes it difficult to study, much less discuss, the benefits and drawbacks of introduced plants .
This post by Dr. Bert Cregg bursts the bubble on some of the native plant superiority myths.
Native soils are not the same as compacted, amended, and otherwise disrupted soils found outside natural ecosystems.
There is no research to support that native plants thrive in soils that have been disrupted by development and urbanization.

***Even native plants will suffer drought stress if they don’t receive sufficient water***

“Native plants are adapted to the state’s environment and more likely to thrive, especially during drought.”

Roadways, state parks, and historical sites are not natural environments (though some parts of parks and historical sites could be).
Plants that can adapt to disturbed environments are most likely to thrive. Some of these are called weeds.
Plants that can survive periods of drought have morphological and/or physiological adaptations for doing so. It has nothing to do with their nativity.

“They support pollinators essential to food production and ecosystem health and boost otherwise declining bird populations that depend on insects associated with native gardens.”

***A lovely, biodiverse landscape that includes native and introduced ornamentals.***

One of the basic tenets of ecology is that new resources are exploited by existing members of a food web. What happens with one species of insect or bird or plant is not the big picture – ecology is the big picture.
This blog post by Dr. Bert Cregg discusses a paper showing that exotic species can grow more quickly than native plants, but they are eaten more by herbivores.
This blog post looks at some of the research on insectivorous birds that contrasts with the claim that native birds require native insects.
The most biodiverse landscapes are those with a high diversity of plants. The vertical structure of a landscape, created by the varying heights of trees, shrubs, and other plants, is crucial for bird habitat. I’ve published both a research article and fact sheet on this topic.

“Native plants, especially grasses, are better able to store carbon, thereby reducing greenhouse gases.”

***Grasses and trees both belong in a landscape, but trees store more long term carbon than grasses can***

Native plants have supercharged photosynthesis? There’s a Nobel Prize waiting for someone to demonstrate that.
Trees and other long-lived woody plants are best for storing carbon. Certainly not grasses. And the nativity of the woody plants is irrelevant to carbon storage.

***Pacific NW native plants like Gaultheria shallon do not thrive in urban sites where environmental conditions are nothing like natural ecosystems***

“Native plants provide habitat for birds and other pollinators, are more resilient, and require less fertilizer and other maintenance.” (Brian Turner, policy director at Audubon North Carolina)

Birds and plants have complex and often unexpected relationships. This post discusses a review article on the interaction between birds and those plants who depend on them to spread their seeds.

In June 2023, North Carolina’s Department of Cultural and Natural Resources installed a new 100% native plant garden in front of their DNCR headquarters. In comparing the before and after photos of the site, I’ve got a few observations.

If storing carbon is important (as stated earlier), then cutting down all those trees and shrubs (which don’t appear to be invasive species) was an interesting decision.
Why not just add a native garden to the existing landscape? That would have increased the plant diversity and retained the vertical structure, which is highly important for biodiversity.
If we want stable, biodiverse landscapes in our urbanized environment, we must include the use of introduced species – especially trees.

“This policy is a big win for birds and everyone who cares about North Carolina’s wildlife. It just makes sense. ” (Brian Turner, policy director at Audubon North Carolina).

Nope. It’s a big win for dogmatic belief systems.

***Vertical structure and plant diversity creates a landscape that appeals to people as well as wildlife***

There are many things that we can do in our gardens and landscapes to maximize biodiversity. Spouting false claims about native plant superiority, garden shaming those who don’t eliminate introduced plants, and forcing communities, cities, and states into lock-step on what can and can’t be planted is not part of that process.

Plant Disease Primer -Part 3: Fight Bac(teria)

Previously in this series I started with some plant disease basics and then covered some common fungal plant diseases. Now let’s turn our attention to bacteria. Just like with fungi before, this installment of the series, I’m going to talk about some of the most common bacterial plant diseases with some suggestions for treatment and prevention. This by no means will be an exhaustive list of diseases (there are so many!), but I hope to cover some of the most common ones that trouble gardeners. Most diseases have similar control and treatment options which will be shared in a section at the end, but special cases are noted in the list of diseases.

