Plant Pathology
By Michelle Grabowski, Extension Educator, University of Minnesota
Edited by Bruce Watt, University of Maine
Introduction
What’s Wrong with My Plant?!
As a master gardener, you will be asked this question many times by the members of your community. Plants may be damaged by many living things but may just as often be damaged by non-living factors. All of these damages can be called disease, which literally means a lack of ease. In this sense, insects and wildlife can disease plants by feeding on them, but they are generally not considered under the topic of plant pathology. All other diseases can be classified under two broad categories:
Abiotic disease is the impairment of normal plant growth and function by a non-living factor. For example, many problems in the yard and garden are the result of environmental factors like drought, flooding, or temperature changes. Problems also arise from poor cultural practices like applying too much fertilizer or planting trees too deep or in a poor location.
Biotic disease is the impairment of normal plant growth and function by a living factor called a pathogen (disease producer). Fungi are the most common plant pathogens, but pathogens are found in every Kingdom. Other pathogens include bacteria, viruses, nematodes, phytoplasmas, and several groups previously considered fungi.
Learning Objectives
- Understand the plant disease triangle and how to use it to understand why disease occurs and how to manage disease.
- Know the five major types of plant pathogens and their basic biology, signs and symptoms, and management of the diseases they cause.
- Know common abiotic disorders of yard and garden plants and how to distinguish them from disease.
The Plant Disease Triangle
In order for disease to occur, three factors must be present; the host plant, the pathogen, and environmental conditions favorable for disease development. This relationship is described by the plant disease triangle. The disease triangle model shows that all three factors are interdependent. Without one of the factors, disease will not occur.
The Pathogen: Fungi, bacteria, viruses, phytoplasmas and nematodes can all be pathogens of plants. Some pathogens are ubiquitous in our environment. They live on plant debris, organic matter, on weeds, or in the soil and are always present. Other pathogens need to be brought into the garden on infected seed or transplants, as windblown spores, on gardening tools, or with irrigation or rain. Some pathogens can be kept out of the garden through careful selection of plant material and other cultural practices.
The Host: Most plants are susceptible to attack by some pathogens. Pathogens, however, are limited and can only cause disease in a specific group of host plants known as the pathogen’s host range. Some pathogens have a broad host range and can attack, for example, many deciduous trees, but not evergreen trees. Other pathogens are more specific and can only attack plants from one plant family. Some pathogens are so specialized that they can only attack one species or even a few cultivars of a single species. Often switching cultivars or replacing a plant with similar plant that is not in the pathogen’s host range can effectively control the disease.
The Environment: The environment plays a key role in disease development, affecting the plant, the pathogen and their interaction. Plants grown in the proper environment for their biology (shade plants in shady areas) are naturally healthier, stronger, and more capable of resisting disease. Plants grown out of their proper environment (shade plants in full sun) will be stressed and more likely to succumb to attack by a pathogen.
In addition, pathogens need specific environmental conditions to thrive and cause disease. Some prefer warmer temperatures, others prefer colder temperatures, and many prefer high humidity. In an unfavorable environment, many pathogens go dormant or are simply too weak to cause disease.
The Infection Cycle
The infection cycle describes how disease develops when a pathogen encounters a susceptible host under favorable environmental conditions.
Infection: The pathogen encounters the host and attacks. Pathogens recognize their hosts from structural and chemical characteristics of the plant. The pathogen then enters the plant tissue through a wound, a natural opening like a stomate, or by directly penetrating plant cells.
Growth: The pathogen utilizes nutrients from the plant and begins to grow and spread throughout the plant tissue. At this stage there are no visible symptoms of the disease.
Symptom Expression: The pathogen has colonized the plant to such an extent that the plant is negatively affected and symptoms like yellowing, dieback, or leaf spotting are visible. The pathogen continues to grow, inducing more symptoms as it spreads to new tissue.
Reproduction: The pathogen reproduces. The new pathogen (bacteria, viruses, fungal spores etc) can move to a healthy plant or plant part and start the infection cycle all over again.
Disease Diagnosis
When abiotic causes of plant disease have been ruled out, the first and most important step in managing disease in the yard and garden is identifying exactly what pathogen is causing the problem. The identity of the pathogen is important because many control measures will work against some pathogens and not others. Fungicides, resistant plants and even some cultural practices are often specific to one pathogen or one group of pathogens (e.g. bacteria only). Using control practices for the wrong pathogen is a waste of time and money.
