March 08, 2008

SURVIVAL TOOLKIT: plant disease primer (repost)

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Dear Organic Gardeners
Further reflections on fatherhood and introducing a new life to gardening...

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Is There a Doctor in the Garden? This first in a series of plant disease primers focuses on tomato plants...

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Farm Report: June '06 Consuming the Farm's bounty in creative ways, evaluating varieties, and collecting data...

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Is There a Doctor in the Garden?
A Plant Disease Primer

by Emily Gatch

Tomato hornworm eating his fillDiagnosing and treating plant diseases in the garden can be a challenge, especially when compared to the unambiguous assault of such insect pests as the tomato hornworm—those fat green caterpillars whose discovery on tomato plants causes an inward squeal of disgust and delight in every gardener. Yellow spots on leaves or a sudden wilting are, by comparison, much harder to pin to an identifiable source and certainly not one that can be picked off and squashed under a boot (oh, the satisfaction). Many gardeners, however, find themselves either reluctantly or enthusiastically drawn to the study of plant diseases and the microorganisms, referred to as plant pathogens, that cause them. Frequently, the motivating force behind a sudden and desperate interest in the world of plant pathogens is an epidemic in the garden. (Why are my tomato plants all dying from the bottom up and what can I do about it?!?) Armed with a little knowledge about how these pathogens work and how best to foil their progress, the disease-savvy gardener will soon find herself dispensing wisdom and Latin names of fungi across the fence. It is to these emerging plant pathologists that the following comments are addressed.

Plant pathogens belong to one of four primary groups:
  1. Fungi: Fungi cause the majority of plant diseases, particularly in humid parts of the world. Fungi obtain nutrients by breaking down and absorbing dead organic matter (a lifestyle referred to as saprophytic) or by parasitizing live tissue. Most plant pathogenic fungi are parasitic and attack living plant tissue, although some pathogens can also survive as saprophytes on dead plant matter. Fungi are composed of branching filaments called hyphae which, when clumped together, form a mycelium. A mycelium is often the visible evidence or "sign" of a plant pathogen, and can look like a fuzzy growth associated with lesions, spots, and cankers, which are considered symptoms. Fungi can reproduce asexually, by forming clumps of hardened hyphae called sclerotia that can survive long periods of time in soil, or by mating and forming reproductive spores. Spores and other reproductive units can be spread by wind, rain, animals, equipment, and insects, and can be found in soil and on seed. There are many variations on these two basic reproductive modes and when and how each occurs, but the important thing to remember is that fungi have evolved complex ways to propagate themselves in response to environmental challenges; for each plant disease, this life cycle must be understood in order to identify proper prevention and control methods. In the specific disease profiles that follow, the basic reproductive cycles are outlined. Various symptoms may be associated with a particular fungal pathogen, including wilting due to the plugging of vascular tissue, the formation of spots or lesions that can coalesce and cause the death of entire leaves and whole plants, and the production of galls and smuts, which are abnormal, "cancerous" growths of plant tissues.

  2. Bacteria: Bacteria are one-celled microorganisms that are second to the fungi in the damage they cause to crop plants. Like fungi, they enter plants through natural openings such as stomata or through wounds, or, in some cases, they are transmitted by insects (e.g., cucumber wilt is a bacterial disease spread by spotted and striped cucumber beetles). Bacteria secrete toxic compounds that kill plant cells and then absorb the contents of the destroyed cells. Plants will often try to isolate an invasion of bacteria by triggering the death of cells in the immediate area of attack, which contributes to the formation of the classic bacterial spot and speck symptoms. A bacterial disease may superficially resemble a fungal disease, but sometimes bacteria can be identified as the causal agent by cutting open diseased tissue and observing a gooey bacterial streaming. Diagnoses of this sort can be complicated by the fact that many secondary microbial invaders will come in after the primary pathogen has begun its destruction.

  3. Viruses: Viruses consist of nothing more than a few strands of genetic material (DNA or RNA) encased in a protein coat. Technically, they are not considered alive, since they do not respire and are unable to reproduce outside a living host. Essentially, they turn host cells into mini-factories for the reproduction of more virus particles. Viruses can be carried on seeds, in the guts of insects, and on infected tools, equipment, the hands of workers, and plant debris. Many of the most damaging plant viruses are transmitted by insects, which are called vectors. Aphids, leafhoppers, and thrips are the most common virus vectors. Viruses can cause yellowing, stunting, and discoloration in characteristic patterns such as mosaics and rings; these symptoms may occur on all plant parts, including fruit. Other malformations, including puckering and rolling of leaves, may also appear.

