Plant Disease Primer-Part 2: Fungus Among Us

In my last post, I talked about the factors leading to the development of plant diseases and some common signs and symptoms of fungal, bacterial, and viral diseases. In this installment of the series, I’m going to talk about some of the most common fungal 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 we see come into the extension office for diagnosis.

Common Fungal Diseases

  • Powdery Mildew
    • Symptoms: White powdery spots on leaves and stems
    • Common hosts: a wide range of plants, but peonies, lilacs, squashes, cucumbers, and roses are what we see most often
Powdery Mildew on Peony
  • Downy Mildew
    • Symptoms: yellowish or whitish spots on the tops of leaves with gray-ish fuzzy growth underneath
    • Common hosts: downy mildew affects many plants, but basil, impatiens, cucurbits, and verbena constitute the most common questions.
  • Rusts
    • Most rust lifecycles require two host plants: a primary and alternate host
    • Common rusts:
      • Cedar-apple rust (Junipers– primary, Apple/pear – alternate)
      • White pine blister rust (white pine – primary, gooseberry/currant – alternate)
      • Hollyhock rust (no alternate host)
    • Symptoms: rust colored (orange/yellow/red) pustules or blotches on leaves, stems, and fruit; may appear as gummy structures on primary host
  • Leaf Blights
    • The most common blights we see are often for tomatoes and their relatives. There are many leaf spots and blights that affect these plants, but early blight seems to be one of the most common.
    • Common leaf blights
      • Early Blight of tomato: irregularly shaped lesions on leaves, often with concentric rings and yellow halos. Eventual leaf curling, necrosis, or dropping. Severe cases can end up with lesions on stems and fruits.
      • Late Blight of Tomato and Potato: Starting as small water-soaked lesions and turning into large, purple-brown, oily looking blotches. Blotches appear on leaves, stems, and eventually fruit.
Early blight of tomato, Source: UMN Extension
  • Anthracnose (multiple species and target species)
    • Anthracnose affects a wide range of plants, but we often see shade trees such as oak and sycamore, dogwoods, beans, peppers, and cucurbits as commonly affected plants.
    • Symptoms: dark, sunken ulcer-like lesions on leaves, stems, fruits, and flowers.
Anthracnose on watermelon, Source: UMN Extension
  • Cankers
    • Common cankers:
      • Cytospera – spruce, pine, poplar, willow
      • Phomopsis – juniper, Russian olive, Douglas-fir, arborvitae
      • Nectria – honey locust, oak, maple
    • Symptoms: sunken necrotic lesions (cankers) on twigs, stems, and trunks of trees. Often leading to death of the plant beyond the canker location. This is especially problematic for trunk cankers which often lead to the death of the whole plant.
Nectria canker on young maple trunk Source: Missouri Botanic Garden
  • Root Rots
    • Common rots:
      • Phytopthora – affects many species to cause root rot
      • Armaillaria – especially problematic for trees
Armillaria root rot (fruiting bodies), Source: UC ANR
  • Fruit rots
    • Common rots:
      • Black rot – many species, but often apple and pear
      • Brown rot – peaches, plums, cherries, and related species
Brown rot on peach, Source: Rutgers

Fungus Treatment

Once a plant is infected with a fungus, it is difficult to eliminate the disease and 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 fungal pathogens, and in woody perennials where symptoms include cankers or rots that affect perennial plant parts such as stems or trunks. Fungal 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. This removes a great deal of the fungal organism from the plant that is likely still producing spores or hyphae to spread through the plant. Removal of affected foliage for foliar diseases is key.

In cankers, removal of whole branches or twigs starting at least a few inches below the canker location is necessary. 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 fungicides may be necessary to reduce spread of the disease further. Often repeated treatments through the season are needed once the disease is established in the nearby environment. Copper sulfate is a common organic option for treatment of fungal pathogens, but may not be effective for every disease. 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.  Chlorothalonil is a widely used conventional fungicide and will help control many, but not all, fungal diseases.  For specific fungicide recommendations for your area, contact your local extension expert.