When attempting to diagnosis a disease problem it is important to observe the signs and symptoms of the disease.
A Sign is a visible part of the pathogen. This can include fungal spores, spore producing structures, mycelium and bacterial ooze. Signs help to tell us ‘who’ the pathogen is.
A Symptom is a visible change in the plant’s normal appearance caused by the reaction of the plant to the disease. Symptoms include yellowing, wilting, leaf spots, abnormal growth and discoloration. Symptoms tell us what is going wrong in the plant. (ex. a wilting plant is not getting enough water to its leaves).
Fungi
What is a Fungus?
The organisms known as fungi in plant pathology are a diverse group of organisms that belong to the Kingdom Fungi and the Kingdom Chromista (Brown Algae). At one point in time all of the fungi were grouped together because they shared similar physical characteristics and life cycles. It is only with modern DNA technology that scientists have learned how diverse the fungi truly are.
Although some familiar fungi, like mushrooms, are large enough to see with the naked eye, most fungi are microscopic. Fungi grow in long filamentous strands, one cell thick, called hyphae (singular hypha). The main body of a fungus is composed of a mass of intertwined hyphae called a mycelium (plural mycelia).
A spore is the reproductive structure of a fungus, much like a seed is the reproductive structure of a plant. Spores can be asexual (have the exact DNA of the parent fungus) or sexual (have a combination of DNA from two different parents). Fungal spores can be produced on simple spore producing hyphae or in very complex fruiting bodies. Many fungi can be identified by their unique spores and spore producing structures.
Fungi as Plant Pathogens
Most fungi are saprophytes and survive by decomposing and digesting organic matter. Only about 11% of fungi are capable of causing disease in a plant and are therefore known as plant pathogens. Fungi are the most common pathogens of plants. More diseases are caused by fungi than all of the other types of pathogens combined.
Fungi are dispersed about the environment as spores or mycelia. Spores can be splashed by rain or irrigation, blown by wind, transferred on seeds or in soil, and can be moved by humans, insects, and animals. Some spores are actively shot out of specialized spore producing structures up into the plant canopy above. Mycelia can also grow actively through plant material, organic matter and soil to reach new plants.
Fungi are capable of infecting all types of plant tissue including roots, leaves, flowers, stems, rhizomes, sepals, and fruits. Fungi often enter plant tissue through wounds or natural openings like stomata and lenticels. By using complex infection structures, fungi are also capable of directly forcing their way into plant tissue. Once inside, fungi often exude a complex mix of enzymes, toxins and other chemicals to digest and absorb nutrients from plant tissue. Some fungal plant pathogens kill plant cells in order to obtain nutrients from them. Other fungal plant pathogens need a living cell to feed on and cannot obtain nutrients from dead plant tissue or other organic matter.
Survival
There are times when a host plant is not available or the environmental conditions are not right for the fungi to cause disease. To live through these times fungi have evolved several complex survival strategies. Most fungi have resting structures comprised of thick-walled cells capable of surviving harsh environmental conditions without a host plant for months or even years. Sexual spores often serve as the primary resting structure of fungi. Some fungi make Sclerotia, a dense mass of mycelia often with a tough outer rind, capable of surviving for long periods of time. Sclerotia are large enough to see with the naked eye and are often about the same size as the seeds of the infected plant. In the garden fungal resting structures can be found on perennial plant parts like rhizomes, bulbs and crowns, in plant debris, in soil and on garden structures like stakes or trellises.
Signs and Symptoms
Signs (visible part of the pathogen)
Fungi often produce microscopic spores on specialized hyphae which grow out to the plant surface. These spores can appear as an obvious powdery covering when dense or they can be very difficult to see when growth is sparse. Some fungi produce specialized vessels which contain spores. These vessels can be seen as tiny dots in or on the diseased tissues. Other fungi produce spores in large mushrooms. Sometimes no spores are produced and the hyphae simply grow into dense pebble-like survival structures called sclerotia.