  4. Nematodes: Nematodes are unsegmented roundworms that are ubiquitous in the natural world and range in size from microscopic to a few inches long; those responsible for plant diseases are soil dwelling and are not visible to the human eye. Although they are somewhat dissimilar to the other three groups of pathogens, they are studied with plant diseases because the symptoms they cause and the methods used to control them are similar. Plant pathogenic nematodes have a needlelike mouthpart called a stylet that pierces delicate plant roots and sucks digested tissue out of them. A severe nematode infestation can lead to yellowing, stunting, and eventually, plant death. They do not move further than a few inches in the soil; so, they are typically introduced into a new area through the movement of infested soil, equipment, or running water.

The presence of a pathogen from any of these groups does not necessarily result in an immediate epidemic. Plant pathologists refer to what is known as the "disease triangle," which describes a disease as the result of the interaction between a pathogen, disease triangle graphica susceptible host (some hosts may have certain levels of resistance to a pathogen), and proper environmental conditions, which in most cases consist of a certain amount of free moisture on plant tissues, high relative humidity, and a specific temperature range. The extent to which all these factors are present—a susceptible host, a virulent strain of the pathogen, and the right environmental conditions, determines the severity of the disease that develops.

The disease triangle can also be a useful tool to identify ways to control diseases: by reducing the contribution of any of these factors, disease can be minimized or avoided. For example, the gardener can easily manipulate the environment through the following cultural methods:
  1. Avoid overhead watering: this reduces free moisture on leaves and relative humidity within the plant canopy, which will create an inhospitable environment for the proliferation of many pathogens.
  2. Prune plants and avoid tight plant spacing to promote air movement, which works against the successful establishment of plant pathogens.
The "size" of the host factor (the extent to which the pathogen is already present in the garden) can be manipulated in the following ways:
  1. Remove every other weedy host or volunteer crop plant that provides a haven for overwintering pathogens.
  2. Rotate crops to prevent the build up of crop-specific pathogens in the soil and in plant debris.
  3. Remove crop debris that may be infested with pathogens upon harvest.
  4. Preserve natural enemies of pathogens, particularly through application of compost to the soil and to plant surfaces in the form of compost tea. Compost is a source of rich microbial diversity, and many of these microorganisms will compete with pathogens for space and nutrients on plant surfaces.
The contribution of the host to disease development is perhaps harder to alter, but can be done by selecting varieties known to be resistant to certain diseases; seed catalogs will often list any known resistance to diseases of their varietal selections. It is beyond the scope of this introductory overview to describe the major diseases affecting every crop group, so we will begin with tomatoes in this issue and continue highlighting a specific crop in each of the coming issues.

Fungal Diseases of Tomatoes:

Early Blight (Alternaria solani):
Life Cycle/Symptoms: Spores of this fungus can be seedborne, but typically the inoculum source is in the soil and spreads to the plant by splashing rainfall. Symptoms are first observed on older leaves of established plants and consist of dark spots with concentric rings. The lesions eventually coalesce and the entire infected leaf dies. The disease can be quite severe in areas of high rainfall and heavy dew.
Prevention/Control: Adequate mulching will prevent splash dispersal of spores, and wider plant spacing will help maintain good air circulation. A minimum two-year rotation and removal of crop residues is recommended for inoculum reduction. Seedborne transmission is rare, particularly if seed is fermented during the cleaning process.

Fusarium Wilt (Fusarium oxysporum f. sp. lycopersicae):
image of fusarium wilt Life Cycle/Symptoms: This fungus is common in soils, but is more problematic in warm, coastal areas. Invasion occurs through the roots and spreads through the vascular system of the plant. The first symptom is wilting, typically of one side of the plant or one stem. A cross section of the infected stem will reveal a discolored ring of vascular tissue. Vines infected with fusarium wilt should be pulled and discarded in the trash, not composted or turned under, as the fungus can persist in soils for up to eight years.
Prevention/Control: Control methods include long rotations of five to seven years, good sanitation practices, including cleaning equipment, tools, and stakes between fields, and use of clean seed, as the pathogen can be seedborne.

