Understanding the mysteries of plant diseases: Prevention, Control and Cure (Part 3 of 3 in this blog series)

Understanding the mysteries of plant diseases: Prevention, Control and Cure (Part 3 of 3 in this blog series)

What next?
You’ve done your research and made a diagnosis—now what? Sometimes the plant has to be removed and never planted there again. Start over, do something else.

Some diseases are difficult or impossible to control (virus diseases) especially when they are new or unknown to science.

Controlling plant pathogens or abiotic disorders can be daunting, frustrating, even impossible. As I mentioned in the last blog early detection gives more options for control because the disease has not advanced to a degree where it can not be controlled. Controlling plant diseases is not just palliative (treating your plant’s pain) it involves understanding where pathogens come from, stopping their movement, arresting their development and preventing their spread. Understanding genetics of resistance can offer amazing control of diseases, and finally biological control limits the development and spread of many pathogens.

Virus particles can only be seen with an electron microscope, since signs are hard to visualize, early detection is very difficult. Shown are TMV virions

What Can What Can’t?
There are some battles that can’t be fought or fought easily with plant pathogens. When plants are infected with viruses, there is almost no control option but removal (roguing). All plants likely contain some kind of plant virus; but not all viruses in plants cause symptoms of disease. Dangerous viruses like tomato spotted wilt, impatiens necrotic spot virus, cucumber mosaic virus, or many others, are devastating to their hosts and once infected there is no controlling these. Removing infected plants at the first symptom of viral involvement is prudent but often infections have already spread. Viral pathogens almost always infect without significant symptoms,  and become systemic in the plant before their more

Once trees with root rot show symptoms it is too late for control measures, the tree is dead and will not recover. Here Phytophthora spp. has killed a eucalyptus tree.

devastating effects become visible.  By late season, in most vegetable gardens, viral titre (concentration) is very high in solanaceae plants (pepper, tomato etc) and in cucurbits, both groups are highly susceptible. When perennial plants get viral pathogens there is no cure, and symptoms will increase over time. In orchids, viruses can sometimes be avoided by tissue culturing the meristem (which is usually virus free) to clean up a rare plant worthy of salvation. Sometimes plants are already dead but don’t look it. In the case of root rotted trees and shrubs, leaves may still hang from the tree, may still be green but the tree is beyond salvation, control may not be possible. In general control measures are best conducted early.  And by early I mean before you obtain your plant!

An old adage goes: “An ounce of prevention is worth a pound of cure” This is especially so when there is no cure!.  Prevention as a control technique really involves several factors such as exclusion, quarantine, and maintaining plant health so that plants are not predisposed to disease. The first tenet of control is exclusion. Don’t bring pathogens to your garden. Gardeners are their own worst enemy where plant diseases are concerned. Since pathogens can be seed-borne, come with insects, be already infected in the nursery, or resident in soil, care must be exercised when new plants are selected for your garden. Practice safe plant swapping!  Gardeners sharing plants with each other may also be sharing their respective plant diseases! Be careful where you buy plants, sloppy nurseries with their plants on the ground in standing water is a red flag.  Also be on the look out for weeds in nurseries since they can harbor insects that vector virus diseases. Plant debris left on the ground and not cleaned up, can be a source of fungal spores.  So consider the source when selecting plants for purchase.  Inspect plants carefully before purchase, especially slipping the container off to look at the root

All plants should be inspected carefully, especially root systems. Always remove a sample from containers to inspect. Here cyclamen are shown with root rot.

system. I don’t purchase anything (even boxed trees) without doing this first. If you are satisfied that you have a healthy plant then you are ready for the next phase of disease control.

Plants are often quarantined before they are released for cultivation or planting. When you bring your plant home, leave it in the pot for some time. Even bedding plants if purchased young can grow for a bit in containers. Remember lack of growth is a symptom of incipient disease.  Observe your new purchase of a few days or even weeks depending on the plant. If normal growth is occurring then move on to garden placement and planting. A little time set apart from other plants, and careful observation, will possibly prevent bringing something bad to your garden.

Once disease is established, and symptoms are apparent, gardeners often turn to pesticides to try to provide therapy. Sometimes fungicides applied to a plant post infection will slow down the spread of the pathogen within or on a plant. Therapeutic approaches can also turn on plant defense systems or enhance them so that the plant can limit the progress of disease. Therapy is usually not an option with most diseases because the pathogen has often gone beyond the point of stopping it by the time disease is recognized.  Some fungicides applied early, can be very therapeutic in turfgrass diseases, blights and powdery mildew diseases. The key to therapy as a control option is to detect the disease early and use an efficacious material that is labeled to control the pathogen you think is causing the disease on a given plant.  All this should be on the label.