Symptoms (visible change in the plants normal appearance caused by the reaction of the plant to the disease)
- Chlorosis – Yellowing of plant tissue
- Necrosis – Death of plant tissue
- Spots – Discolored or dead areas on leaves, stems, flowers, and fruit
- (e.g. Chlorotic leaf spots are yellow leaf spots)
- (e.g. Necrotic leaf spots are spots of brown dead leaf tissue)
- Wilt – Loss of plant rigidity; leaves and stems flop
- Blight – Dieback of plant tissue
- Canker – Lesion on a plant stem or branch, often sunken and discolored
- Gall – Malformed overgrowth of plant tissue, tumor like
- Rot –Soft, mushy, and discolored tissue; includes fruit rot, root rot, crown rot etc.
Examples of Fungal Disease
- Apple Scab on crabapple and apple
- Black Spot on Rose
- Powdery Mildew on many plants
- Common Smut on sweet corn
Bacteria
What are Bacteria?
Bacteria are single celled, microscopic organisms. Most plant pathogenic bacteria are rod shaped and have a cell wall. Outside the cell wall, bacteria have a polysaccharide slime layer called a capsule that is believed to be involved in the disease process. Some bacteria have a flagellum (singular) (flagella plural), a small tail like structure used to propel the bacterial cell short distances.
Bacteria reproduce by fission, the division of one bacterial cell into two identical cells. Under the right environmental conditions, bacteria can reproduce very rapidly, doubling their population in minutes.
Bacteria as Plant Pathogens
Bacteria are favored by high humidity and free moisture. Most bacteria need free water on the surface of the plant tissue to be able to infect. Bacteria are easily transferred from plant to plant in irrigation water, on cutting tools, and on the hands of gardeners. Bacteria need a wound or natural openings like stomata to infect a plant.
Inside the plant tissue, bacteria live and multiply in the spaces between the plant cells. They exude enzymes and toxins to break down plant tissue and make nutrients available for the bacteria to absorb and digest.
Symptoms often develop very rapidly when bacteria infect plants because the bacteria can multiply so quickly under favorable conditions.
The Exception
One group of bacteria, called the fastidious bacteria, live inside xylem or phloem cells (the vascular cells of the plant) of diseased plants. These bacteria are transferred from plant to plant by the feeding of insects with sucking mouth parts and can only survive inside a host plant or an insect vector. Fastidious bacteria are incredibly difficult to culture in the laboratory. For many years scientists believed that fastidious bacteria were actually viruses!
Survival
Most bacteria that cause disease in plants are able to survive for a short time on plant surfaces or in plant debris when conditions are not favorable to cause disease. Plant pathogenic bacteria do not form specialized resting structures like fungi, so their survival is limited. Many bacteria survive within living or dead plant tissue where they are protected from environmental extremes. A few plant pathogenic bacteria are able to survive by living freely as saprophytes (feeding on dead organic matter) in soil for several months.
Signs and Symptoms
Signs (visible part of the pathogen)
- Bacterial ooze – a slimy mass of bacteria oozing out of infected plant tissue.
Symptoms (visible change in the plants normal appearance caused by the reaction of the plant to the disease)
- Spot – Discolored or dead areas on leaves, stems, flowers, and fruit
- Soft rot – Soft, mushy, and discolored tissue; e.g. fruit rot
- Stem and leaf blight – Growing shoots turn dark and dieback from the tip
- Galls – Malformed overgrowth of plant tissue, tumor like
- Cankers – Lesion on a plant stem or branch, often sunken and discolored
Examples of Bacterial Disease
- Fireblight of Apples
- Crown Gall on Rose
- Bacterial Leaf Spot on Geranium
Phytoplasma
What is a Phytoplasma?
Phytoplasmas are single celled, microscopic organisms very similar to bacteria. They differ from bacteria because they do not have a cell wall and are significantly smaller.
Phytoplasmas as Plant Pathogens
Phytoplasmas infect phloem cells (cells in a plant’s vascular system responsible for transferring nutrients through the plant). Phytoplasmas are carried from plant to plant by an insect vector. The vector feeds on an infected plant and then carries the phytoplasma with it to subsequent healthy plants. Common insect vectors include leaf hoppers, psyllids, and stink bugs. Although it is rare, phytoplasmas have been transmitted through pruning tools and a few plant parasitic plants. Phytoplasmas actively reproduce in both their insect and plant hosts and will remain with both until the plant or insect dies. Phytoplasmas are not transmitted to the seed of the infected plant. The only way for a phytoplasma to survive is inside a living host, insect or plant.