Fungus Prevention through IPM

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

  • Use correct plant spacing and pruning to ensure airflow around plants. This can reduce humidity within the plant structure and moving air can reduce the number of fungal spores that land on the plant.
  • 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
  • Remove rust alternate hosts (not always possible), such as junipers if you’re growing apples (or vice versa)
  • Utilize biofungicides as a preventative measure. Products containing different types of Bacillus bacteria can be competitive with disease-causing organisms and limit their ability to form on leaves.
  • Practice good hygiene in the garden by cleaning up any fallen or diseased leaves, fruits, etc.

Wrapping it up

There are lots of fungal 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 bacterial diseases. Stay tuned!

Plant Disease Primer: Part 1 – Shaping up the causes, signs, and symptoms of disease

Throughout the garden season, extension professionals all across the country get to play detective when trying to diagnose plant diseases and recommend specific controls or preventative measures.  We often have to put on our Sherlock Holmes-esque thinking caps and our standard issue detective’s magnifying glass (or microscope) to diagnose plant maladies.

Having a basic understanding of diseases, how they function, and what they look like is key. Gardeners who bring samples or pictures into our office often get exasperated when we play twenty questions trying to figure out if it is a fungus, bacteria, or virus (or something else) causing the issue. Knowing about placement, environment, planting, etc. can all be keys in discovering what might be causing the issue.  Sometimes we can’t identify an exact disease at a glance and have to send things to the diagnostic lab on campus, but by looking at signs and symptoms and identifying factors about the plant we can often figure out the type of pathogen causing the issue, or whether it might be environmental, abiotic, or insect related. 

What leads to plant diseases?

Of course, the thing that causes the disease is a pathogen or a causal agent such as a fungus, bacteria, or virus (or a few other odds and ends like phytoplasmas). But there are other factors at play to get a disease infection started and sustained. You need all of the factors in place for infection. This is often represented as a triangle, where a causal agent (pathogen) must be present with the right environmental conditions and a host plant that can actually be infected by the pathogen. I’ve also seen a plant disease “pyramid” where time is added as another factor (as in, the correct conditions must be present at the same time and for a long enough period for infection to start).  And still yet in researching this article I found the PLANT DISEASE TETRAHEDRON, which adds human activity as another factor.  What’s next, the plant disease fractal? 

But I digress. When a sample comes in to our office, we play twenty questions with the gardener asking about these different factors. Like is it irrigated (many diseases need water to be spread or to develop), does it get shade or full sun, have you seen any insect activity, when do you usually work in the garden, etc.  These questions can help us identify parts of the disease triangle/pyramid/tetrahedron that could inform the diagnosis. 

Keeping these factors in mind can also help gardeners reduce the likelihood of disease through IPM.  For example, viruses require a vector – usually an insect, animal, or human to spread the disease. Many viruses are spread through leafhoppers, bacterial wilt in cucurbits is spread through cucumber beetles, and mosaic viruses like tobacco mosaic virus has many vectors including humans and garden tools (which is why many green industry businesses have strict sanitation rules, including rules for tobacco users and hand sanitation). Knowing that fungi and bacteria can be airborne with spores or splashed by “wind splashed rain” or irrigation water can lead to improved practices like mulching, pruning for good air flow, and plant spacing. 

How can you tell which diseases are which?

Let’s face it, many plant diseases look very similar. There are usually what we call spots and rots that can be very similar.  But there are some identifying characteristics that help us at least determine what type of pathogen or causal agent is causing the issue.

The first thing to keep in mind is that plant diseases have both signs and symptoms.  Signs are the presence of the actual disease causing organism, visible to the eye.  Fungal diseases often have mycelia, or fungal threads, and reproductive structures like pycnidia present. You may also see the causal agent itself, such as leaf rust or powdery mildew. Bacteria will often have exudates that ooze out of plant parts, water soaked lesions, and bacteria that stream out of cut stems parts. You can actually diagnose some bacterial infections by suspending a cut stem in water and watching bacteria stream/ooze out of the cut.  Viruses are not visible to the human eye, therefore do not have signs.