Pruning out branches with cankers is a form of eradication.

An immediate response of many gardeners when disease is discovered is to kill the pathogen. This is eradication. Eradication takes several forms. There are eradicant pesticides, that kill the pathogen on contact. Usually these cause some degree of harm for the host since most pathogens have a host relationship that is destroyed when the pathogen is killed. Eradication can also be  or removing plants from the garden that are a source of disease. Picking up and disposing of fallen plant debris is eradicating a source of potential inoculum from the garden. Pruning cankered branches from a tree is a form of eradication.

One of the best forms of disease control is resistance. Selecting plants that resist disease is built in control. Diseases such as rust and powdery mildew have wide ranges of interaction with their hosts. By selecting plants that are resistant, there is no need for other control measures such as sprays. Resistance to plant disease comes as two types. Horizontal or multigenic resistance is partial or incomplete resistance and is conferred by several genes or their interactions in the host with the pathogen. Vertical or complete resistance is resistance conferred by a single gene in the host. Plants with horizontal resistance will get some of the disease but it won’t be overwhelming, often in grains, lack of resistance will result in complete crop failure. Plants with vertical resistance show no symptoms and are completely immune to the pathogen. While this complete resistance is appealing, it is only conferred by a single gene, and the pathogen can easily break down this resistance and cause disastrous disease. Horizontal resistance while not complete, is called “durable” resistance because it takes more time to overcome resistance conferred by multiple genes. The nature of resistance in garden plants is rarely detailed by growers or seed sellers. Often we are lucky to see any labeling for resistance. Crops like rose, crape myrtle and snap dragon are often sold as resistant to powdery mildew or rust and in some cases plant breeding programs strive to incorporate disease resistance into their breeding lines but then fail to label the product as disease resistant!

Cultural control often involves doing the right horticulture. Keeping turfgrass away from the stem of trees, applying arborist chips, planting at the right depth, are all cultural controls for root disease.

Cultural control is using good horticultural practices to limit the development of disease. Since many plant pathogens require a host to be predisposed, we have the opportunity through good horticulture to avoid the disease development. Planting woody ornamentals at the right depth is a cultural control of Phytophthora collar rots.  Appropriate application of water, reduces stress and prevents plants from being predisposed to both root rots and canker diseases. Correct pruning cuts limit the development of decay in trees.  Appropriate horticulture as discussed in the Garden Professors page will go far toward cultural control of common garden maladies.  Proper plant selection is also a form of cultural control. Choosing plants adapted to the growing area climate, and soils selects plants that are less likely to be predisposed to disease. Poorly adapted plants are more susceptible to pathogens and thus more likely to become diseased.

Biological control is the effect of one organism limiting the development of another thus preventing disease. Classical bio-control is when an exotic pest is introduced and there no natural enemies or parasites to regulate it. The pest/pathogen multiplies rapidly killing or affecting a large plant population. Research in the native range of the host looks for native organisms to control the pest. They are brought to the infestation, released and the pest/disease is brought into control. This works well with insects and the damage they cause. It has also been achieved with exotic plant pathogens.  For our native pathogens, there may already be a community of organisms that limit its development.  This is especially true for soil-borne pathogens. This is why the GP professors so often recommend fresh wood chips as mulch.  Fresh wood chip mulches supply carbon for organisms in soil that interfere with soil-borne pathogens; a kind of mulch-mediated bio control for root diseases.

I find controlling diseases is a lot more difficult than understanding or identifying them.  Usually by the time you have observed disease in the garden it is too late to stop its progress. You can take mental notes not to plant that variety again, or prune more diligently etc. but diseases are largely regulated or advantaged by the environment and our good or bad gardening practices. Of course the pathogen has to be present for biotic disease to happen, as we know that organisms don’t spontaneously generate.  Disease control starts with identification then research and finally gardening actions that help prevent, limit or eradicate disease propagules.

Supplemental Lights for Home Seed Starting and Indoor Growing: Some Considerations

Whether you’ve already got seedlings growing away or getting ready to start your annual indoor seed starting, one of the important factors in seed starting is light.  (Last month I covered heat, which you can see here).  Questions like “Do I need to use supplemental light or can I use a window?” and “What kind of light do I need to use?” are ones we often get from gardeners – new and seasoned alike.  So I thought I’d take a little time to talk about light – the factors that are important for plant growth some ways that you can make sure you’re providing the right kinds and amounts of light to your new seedlings.  Keeping these ideas in mind can help you choose lights for your seeds starting (or other plant needs), whether it is a simple shop light ballast from the hardware store, a pre-fab light cart system, or even higher-tech LED system.