Signs and Symptoms
Signs (visible part of the pathogen)
- Phytoplasmas are not visible with the naked eye or even a common light microscope. A specialized laboratory test is usually necessary for accurate identification.
Symptoms (visible change in the plants normal appearance caused by the reaction of the plant to the disease)
- Yellows – Leaves, stems, flowers and other plant parts are evenly yellow or yellow green
- Witches brooms – Multiple side shoots originate from one point on the stem
- Stunting – Plants or plant parts are unusually short
- Distorted Flowers – Flowers may be malformed or unusually shaped
Plants with phytoplasmas are not curable and so infected plants should be removed to stop the disease from spreading to other plants.
Examples of Phytoplasma Disease
- Aster Yellows
- Ash Yellows
- Elm Yellows
Virus
What is a virus?
Viruses are the smallest plant pathogen, consisting only of genetic material (RNA or DNA) and a protein coat. Whether viruses are living organisms or not is debatable. Viruses need another organism for reproduction and movement. Many plant pathogenic viruses can only survive for a short time outside of a host cell.
Viruses as Plant Pathogens
Viruses enter plant cells through wounds or are transmitted by a vector. Vectors are organisms that are able to spread viruses by feeding on an infected plant and transferring the virus to a healthy plant. Mites, insects (especially insects with sucking mouth parts), fungi, nematodes and plant parasitic plants are all vectors of plant pathogenic viruses. Just as viruses only infect specific host plants, most viruses can only be transferred by a specific vector. (Example – cucumber mosaic virus is spread by several species of aphids, but will not be spread by nematodes, thrips or other vectors).
Viruses can also be transferred mechanically from tasks that wound and move sap, such as pruning. Vegetative propagation from a virus infected stock will result in virus infected plants. Some, but not all viruses, can also be spread through seed from an infected parent plant.
Once in a host cell, the genetic material of the virus is released from the protein coat and the virus uses the resources of the host cell to build hundreds or even millions of new virus particles. Viruses move easily from cell to cell within a host plant through plasmodesmata (naturally occurring tube like connections that join plant cells to one another). The entire plant is often infected.
Survival
Viruses have no specialized survival structures. Outside of a host cell, most plant pathogenic viruses only survive a few hours to a few days. One exception is Tobacco Mosaic Virus, an unusually stable virus that can survive for years in plant debris, and has even been found in cigarettes and compost. Typically viruses survive in living plant parts like seeds, tubers, and stems or inside an insect vector.
Signs and Symptoms
Signs (visible part of the pathogen)
- Viruses have no signs. Viruses are so small that they cannot be seen even with a common light microscope. Complex electron microscopes, which are far more powerful than a light microscope, are needed to see virus particles. A lab test is usually necessary to confirm identification of viral infections.
Symptoms (visible change in the plants normal appearance caused by the reaction of the plant to the disease)
Symptoms caused by the same virus can vary depending on temperature, cultivar and many other factors. Viral symptoms are generally most easily seen in young tissue that is negatively affected by the virus as it grows and develops. Older tissue may not exhibit symptoms. Common viral symptoms are listed below.
- Stunting – Leaves, flowers, fruits and stems can all be unusually small, short and underdeveloped.
- Mosaic or Mottling – Irregular patterns of yellow, light green and dark green across a leaf surface.
- Streaking – Long uneven stretches of a different color
- Yellows – Leaf veins first turn yellow or white, then the entire leaf turns yellow. Occasionally there are yellow spots on green leaves.
- Ring spots – Single or multiple rings can be light green, yellow or brown. In some plants these look more like wavy lines.
- Necrosis – Dead sections of leaves, stems, and flowers. Necrosis occurs as dead brown spots, streaks, or large sections of plant tissue.
- Unusual growth – Leaves, flowers, stems and fruit can all be deformed, twisted, curled, and misshapen due to viral infection.
- No symptoms – some plants can be infected and show no symptoms of the virus. This is common for some garden weeds. These plants are problematic because they can be a hidden source of the infection.