Powdery mildew on peony Source: Douglas/Sarpy Extension – Nebraska

Symptoms, on the other hand, are the effects of the pathogen on the plant. Common symptoms of fungal infections are leaf spots, spots or rots of fruits, chlorosis, and damping off in new seedlings. Bacterial symptoms include leaf spots (often with a yellow halo around them), crown gall, stem/trunk cankers, wilting, shepherd’s crook (like fire blight), and fruit rots. And since bacteria usually depend on streaming through liquids, they often leave definitive patterns for leaf spots that align with vein structures. For example, leaf spots will often be angular because they are “trapped” in between veins on the leaf. The most common symptom of viruses is a mosaic pattern on leaves and fruit, but also crinkling and yellowing of leaves, necrosis, and stunting.

Angular leaf spot on cucumber – the symptom pattern falls within veins, giving the angular appearance. Source: UMN Extension

Special mention- phytoplasma: Phytoplasmas are single-celled organisms that aren’t really bacteria, but are descended from them. They don’t have cell walls and are transmitted to plants through an insect vector like leaf hoppers.  The most common type of phytoplasma diseases is called yellows (aster yellows, ash yellows, etc.) because plants often turn yellow. The symptoms are often interesting. Witches brooming, which is irregular growth that makes branches look like brooms is common. Aster yellows is a common disease affecting many plants in the aster family. Most commonly, flowers of these plants look distorted and may grow leafy structures instead of flower structures. 

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

To wrap it up

It can be difficult to figure out what diseases are affecting plants, if it is a disease at all.  Getting help in determining what disease might be affecting plants can help you treat or prevent the problem in the future.  In my next installment of this series, I’ll talk about common diseases, their signs and symptoms, and treatments and preventative measures. 


Pest Profile: Spotted Lanternfly

We have seen many high-profile examples of insect invasions, and as gardeners, we have probably come across some of these species in our very own landscapes and experienced their impacts first-hand.

If you live in the Eastern part of the United States, you have probably already heard about one of these invasive insect species that is currently wreaking havoc. The Spotted Lanternfly (SLF), Lycorma delicatula, is a 1 inch long planthopper native to China, and has since spread to Japan, South Korea, and the United States. This is a piercing/sucking insect (Order: Hemiptera) that feeds on the phloem of plants and excretes a sweet and sticky product called honeydew. This feeding damage, especially in large populations, can impact the health of certain plant species. Not to mention the nuisance potential, as any objects under infestations of this insect will find themselves coated in a sticky layer of honeydew.

Picture of a pinned adult Spotted Lanternfly (Photo: Lawrence Barringer, Pennsylvania Department of Agriculture, )

It was first detected in Pennsylvania in 2014, and can now be found in several surrounding states including Delaware, Indiana, Maryland, Massachusetts, Michigan, New Jersey, New York, North Carolina, Ohio, Rhode Island, Virginia, and West Virginia, although most states are considered at risk for SLF invasion. Although the insect itself can’t fly long distances, it can be easily spread by moving infested materials and through their egg masses which look fairly nondescript (like a small smear of mud). Several states are currently quarantining this pest, so follow regulatory guidelines by visiting your state’s department of agriculture. Inspect your vehicles and personal effects for the insects and their egg masses (and scrape them off/squish them) especially if you are traveling through these quarantine areas to prevent spreading them to new locations.

Spotted Lanternfly egg mass on the bark of a tree (Photo: Pennsylvania Department of Agriculture , )

This insect has over 100 potential host species, and this wide dietary breadth adds unique challenges to this insect’s pest potential. Its preferred host plant is another invasive species: Tree of Heaven (Ailantis altissima), which is currently widespread in the US and parts of Canada.

A group of Spotted Lanternfly adults (Photo: Lawrence Barringer, Pennsylvania Department of Agriculture, )

SLF can also be problematic for some important fruit crops such as grapes, where it has the potential to reduce fruit yield, impact fruit quality, and potentially reduce hardiness and winter survival. There are also other economically important trees that this insect feeds on, including apple, maple, black walnut, birch, willow, etc.. Feeding damage can stress plants leaving them susceptible to other pests and diseases. If this pest continues to spread it could have significant impacts on the US grape, horticulture, and forestry industries.