Plants require light for several of their functions, most importantly the process of photosynthesis.  The green pigments in plants (Chlorophyll A and B) act as receptors, gathering electrons from the light to use as an energy source to manufacture glucose, which is stored in the plant in a number of ways and then ultimately broken down in respiration to release energy to support plant functions.  There are three aspects to light that gardeners should keep in mind for supplemental lighting: quality (color), quantity (brightness/intensity), and duration (day/night length).

Duration is a relatively simple concept when it comes to seeds starting and light set-ups.  Gardeners will want to try to mimic the natural environment that will be provided by the sun.  For the most part, aiming for 16 hours of light and 8 hours of dark is standard.  This gives the plant sufficient light, but also provides a rest period which can be important for plant functions.  Most gardeners find it handy to invest in timers to turn lights on and off, rather than trying to remember to do it themselves.  This can be a simple on-off set up from the hardware store (after-holiday shopping can be a good way to pick them up on sale in the string light section) to something more elaborate from grower suppliers.  Duration could be more important if you’re doing longer term growing beyond seeds starting, as day length affects initiating of flowering in some plants.

Intensity refers to how bright the lights are.  Some lucky people have big windows with lots of bright light for starting seeds, but even for them intensity (and duration) may not be enough during the shorter, grayer days of winter. Growing in bright windows can sometimes be a challenge because the light is coming from the side rather than above, so plants often grow toward the window and need to be rotated.  Supplemental light can increase intensity and lengthen duration, even for plants grown in windows.

Most commonly, light bulbs are sold by wattage as a measure of their energy (light) output.  Standard tube florescent lights are generally around the 40 Watt level, but some of the full spectrum plant lights come in 54W options.  If you can find it, the higher wattage can make a big difference in the intensity of light and thus the production of your plants.  Even at the higher wattage, you’ll want to get a ballast that holds at least two bulbs (and some grow light ballasts hold more).  You can further control the intensity of light reaching your plants by increasing or decreasing the distance between the plants and the lights.  This is why the pre-made plant carts have a chain or other mechanism for you to raise and lower the lamps.  For fluorescents, lights are sometimes lowered to around an inch above the canopy of the lights.  For high intensity LEDs, the distance may need to be more.  (If you’re using lights for long-term growth of, say houseplants, you’ll have to experiment with the distance to meet the intensity needs of the plants – closer for high light plants and farther away for low light plants).

Light Quality: The Rainbow Connection

Sunlight, or white light, is composed of all of the colors of the spectrum. Think back to art class and our friend ROY G BIV – the colors of the rainbow.  There’s also parts of the spectrum that we don’t see like ultraviolet and infrared.  For photosynthesis, plants mostly use light in the red and blue spectrum (referred to as Photosynthetic Active Radiation, or PAR), though almost all of the colors have some sort of effect or function on plants.  Blue light has a role in promoting vegetative growth in plants, while red has a role in promoting flowering.

Image result for plant light spectrum

For most applications, supplemental light for seed starting or other indoor growing should be full-spectrum.  You can achieve this in a variety of ways – buying specific full-spectrum plant light bulbs is the best, but you can buy non-plant specific full spectrum bulbs as well.  For small-scale home growers and beginners, it can be as simple as buying a shop light ballast at the hardware/box store with a full spectrum bulb.  For more intensive or large-scale growers, there are lots of sources for higher-end, full spectrum grow lights that you can buy from specialty garden retailers, but these are often more than what home gardeners starting seeds indoors need.

Fluorescent vs LED

Image result for fluorescent shop light
Typical shop light ballast

These days you might be presented with a choice of lights – fluorescent vs. LED.  There are some positives and negatives to each.  While they have a higher up-front cost, LEDs use much less energy than fluorescents and can save money over several seasons of use.  The reduced energy usage also means there’s less energy loss in the form of heat, which can be a positive if you are always struggling with creating excess heat that burns your plants, but a negative if you’re relying on that heat to help keep the temperatures up (see my article from last month on heat and seed starting) or have issues with drying out your growing media.  Fluorescents on the other hand can be more affordable up-front, but have a higher energy usage that will result in higher electric bills over time.

understanding the basics of grow lights for indoor plants and indoor gardening
LED grow light via Shutterstock by nikkytok

You might have noticed in your searching or in visiting some growers that LED lights for plant growth come in either white (full spectrum) or a red/blue combination which end up giving a purple light.  Since LEDs give a larger control over the spectrum of light, growers, especially larger scale intensive operations, use these red/blue combinations as a means to add further energy efficiency since it is the blue and red spectra that are the photosynthetic. By eliminating the spectra that are largely reflected rather than absorbed, less energy is used.  This is useful in hydroponic and vertical farming systems where short-term crops are being grown quickly and where profit margins can be slim.