Diagnosis
A few viruses have distinct symptoms and can be diagnosed based on symptoms alone. Many viral symptoms, however, are easily confused with nutrient deficiency, abiotic stress, or attack by another pathogen. To be certain a plant is infected with a virus and to learn which virus it is infected with, a lab test is necessary.
Control
Plants can rarely be cured of a virus. Infected plants must be removed and destroyed. Since viruses do not survive long outside of a host plant, removal of all infected plant material is usually enough to stop the disease from spreading. Taking care to clean tools when pruning, pinching, propagating, or other tasks involving open wounds and sap is important to prevent the spread of viruses from one plant to another. Soap and water is usually sufficient to clean tools that may have been in contact with a virus infected plant. A household disinfectant, like Lysol, will also clean the tools of other pathogens.
Examples of Viral Disease
- Rose mosaic virus
- Hosta virus X
- Squash mosaic virus
- Impatient necrotic ring spot virus
Nematodes
What is a Nematode?
Unlike other plant pathogens, nematodes are animals. Nematodes are small round worms that live in water, soil, or as parasites inside plants and animals. Although there are over 1 million kinds of nematodes, most are very small, or microscopic and frequently go unnoticed. Most nematodes live in water (the ocean and fresh water) and in soil and feed on fungi, bacteria, and organic debris. A few are parasites of animals, and even fewer are parasites of plants.
Nematodes start their life as eggs, and go through four juvenile stages before becoming adults. Most are long, round and tapered at each end. In some species, females swell up to become round or kidney shaped when they reach the reproductive stage. One female typically lays 50-500 eggs, but under the right conditions, some species can lay up to 1,000 eggs. Plant parasitic nematodes can easily be distinguished from nonparasitic nematodes because only plant parasitic nematodes have a stylet, a specialized piercing mouth piece which is used to puncture plant cells and feed.
Nematodes as Plant Parasites
Nematodes feed on plant cells by piercing the cell with a straw like mouth part called a stylet. The nematode injects enzymes into the host cell through the stylet. These enzymes partially digest the contents of the cell allowing the nematode to suck them out through the stylet.
Different nematodes parasitize plants in different ways. Some nematodes move around the outside of the plant feeding on different cells (typically root cells) along the way. Other nematodes burrow into plant tissue (ex. roots, leaves, & bulbs), feeding on and killing plant cells as they go. Some nematodes attach to the plant at one point, establish a feeding site, and stay there absorbing nutrients from the plant for the rest of their life.
Nematodes can attack leaves, seeds, buds, stems, and roots. Root damage is the most common form of nematode damage, so it is often difficult to recognize nematode damage from observing above ground symptoms. Foliar nematodes, which feed on leaf tissue, are common parasites of hostas, chrysanthemums, and other ornamental plants. Bulbs and rhizomes can also be infected with nematodes.
Survival
All plant parasitic nematodes are obligate parasites – they feed only on a living plant. Nematode eggs can survive the winter in soil, but many die out during unfavorable environmental conditions such as freezing and drying. Most nematodes survive from season to season in perennial plants, weeds, and inside corms and bulbs. A few species of nematodes can survive without a host plant in a dried out, dormant state called cryptobiosis.
Signs and Symptoms
Signs (visible part of the pathogen)
- The nematode – These can be seen with a dissecting microscope or other magnification
- Cysts – The swollen, egg filled bodies of female nematodes. These may be attached to roots or loose in the soil. On roots they often resemble the N fixing root nodules seen on legumes.
Symptoms (visible change in the plants normal appearance caused by the reaction of the plant to the disease)
- Leaves yellow or turn brown at the edges
- Wilting
- Stunted plants
- Root galls
- Stunted roots
- Dark sunken lesions on roots, bulbs or rhizomes
Diagnosis
Nematodes can be seen with a microscope, although sifting them out of soil or plant material may be difficult. Several labs will identify nematodes for a fee. Plant tissue or soil samples can be used to identify plant parasitic nematodes’ depending on the nematodes feeding habits.