Invasive insect species can also have significant impacts on natural ecosystems, and can tip the balance of a well-functioning food web. Adding a pest that often has very few adapted natural enemies, and especially those that can reduce the availability of an important food and shelter source for other native organisms can result in cascading ecological effects that can be difficult to understand and manage.


It is important to stay vigilant in keeping an eye out for invasive species such as Spotted Lanternfly, so if you see this insect outside of a currently quarantined area, before you squish the bug; take note of where you spotted it and report it!

State-specific reporting guidelines for Spotted Lanternfly can be found here:

If you are curious about other current/potential invasive pests in the US (and state specific guidelines for invasive pests) visit:

To learn more about this insect, visit:

You can also reach out for more information to your state department of agriculture, or your local and regional extension offices.

Water: Garden Friend….and Foe? – Water, Relative Humidity, and Plant Diseases

We all know that water is essential for life and that we have to ensure our landscapes, gardens, and houseplants all have a sufficient supply of the stuff.  Forget to water your garden during a hot, dry spell and it could mean disaster for your plants.  But water can also create issues for plants, usually when it is in an overabundance – water helps spread and develop diseases on foliage and excess soil moisture can damage roots, creating opportunities for root rots and other diseases.  How do you meet the water needs of the plant while also avoiding issues associated water?  Understanding how water affects disease organisms will help, along with some tried and true Integrated Pest Management Strategies.

Water and Pathogenic Microbes

Both bacteria and fungi require water to grow and reproduce.  Most do not have a mechanism to actively take up and manage water, so they uptake water mainly through osmosis.  This means there must be some form of water present for those microbes that are actively growing and especially for processes like reproduction which use not only a lot of energy but might also be required to carry spores in order to spread.

File:Septoria lycopersici malagutii leaf spot on tomato leaf.jpg -  Wikimedia Commons
Septoria leaf spot, a common fungal disease of tomato that requires water for initiation and development.

Both pathogenic microbes and beneficial (or neutral) microbes require water to thrive.  It is one side of what we refer to as the disease triangle.  Water (along with temperature) are major components of the “favorable environment” side of the triangle, with the other sides being a plant capable of being infected and a population of pathogens capable of infecting.  Those last two sides meaning you have to have a population of the pathogen big enough to initiate or sustain an infection and a plant that can actually be infected by that pathogen.  For example – one disease spore may or may not be enough to start an infection (depending on the pathogen), but several hundreds or thousands definitely can.  And the pathogen has to be one that can actually infect the plant – it doesn’t matter if you have a million spores of Alternaria solani (one of two closely related fungi that cause early blight in tomatoes) on your cucumber plants, they likely won’t get a disease.  But if there are spores of A. cucumerina, a different species, you’ll likely get leaf spot on those cucumbers.  But it doesn’t matter if you have both a susceptible plant and a pathogen, there has to be a favorable environment (water and temperature) for there to be a disease infection. 

As this paper points out, water in the form of liquid (rain, ground water, dew, etc) and vapor (air humidity, fog) can provide the environment for microbe development in the soil and on foliage.  Microbes in the soil are ubiquitous as water is typically available in most soils (except in droughty or arid areas) , but excess soil moisture can create booms in populations for both the “good” microbes and the “bad” ones.  Microbes that live on foliage (sometimes referred to as epiphytic since they rely on moisture from the atmosphere) are much more likely to be water stressed since they are exposed to the atmosphere.  When there isn’t water available on the surface of leaves (from rain, fog, etc.) microbes tend to colonize around areas where water leaves the plant – stomata and to a lesser extent around tricomes and hairs. 

The paper also points out high atmospheric humidity is positively correlated with the number of fungi on a leaf surface. It’s also a requirement for diseases microbe spores to germinate, for filamentous fungi to break dormancy, for pathogen survival, for microbe movement on the leaf surface, and for disease infections to be sustained.  It is also shown that heavy precipitation increases water availability to these microbes thus hastening their growth.  Precipitation also dislodges and disperses pathogen spores and cells to adjacent plant tissues, and to leaves of nearby plants.  High humidity also makes leaf cuticles more permeable and promotes opening of the stomata, which can serve as an entry point for pathogenic infection.