Fireblight (Erwinia amylovora)

Signs & Symptoms: Wilting and blackened young shoots with “shepherds crook” bend, brown/burnt blossoms
Host plants: Apple, pear, quince, and relatives
Treatment notes for Fire Blight: The bacteria is spread through pollen during bloom. Most common treatment is pruning out affected branches. Some application of the antibiotic streptomycin may help, but it must be applied to flowers and care must be taken to avoid potential side effects for pollinators. Use of copper fungicides in the dormant season may limit spread.

Classic fire blight looks like the ends of branches have been burned

Bacterial Leaf Spot (Xanthomonas spp.)

Signs & Symptoms: Small, water-soaked lesions on leaves; dark lesions with yellow halos, premature leaf drop and reduced plant vigor
Host plants: Various, including tomato, pepper, and crucifers

Bacterial wilts (Ralstonia solacearum, solanaceous crops; Erwinia tracheiphila; cucurbits; others)

Signs & Symptoms: wilting of leaves, usually rapidly and whole branches at a time; yellowing and browning of leaves; vascular tissue discoloration; rapid death
Host plants: solanaceous crops, cucurbits, many others

Crown Gall (Agrobacterium tumefaciens)

Signs & Symptoms: tumor-like growths/galls on stems, roots, and crowns; stunted growth and reduced yield
Host plants: many
Fun fact: A. tumefaciens transmits a small bit of DNA to the host plant, a circular plasmid, that causes the tumor-like growth. This was used as one of the first methods (and is still used) to genetically engineer plants by introducing new DNA.

Bacterial Soft Rot (Pectobacterium and Dickeya spp., others)

Signs & Symptoms: water-soaked, mushy, and foul-smelling lesions on plant parts, especially fruits and tubers (the classic “rotten potato” smell); rapid decay, slimy
Host Plants: potatoes, others

Angular Leaf Spot (Pseudomonas syringae)

Signs & Symptoms: angular, water-soaked lesions on leaves; lesions are often limited by leaf veins; premature leaf drop and reduced vigor
Host Plants: cucurbits, most commonly cucumber

The flow of bacterial cellular streaming is impeded, or limited, by leaf veins, resulting in “angular” spots

Bacterial Canker (multiple)

Signs & Symptoms: Canker-like lesions on stems and other plant parts; lesions may or may not ooze bacterial exudate; for citrus canker, lesions are common on fruits as well
Host Plants: many, usually pathogen specific

Bacterial Ring Rot (Clavibacter michiganensis)

Signs & Symptoms: brown ring/rot inside tubers; foliage yellowing and death
Host Plants: Potato, sometimes tomato

Treating Bacterial Diseases

Just like I discussed with fungi last month, it is difficult to eliminate bacterial diseases once present. Treatment focus should be on slowing down the spread of the disease to the remaining plant. Treatment is important for annual plants, which may be killed entirely by pathogens, and in woody perennials where symptoms include cankers or rots that affect perennial plant parts such as stems or trunks. Bacterial diseases that affect only foliage on perennial plants are less of a threat and often the damage is limited to aesthetics.

For the most part, removal of the diseased plant parts is an important first step in treating the disease. Bacteria stream, or ooze, through plant parts (this is why signs and symptoms are sometimes limited by plant structures, like angular leaf spot lesions being stopped by leaf veins). Therefore the organism may be present a distance away from the visible sign or symptom. For cankers and other stem infections, removal should include “healthy” tissue below (between the callus and main plant).

Sometimes this may require removal of large parts of plants, at which point decisions should be made about removal of the entire plant. Cankers occurring on main stems or trunks are especially devastating.

Once affected plant parts are removed, a treatment with a copper-based product may reduce spread. While often used as a fungicide, copper does have some effect on bacteria. Except for in the case of fire blight (usually in commercial orchards), treatment with an antibiotic is not practical or possible. Often repeated treatments through the season are needed once the disease is established in the nearby environment. Care should be taken to not overuse copper sulfate, as it will not break down in the environment and can build up in the soil, causing damage to populations to good fungi and bacteria in the soil.

Bacterial Prevention through IPM

Just as in fungal diseases, there are several Integrated Pest Management strategies that can be used to reduce the likelihood of bacterial infection in your garden or landscape. Below are some strategies that can be used for general fungal prevention:

Use mulch to limit splashing of soil onto plants
Eliminate overhead watering to reduce foliar moisture
If overhead watering is necessary, water early in the day so plants dry out before the dew point drops in the evening
When possible, plant disease resistant cultivars
Reduce nearby weeds to eliminate potential secondary hosts
Practice good hygiene in the garden by cleaning up any fallen or diseased leaves, fruits, etc.