Control
Healthy plants can tolerate a moderate population of plant parasitic nematodes. Providing plants with optimal water, proper fertilization, and good growing conditions will help the plant thrive even when parasitized. Crop rotation can play an important role in reducing populations of nematodes since plant parasitic nematodes can not survive without a host plant. Removing weeds is essential for nematode control since many nematodes survive on weeds as an alternate host. Nematode resistant cultivars are available for a few plants but not all. Nematicides, pesticides used to kill nematodes, are very toxic and require a special license to apply. These chemicals are not typically available to gardeners.
The Plant Disease Triangle and Disease Management
Once the pathogen responsible for causing disease is identified, several management options are available. The plant disease triangle can be used to understand disease management. The triangle shows that all three factors must be present in order for disease to occur. Therefore if only one of those factors is changed, disease will not occur. Disease management can be achieved from any side of the disease triangle.
Managing Disease by Managing the Host Plant
Hosts can be categorized using several terms that describe how likely they are to have problems with any particular disease.
An Immune plant will not become infected with a disease even if the pathogen is present and the environmental conditions are right. Immune plants are not host plants for the specific pathogen.
Example: A maple tree is immune to the disease fire blight, because the bacteria that cause fire blight can only attack plants in the Rosaceae family (The maple is not a member of this family).
A Resistant plant is included in the host range of the pathogen. Resistant plants have a genetic makeup that in some way blocks or represses the pathogen. Resistant plants will not become infected with a disease even if the pathogen is present and the environmental conditions are right. This is common in specific cultivars bred for disease resistance and can also occur naturally in plant populations.
Example: ‘Rose Queen’ Bee Balm is resistant to powdery mildew and will not get the disease. Many other cultivars of Bee Balm are not resistant and will be severely infected.
A Tolerant plant will become infected but the disease will not affect the growth or productivity of the plant as severely as it would a susceptible plant.
Example: ‘Chieftain’ butternut squash are tolerant of powdery mildew. The powdery mildew fungus will infect the leaves of these plants, but the disease will not be severe enough to reduce the number or weight of squash produced.
A Susceptible plant will become infected with the disease if the pathogen is present and the environmental conditions are right.
Example: Chokecherry trees are susceptible to black knot and become infected in spring when spores are being released.
Finding Resistant, Tolerant and Immune Plants
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Many ornamental plants are not tested for disease resistance so information can be hard to come by. In cases where a widespread disease becomes a problem on a common garden plant (like powdery mildew on phlox or apple scab on flowering crabapple) specific cultivars may be developed that have disease resistance or disease tolerance. Disease resistant or tolerant fruit and vegetable varieties are more commonly available but not for every disease. If resistance is available, it will most likely be advertised on the plant label, the seed package, or in extension publications. Information may also be available from the seed company or the nursery producing the plant. If a specific disease has been a problem in the past or it is a commonly occurring disease in your area, a little research to look for resistant cultivars can save time and money by avoiding future disease problems.
To find a plant that is immune to a disease, it is best to learn more about the pathogen. Reference books about common garden diseases and extension publications will often list the ‘host range’ of a pathogen. All of the hosts in the ‘host range’ may become infected by the pathogen under the right conditions.
Some host plants may be more seriously affected than others. Plants not listed are non-hosts and should be immune. Warning – make sure the article lists the complete host range. Some books and articles focus on just one plant and do not list the full host range of the pathogen.
Plant Defense
Although plants do not have an immune system like humans and other animals, plants do have structural and biochemical defense mechanisms to help them fight off an attack from a pathogen.
Structural defenses include bark, the cuticle, the wax layer on fruit and leaves, and trichomes (hairs) on plant surfaces. These structures act as a barrier to pathogens attempting to attack the plant. Once attacked, the plant is capable of quickly constructing new barriers like a cork layer, tyloses (a plug in the vascular system), gums and even a layer of dead plant cells that block further progression of the pathogen into the plant tissue.
Biochemical defense mechanisms include phenolic compounds, phytoalexins, enzymes and other proteins. Some of these compounds are present in the cell before infection and others are only produced after infection by a pathogen. Production of all defense chemicals increases when a plant is under attack.