Once inside the plant, microbes such as fungi and bacteria can thrive on the aqueous environment inside a plant, moving easily between cells or into the vascular tissue (depending on disease).  Pathogens that thrive in wet conditions, however, may initiate water soaked lesions on the plant to develop conditions favorable to their growth. 

Water, water everywhere – so is there anything you can do?

Of course, water is naturally occurring and in most places falls from the sky in some form or another.  In some places very little precipitation falls, in others there’s a lot. And don’t forget about the humidity, dew, and fog (which are often more common in places that get more rain, but provide moisture even in dry climates).  There are a few places where the atmospheric moisture levels are in that “just right” zone to support plant growth but not pathogen growth, which makes agricultural production of certain crops easier.  You could consider these areas the “Goldilocks” zone for crop production.  For example, a lot of seed crops are produced in the Midwest and arid north West, potatoes in Idaho, apples in Washington, etc.  The conditions there mean that, at least when those crops were getting established (before the advent of modern pesticides) in those regions, disease pressure was low. 

You can’t stop the rain, of course, if you’re in a place both blessed and cursed with abundant rainfall or atmospheric humidity.  But there are some things that you can do reduce the likelihood of diseases spread or supported by that water and humidity.

  • Evidence shows that there is a positive correlation between the density of planting and disease incidence.  Therefore, proper plant spacing and pruning can do at least three major things.  First, having space between plants, especially in the vegetable garden, can reduce the splashing of pathogens from one plant to the next during a precipitation event.  Second, it increases air flow through the plant, which can reduce the likelihood of pathogen spores that might float in and land on foliage.  Third, it reduces humidity in the immediate microclimate around the plant. The increased air flow in addition to the reduced amount of foliage that is releasing water through transpiration can have a significant effect on the humidity, which can have a big effect on the germination, establishment, and survival.  
  • Utilize diverse planting plans in the vegetable garden and the landscape.  Research shows that while having a variety of plants increases the diversity of disease organisms, it actually reduces the infection rate possibly because pathogens splashing from plant to plant are less likely to find a host plant if they are surrounded by non-host plants.  This practice is promoted in intensive vegetable plantings such as square foot gardening. 
  • As stated earlier, precipitation can drastically increase the population of microbes on foliage.  This also includes water from overhead irrigation.  For example, this study found that overhead watering of cabbage led to significantly higher and faster rates of spread of the black rot fungus as compared to drip irrigation.  Therefore, reducing or avoiding overhead watering can reduce the likelihood of disease incidence. 
  • Timing of watering may also contribute to disease development.  The dew point, which usually happens during the night time hours, is when the air is totally saturated at 100% relative humidity and therefore cannot hold any more water.  This is the point where excess moisture is deposited as dew on surfaces (another source of water on the foliage) and little to no evaporation of water already on surfaces happens (learn more at  As shared in this book chapter review, lower temperatures resulting in reaching the dew point can extend the time leaves are exposed to high moisture and result in higher disease incidence. 
  • As our own GP Linda Chalker-Scott points out in this review, mulching not only retains soil moisture, reduces erosion and more but also reduces the incidence of disease in plants by reducing the splashing of soil or spores from rain or irrigation onto the plant.  This drastically reduces disease spread from pathogens found in the soil or on plant debris.  The organic matter from organic mulches also has the benefit of increasing the population of beneficial microbes, which out-compete the pathogenic microbes. 
Mulching and drip irrigation can both significantly reduce disease incidence in gardens.
  • Crop rotation, where crops are not grown in the same soil or plot for a number of years, also reduces disease incidence by reducing pathogen loads in the soil or from crop residues left in the garden.  This study shows significantly reduced disease incidence on potato and onion when a crop rotation plan of four years is utilized (meaning that either onions or potatoes are not planted in the plot for a minimum of four years, with other crops planted between those years). 
  • If root rots and pathogens are a problem, try improving drainage around the garden. Adding organic matter can help with water permeability of the soil over time. Raised beds can also drain faster than in-ground gardens.
  • Of course, if you’re having lots of problems with certain diseases on your plants, these cultural controls may not be enough.  Finding resistant varieties may be a necessary step in breaking the disease cycle in your garden.