Wrapping it up

There are lots of bacterial diseases that can damage or kill plants in our gardens or landscapes. Prevention is key, as treatments only help slow the spread of disease. In the next installment, we’ll talk about viruses and virus-like pathogens. Stay tuned!

Fall is for planting?

Fall is for planting they say when folks talk about shade trees. But is it? When is the best time to plant a tree? In this blog I will cover tree planting times and other particulars, the drawbacks and good points of these decisions.

This young Emory oak was planted in the fall. It will still need inspection of its roots and possible root washing. No amendments of the backfill are required.

So is fall the best season to plant a tree? Of course like so many questions it depends on many factors. Where you live (latitude) is a big part of this equation. I reside in Southern California and Southern Arizona. Both mild climes by any standard, but the Arizona property is at a higher elevation (4500ft) and gets cold sooner than the Southern California location. In Alaska, for another example, the planting seasons are much shorter or narrower as the onset of cold weather can be sooner in the calendar.

Nursery Stake Removed, good! Mulch, good! No turf next to tree, good! Air gap between base and stem BAD!

I think it is important to consider things from a “tree perspective”; or when is it best to plant from a tree’s perspective. Planting is not only the act of installing the tree correctly (see other GP blogs posts for correct planting technique) but it is also an acclimatization process. It is a good idea to purchase your tree beforehand and give it time to get used to the temperatures, light levels and water in the new site. Most fall planted trees are in containers or B&B which requires we do root inspections in order to not plant a tree that has root defects. These root inspections include removing all the old growing medium and root washing (search the GP blog for root washing). Locally sources fall planted trees will automatically be acclimated to reduced light and cooler temperatures. In fact if you plant a deciduous tree it may already be preparing to drop leaves. Fall planted trees still need root ball moisture to establish and thus will need some irrigation, but fall is also a time of reduced water use. One benefit to fall planting is that the trees will grow some roots over the winter and be ready for a big growth push in the spring. They will be partially established and take full advantage of longer days, moist soil and warming temperatures.

Root washing exposes root defects and is recommended before trees are planted into landscapes, especially if trees have been container grown. Bare root stock may also require root pruning to fix injured or girdling and circling roots.

What about winter planting? I have a colleague that described in great detail his ambition to move to California and seek academic employment after not getting a $.50/hour raise at his landscaping job during a long stint of chipping ice in Minnesota to plant conifers in frozen ground. My colleague just retired from a nice career in Cooperative Extension, but that winter planting helped him make the move. You can plant trees in frozen soil but winter kill is a thing and the success rate of such efforts is less than for fall planting. In Southern California and other areas of the southwest and southern USA, winter planting is preferred for fruit trees because you have great access to bare-root stock (only in Winter actually) and we don’t contend with frozen soils. If it ends up being a super wet winter (see the previous blog by Pam for insight on that) it can be a problem when newly planted trees sit in saturated soils for weeks on end.

Spring planting is a thing because Arbor Day is in spring and everyone wants to plant trees on Arbor Day right? Spring planting is sometimes limited by availability, bare root stock is usually sold out or moved into containers. I don’t like buying left over stock because the leftovers are often not the best. And, trees may be in a new growth or flowering phase and their root systems are activating.

This leaves summer. In Southern California shade trees are planted all year. Fruit trees planted in summer will be the left overs from winter and again I don’t like left overs, so I generally don’t plant fruit trees then. Subtropicals establish well in warm weather so mangoes, avocados and citrus are easily planted in summer if irrigation is assured.

All container stock, even boxed trees, should be inspected for girdling roots. Planting large trees requires careful monitoring after planting to assure success.

So the harsher the climate, the more restrictive the planting dates, but Fall is still best. In mild climates of southern states you can plant when you want in most cases. But avoiding months with frost is usually helpful as nursery stock often has tender growth. In almost all cases follow your plantings with a generous ring of arborist chips, avoid planting directly in turfgrass and irrigate your tree like it is still in the nursery for the first few weeks until it roots into the native soil. Do not amend the backfill and PLEASE remove the nursery stake at planting. Provide whatever the tree needs to stand upright with loose ties to poles outside the rootzone. Plant trees where they have room to grow and access to sunlight for most of the day. Plant HO!