Plant Stress and Disease Resistance
It takes energy for a plant to produce both biochemical and structural defense mechanisms. If a plant is under stress, it has less energy to defend itself and will be more susceptible to disease. Wounding a plant with a lawn mower, weed whip, or through carelessness in the garden will create an entry point for a potential pathogen that may not have otherwise been able to break through a plant’s natural structural defenses like bark and the cuticle. Plants that are stressed by lack of water, compacted soils, over or under fertilization, and many other factors are more likely to have disease problems. In addition disease symptoms will often be more severe in a stressed plant than in a healthy plant. Some pathogens (called opportunistic pathogens) are only able to attack a plant that is under stress. Healthy plants of the same species are resistant to opportunistic pathogens!
As master gardeners it is important to educate the public that using good cultural practices to avoid plant stress will result in the healthiest, strongest plant, which is most likely to be able to defend itself from disease.
Plant Diversity and Disease
Plant pathogens can attack a limited group of plants (the host range of the pathogen). Because of this, plant diversity in the yard and garden can play an important role in disease management.
Plants outside of the host range of a pathogen are immune to the disease. By mixing plants of many species and families in the garden, the number of plants any one pathogen can attack is reduced. For example, a pathogen may attack the sunflowers in the garden but this same pathogen will not be able to attack the native grasses or daylilies.
Pathogens move from a diseased plant to a healthy plant by wind, splashing water, insect vectors, carried on tools or clothes of garden worker, or simply by growing through the soil or plant tissue. Any interruption of this movement will slow the spread of disease and reduce the number of plants affected by the disease. By planting native grasses and daylilies in between groups of sunflowers, it is more difficult for the pathogen to spread from one group to the next.
Case Study
A classic example of lack of diversity resulting in huge disease problems is the case of Dutch elm disease. In the 1940’s America’s streets were lined with rows of American elm trees. These trees were tolerant of heat, salt, soil compaction and many other stresses common among urban trees. As a result cities planted elms in long continuous rows in many areas. The fungus that causes Dutch elm disease is transmitted by beetles and through the root grafts that form between trees. When this fungus was introduced to the United States, it rapidly killed thousands of elm trees. In a typical city street if a beetle transferred the pathogen to just one tree, all of the trees would be killed by the disease. With no break in the row of elms, the fungus was able to move through root grafts from tree to tree until the entire street was infected. Had another species of tree been planted in between the elms, it would have been harder for the pathogen to move so quickly from elm to elm. City managers would have had more time to remove diseased trees and to protect healthy trees.
Managing Disease by Managing the Environment
There are many environmental factors, like rain and daily temperature, which cannot be changed. Many cultural practices, however, affect the microclimate around garden plants. Because many plant pathogens need free moisture on leaves and other plant parts to cause disease, reducing moisture on and around the plant can significantly reduce disease problems. The following cultural practices will increase air movement in the garden, reduce humidity and leaf wetness, and reduce disease.
- Use drip irrigation, soaker hose, or other method to apply water to the soil and not the foliage.
- Water plants early in the morning so leaves dry quickly in the sun.
- Always consider the mature size of a plant when choosing a planting location, and allow for adequate space between plants, buildings and other structures.
- Mulch the soil with 2-3 inches of organic material like woodchips or straw.
As you learn more about the biology of the specific pathogen and the biology of the host, other cultural practices that help to reduce disease severity will be pointed out.
Managing Disease by Managing the Pathogen
Cultural Control
Exclusion is any activity that prevents the introduction of a pathogen to a yard or garden, where it was not previously located. Some pathogens live in soil and plant debris and are already in the garden. These pathogens cannot be controlled through exclusion.
Other pathogens are brought into the garden on soil, tools, plants and seeds. To exclude these pathogens buy plants, seeds and supplies from a reputable company. Inspect every plant before purchase. Look for dark, discolored, sunken, or soft tissue. Look at every part of the plant including roots, stems, and both sides of the leaves. Purchase only healthy looking plants.
Clean garden tools on a regular basis and especially after working in areas with diseased plants. Bacteria, fungi, and some viruses can be carried from plant to plant on garden tools. A 10% bleach solution (one part household bleach to nine parts water) will sterilize most garden equipment but can be corrosive to metal tools. Household disinfectants like Lysol and Listerine have been found to be equally effective at removing disease organisms without the corrosive effects of bleach.
Do not carry firewood from one part of the state to another or bring firewood into Maine from other states. Many very serious tree diseases like Oak Wilt and Dutch elm disease have been carried on firewood to areas that did not previously have the disease.