You can read (and listen to) more about light in the Joe Gardener podcast and article on seeds starting I was interviewed for last year with Joe Lamp’l.

However, research has emerged in the last few years that expanding the spectra of light in LED systems increases production. Research has shown that incorporating green LEDs significantly increases production over just red/blue LEDs (some of that research was by Kevin Folta, who is one of the leading science communicators on biotechnology). While green plants largely reflect rather than absorb green light, it does have some effect on plant functions.   (Research also shows that adding the green makes the light appear a little more natural to workers in facilities like greenhouses and makes it easier to see issues with the plants – the purple of the red/blue systems washes out the plants and makes it hard to see differences in leaves like diseases).

So if you’re looking at LEDs for seeds starting, and especially if you’re looking at them for longer term indoor plant growing, stick with full spectrum or explore one of the LED systems that incorporates green.  Though don’t be afraid to experiment with the colorful LED options – I have a small red/blue system to supplement light to my office potted lime.  The key is to experiment and shop around – every gardener’s need for supplemental light is different and the solutions to those needs are different.  Don’t be afraid to start small with that shop light from the hardware store before working your way up – especially if you’re just starting a small amount of seeds in the spring.

 

Is it good advice? Or is it CRAP?

In my educational seminars I’ve long shared a version of the CRAAP test (currency, relevance, authority, accuracy, and purpose) for analyzing information related to gardens and landscapes. My version is CRAP (credibility, relevance, accuracy, purpose), and we’ve published an Extension Manual that explains in detail how to apply it. This past week I was at the Philadelphia Flower Show participating in Bartlett’s Tree Care Update panel. Given that the theme of the show was “Flower Power,” I figured that a talk on Magical Mystery Cures was in order. And the 1960’s was the decade where the late Jerry Baker gained prominence as a garden authority – and whose presence is still widely felt nearly 60 years later.

And anthropomorphizing of plants begins….

 

 

 

 

 

 

 

 

 

Now, I could spend the rest of the year discussing all of Jerry’s advice, tips, and tonics for gardens – but it’s more useful to determine whether he is a credible source of reliable information. So let’s apply the CRAP test.

C = credibility. What are Jerry’s credentials as a garden expert? It’s easy to find this information from the internet, including the Jerry Baker website. He had no academic training in plant or soil sciences but started his career as an undercover cop who often posed as a landscaper. His books are all popular publications, meaning they have not gone through critical review by experts before publication.

R = relevance. For our purposes, his information is relevant to our focus of managing gardens and landscapes (as opposed to production agriculture, for instance).

A = accuracy. Jerry’s advice is not based on any scientific source. He relies on common-sense approaches, folklore, and his grandmother’s advice. In fact, many of his assertions are at odds with published scientific evidence. Now, science evolves, and older scientific publications are sometimes found to be inaccurate after new information comes to light. If Jerry’s books were meant to be accurate sources of information, they would be updated with new findings as subsequent editions were published. This is what happens with textbooks, for example.

P = purpose. What is Jerry’s ultimate purpose? It’s sales. There’s no way around this conclusion. Over twenty million copies of his books have been sold, and during his career he became the spokesperson for several gardening products. Probably the most well-known of these was the Garden Weasel (which parenthetically is a great way to destroy fine roots and soil structure). There’s no doubt he was a brilliant self-promoter and marketer. But he was not a reliable resource, and many of his “tips and tonics” are extraordinarily harmful to plants, pets, and the environment.

“Garden Weasel” courtesy of Wikipedia

While I was wrapping up my research on Jerry Baker I was particularly taken by a chapter in one of his books (one of his Back to Nature Almanacs) called “The Tree Quacks.” I thought some of these quotes were particularly ironic:

The source…
…and the quotes

 

 

 

 

 

 

 

Imagine my surprise when I discovered that these quotes were actually not his own. In fact, the entire chapter was plagiarized from a 1964 article by John Haller in Popular Science, which is online. This action is uncomfortably similar to his 1985 trademarking of the phrase “America’s Master Gardener,” 12 years after the Master Gardener program was formed (but not trademarked) at Washington State University.

Text from 1964 Popular Science article

I hope this post has helped you learn to analyze the credibility of information and information sources. If so, you can claim the of America’s Master CRAPper ™!