While water is required for plant growth, it can cause issues with plant diseases if there is too much or if it lingers on the wrong parts of the plant for too long.  Water from rainfall, irrigation, high humidity, fog, and dew can all lead to the initiation, development, and longevity of plant fungal or bacterial diseases.  Reducing the amount, persistence of water or humidity on or around foliage can significantly reduce the likelihood of plant disease incidence.  Methods such as reducing overhead irrigation, timing of irrigation, mulching, and crop rotation are key cultural methods in reducing diseases spread by water. 


Aung, K., Jiang, Y., & He, S. Y. (2018). The role of water in plant–microbe interactions. The Plant Journal, 93(4), 771-780.

Burdon, J., & Chilvers, G. A. (1982). Host density as a factor in plant disease ecology. Annual review of phytopathology, 20(1), 143-166.

Café-Filho, A. C., Lopes, C. A., & Rossato, M. (2019). Management of plant disease epidemics with irrigation practices. Irrigation in Agroecosystems, 123.

Chalker-Scott, L. (2007). Impact of mulches on landscape plants and the environment—a review. Journal of Environmental Horticulture25(4), 239-249.

Krauthausen, H. J., Laun, N., & Wohanka, W. (2011). Methods to reduce the spread of the black rot pathogen, Xanthomonas campestris pv. campestris, in brassica transplants. Journal of Plant Diseases and Protection, 118(1), 7-16.

Rottstock, T., Joshi, J., Kummer, V., & Fischer, M. (2014). Higher plant diversity promotes higher diversity of fungal pathogens, while it decreases pathogen infection per plant. Ecology95(7), 1907-1917.

Wright, P. J., Falloon, R. E., & Hedderley, D. (2017). A long-term vegetable crop rotation study to determine effects on soil microbial communities and soilborne diseases of potato and onion. New Zealand Journal of Crop and Horticultural Science, 45(1), 29-54.

Understanding mysteries of plant diseases

(Post 1 of 3 in this blog series)

Gardeners, especially those new to gardening may find they have a “black thumb.” Plants die for no reason! “Oh well chuck it in the greenwaste recycling can and start again.” Or… “Oh I can’t grow cyclamens!… They always die in my garden for some reason.” For many gardeners it is mysterious why some plants fail to thrive or die suddenly. Plant disease processes are complicated, and it requires some knowledge of botany (anatomy and physiology), genetics, and microbiology to really understand what is happening. Also, since microbes are microscopic and most pathogens are microbial we can’t always see them at work, especially before symptoms develop. Symptoms are plant responses to the action of a disease agent. In this post I will try to describe the different kinds of diseases, and where they come from.

There are two broad categories …
of plant disease possible in gardens: biotic  diseases and abiotic diseases. Biotic diseases have a disease agent called a pathogen. The pathogen can be microbial, or a nematode or a virus, or a parasitic seed plant. Bacteria and fungi are the most common microbes. It is debatable whether viral particles are living, so also debatable whether or not they are considered microbes. Of the biotic pathogens, fungi cause  most diseases in gardens.  Many pathogens rely on environmental conditions to favor their lifestyle, this is particularly true of bacteria which like moist, warm environments.

The other category of disease is the abiotic category. Abiotic diseases have no pathogen. An environmental condition such as an excess or lack of an environmental condition causes physiological changes in plants that develop symptoms. Extremes of temperature, light, humidity, soil or water chemistry, soil physical conditions, air quality, and pesticide residue can all lead to abiotic diseases. Since there is no pathogen there is no epidemic, and abiotic diseases are not infectious. So spread, occurrence and movement of abiotic diseases are usually different than biotic disorders

So how does disease happen?
I have heard many gardeners make sweeping statements like “overwatering killed my plant” or “It just died of neglect” or “insects killed it”. Plant pathologists describe the disease process with a cartoon called the disease tetrahedron. It describes the interaction of four things: the pathogen, the environment, the host and time. Of course it is only a triangle for abiotic diseases since there is no pathogen.

For disease to occur there must be an active pathogen present that is virulent (has genes to cause disease). The pathogen must have enough inoculum present to begin the disease process. A single spore rarely leads to a successful disease (although it can in some systems).  Most importantly the pathogen must have the right genetics to recognize its host.