Sanitation and Eradication refers to removing the pathogen from the yard and garden once it has become a problem. Eradication, the complete elimination of a pathogen, is almost impossible in a garden setting. Sanitation, the removal of infected plants or plant parts, however can greatly reduce disease problems in the yard and garden.
In herbaceous plants, diseased leaves and stems can be cut off at any time during the growing season to reduce the spread of the disease. This should always be done when the foliage is dry. Never remove more than a third of a plant’s tissue at any one time. At the end of the growing season it is important to remove any diseased plant parts because many pathogens survive the winter on plant debris.
In trees and woody ornamentals, infected branches can be pruned out. It is best to prune out infections in February through March. Pruning out diseased branches during the growing season opens a wound that can be infected by the same or a different pathogen. In February and March the pathogen will be dormant and is not as likely to be spread through pruning practices.
Diseased plant tissue should be removed completely from the garden. It can be composted if the compost heats to 160F. This is difficult to achieve in most home gardens, so a municipal compost or yard waste disposal site is often a better option. If complete removal of the plant tissue is not possible, burying the plant debris can help to speed up break down and reduce the numbers of pathogens that survive from one year to the next.
Pathogens can survive on pots, trellises, stakes, and garden tools. These can all be cleaned with a 10% bleach solution before being used again.
Chemical Control
Pesticides are products applied to a plant in order to prevent, suppress or kill a pest.
Fungicides generally control only fungal pathogens. A few products are available to control bacteria, occasionally called bactericides. Most nematicides (controls nematodes) are highly toxic and not commonly available to gardeners. Chemical control is not an option for control of viral pathogens or phytoplasmas.
Before purchasing and using any pest control product READ THE LABEL! In Maine, the label is the law. Pesticide applicators can be fined for damage they cause by applying pesticides incorrectly.
The label will tell you:
- what plants the product can be applied to
- what rate the pesticide should be used at
- how often to spray the product
- what diseases the pesticide should control
- safety equipment the applicator should wear
- cautions about environmental and human safety
Most fungicides are preventative. They are able to block new infections, but cannot cure existing infections. A few are able to kill fungal tissue on the outer surface of the plant, and some have ‘kick back action’ that can stop recent infections, but fungicides primarily are used to protect healthy plant tissue.
Fungicides can be either contact or systemic. Contact fungicides act by creating a barrier on the plant surface that protects the plant from infection. In order for a contact fungicide to be effective, it must be applied before infection takes place and it must be applied to completely cover the surface of the plant. Contact fungicides may be washed off the plant by rain or irrigation, broken down by the sun, and will wear off over time so they need to be reapplied according to the label instructions. The majority of fungicides available to gardeners are contact fungicides.
Systemic fungicides are absorbed into and move through the plant tissue. Spraying for 100% plant coverage is not as important with these fungicides because the chemical will move through the plant tissue to protect unsprayed areas. A few systemic fungicides are available to homeowners.
Both conventional and organic fungicides are available to homeowners. Organic fungicides can often be recognized by the OMRI or three leaf label in figure 5. All types of fungicides need to be applied according to the label instructions.
Many gardeners are concerned about the affect of pesticides on human health and environmental safety. Information about specific pesticide products can be found at EXTOXNET (http://extoxnet.orst.edu/), a pesticide database maintained by several universities.
Biological Control
Biological Control is the use of a beneficial organism to control a pathogen. Most biological control organisms that are effective against plant pathogens are microorganisms like fungi and bacteria. These are commonly isolated from natural sources like plant, soil and water environments and tend to have lower human and environmental health concerns than synthetic chemicals.
A biological control organism controls disease in several ways. They can:
- Compete with the pathogen for nutrients and space
- Produce antibiotic compounds that suppress the pathogen
- Directly parasitize the pathogen
- Stimulate the plant to increase its natural defense mechanisms (Systemic Induced Resistance)
Biological control products for disease control are sold as a pesticide with the biological control organisms name listed as the active ingredient. Like any pesticide, the instructions on the label must be read and followed. Because biological control organisms are living creatures they may have special storage and application requirements. Many will not survive extreme heat or cold. Some biological control products have a shelf life of less than 6 months. All of these special requirements will be written on the label.