Next the environment must be conducive to the pathogen and its development and/or  harmful or stressful to the host. The environment can cause the host stress while favoring the pathogen.  An example would be oxygen starvation in flooded roots. The environment must favor the pathogen’s build up and dispersal of its inoculum (infective propagules such as spores, cells or seeds). Often splashing rain during the warming spring period is important for their spores to reach a susceptible host.

Finally for disease to happen, the host must be susceptible to the pathogen and possibly predisposed in some way to its attack.  Pathogens also have phenotypic synchronicity, that is the ability to produce inoculum at the same time as the host is producing susceptible plant tissues (leaves, buds or stems).

The final facet of the tetrahedron is time. Diseases do not occur instantaneously (even though we may only notice them instantly) – it takes time for them to develop.  Disease life cycles or life histories describe how pathogens survive, reproduce and disseminate themselves through the environment over time. The tetrahedron can be used to understand the factors that lead to disease but also can be used as a way to stop or control diseases (more on that in another post).

So where do diseases come from and where are they going?
Abiotic diseases are caused by environmental extremes.  Another way to look at them is that they occur when there is a violation of the adaptations of the host.  In this regard when we grow plants not well adapted to our climate or environment they can be harmed. A good example is my papaya tree. Right now it has been harmed by low temperatures. Growing a papaya in Ojai, CA is a violation of its adaptations.

Jim’s papaya tree is intolerant of Ojai winter temperatures–a violation of its adpatatons

If abiotic factors don’t cause actual symptoms, sometimes they are able to weaken the host so that a pathogen can enter, and begin disease formation.  So abiotic conditions are often predisposing factors for the development of biotic pathogens. Many of the root rot pathogens such as Phytophthora or Armillaria

Wet soils and fine texture (clay) predispose trees to root rot. Note the “root snorkels’ did not prevent the problem

require a predisposing abiotic factor such as drought, saturated soils, high salinity or compaction to facilitate disease development.

Many canker diseases such as those caused by Botryosphaeria are predisposed by drought conditions

So where do pathogens come from?

I like the hospital analogy. Where do you go to get sick? A hospital! They certainly have a difficult time controlling the spread of disease there because that is where sick people go. So where do sick plants come from? Often a nursery!  Nurseries import plants from wholesale sources, propagate from their own stock, sometimes reuse their container media, and grow many hosts in a concentrated place over time. There is no better place for diseases to occur than in nurseries.

This is especially true of root diseases because roots are inside the container and often not observed at the time of purchase

Inspect nursery stock for healthy roots before purchase

(but you always should inspect roots of all purchased plants from six packs of garden flowers to boxed trees). Also, some nurseries suppress diseases with fungicides that do not eradicate the pathogen, so when fungicides wear off (after you purchase your plant), disease can develop from now unsuppressed pathogens. Nurserymen relax! I’m not saying that all nurseries sell diseased plants (at least knowingly), but consumers should take extra care when selecting plants and when bringing new plants to their property.

While landscape mulches don’t likely spread viable pathogens they can change soil moisture status enhancing collar rots if irrigation is not adjusted. This tree was also planted deeply, another predisposing factor for disease

Once pathogens establish in the landscape, they may continue to harm new plants. Some pathogenic spores blow in on wind or inoculum moves in water courses along streams or other water paths. Animals, people and equipment can move infested soil onto a property.   Once diseases have run their course, pathogens often survive as saprophytes in the diseased tissues. They overwinter or over-summer in debris on the ground. So sanitation is critical in disease control (more on this in another post). Fruiting bodies can be moved in the greenwaste stream but there is very little research showing that disease is initiated by contaminated greenwaste, even though some pathogens may survive there. We do know that when greenwaste is chipped, it dramatically reduces pathogen and insect survival. Stockpiling wastes for as little as seven days will reduce chances pathogen survival by an order of magnitude. Certainly our favored arborist chips are very unlikely to have viable pathogens especially when sourced locally.

Understanding that diseases are not usually caused by gardening practices but by a pathogen or an environmental factor is the first step in diagnosis and control. In my next post I will talk about disease diagnosis and detection…