Septoria Leaf Spot (Septoria lycopersici):
image of septoria Life Cycle/Symptoms: This fungus is dispersed by splashing rain or irrigation water from the infected debris in the soil, and can be spread by workers and equipment once introduced. Symptoms occur first on older leaves and are circular lesions with dark-brown margins and tan to gray centers. The disease progresses to newer tissue and can become severe during conditions of high humidity and high temperatures.
Prevention/Control: Field sanitation, crop rotation of one to two years, removal of host weeds such as horsenettle, removal of crop debris, and drip irrigation will reduce occurrence of this disease.

Verticillium Wilt (Verticillium albo-atrum and V. dahliae):
image of verticillium wilt Life Cycle/Symptoms: These fungi occur widely and affect many species of vegetables. Symptoms can be confused with fusarium wilt; the first indication may be a diurnal wilting pattern, with plants wilting during the hottest part of the day and recovering at night. Lower leaves may develop a distinctive, interveinal, v-shaped yellow lesion. Vascular discoloration is also evident in stems. The pathogen survives in soil for up to eight years and infects through the roots. This is a cool-weather disease that is more severe in alkaline soils. It is not known to be seedborne.
Prevention/Control: Rotation is of limited use because of the wide host range of the pathogen. Removal and destruction of infected debris will reduce the amount of inoculum in the soil. In severely infested soils, soil solarization (not sterilization) is an option to consider.

White Mold (Sclerotinia sclerotiorum):
image of white mold Life Cycle/Symptoms: This disease is favored by cool, moist conditions. Symptoms usually appear at the time of flowering and are characterized by water-soaked lesions in stem joints. Infected stems become bleached and dry, similar to animal bones. A white, cottony mold often appears on stems. The fungus survives in soil as small, hard, black masses of mycelium called sclerotia, typically on the surface or in the top inch of soil. When conditions are cool and moist, spores are released from these sclerotia which colonize the stem at the soil line. Infected fallen flower petals become a secondary source of infection. The disease is not seedborne.
Prevention/Control: Increasing plant spacing to ensure maximum air circulation will reduce white mold severity.

Bacterial Diseases:

Bacterial Canker (Clavibacter michiganensis subsp. michiganensis):
image of bacterial canker Life Cycle/Symptoms: This disease occurs worldwide, including in major seed-producing areas. The principal symptom is a systematic wilt, characterized by downward turning of leaves (starting with lower leaves), and leaf-margin necrosis. Symptoms on the fruit are described as bird's-eye spot: lesions with raised brown centers surrounded by a white halo. Sources of inoculum include survival in plant debris, weed hosts and volunteer plants, stakes, and seed. Secondary spread can occur through poor sanitation and pruning/staking procedures.
Prevention/Control: Disease-free seed is critical for prevention of bacterial canker. Disinfect pruners and clippers between plants. If an infection has occurred and stakes are to be reused the following season, wash the stakes in a 1% bleach solution (notify certifying agency). Incorporation of plant residue into soil hastens decomposition of the bacteria. Crop rotation is also recommended.

Bacterial Speck (Pseudomonas syringae pv. tomato):
Life Cycle/Symptoms: This disease is widespread but not typically severe. It is favored by low temperatures and high moisture, and it is characterized by minute lesions on fruit and slightly larger lesions on leaves. Early lesions lack a halo and have a raised appearance. The bacteria are seedborne; reports of incidence have been increasing worldwide. It overwinters in crop residue and on weed species and is disseminated by splashing rain and pruning/clipping instruments.
Prevention/Control: Disease-free seed is critical for prevention of bacterial speck. Disinfect pruners and clippers between plants. Remove crop debris, weeds, and volunteer plants from production fields. Incorporation of plant residue into soil hastens decomposition of the bacteria. Crop rotation is also recommended.

Viral Diseases:

Curly Top Virus:
This virus infects a wide range of crops throughout the western U.S. This virus is transmitted by leafhoppers, which overwinter on weed hosts and move to tomato plants when weeds start drying up in late spring. Russian thistle (tumbleweed) is a common weed host for the virus and its vector. Plants infected when young are typically killed, while older plants survive but with reduced vigor and with symptoms that include stunting, thickened leaves that roll upward, purplish veins, and prematurely ripened fruit. The virus is not normally transmitted mechanically. The virus is not seedborne. Control measures may include eliminating weed hosts to the extent possible and avoiding planting fields near overwintered beets.

Tomato and Tobacco Mosaic:
These viruses are very similar and are common in production systems requiring frequent plant handling. Infected seed and plant debris are the most common sources of inoculum (which can survive for up to two years in buried root debris), and spread occurs most readily by human activity. The characteristic symptom is light- and dark-green mottling on the leaves, with some curling and stunting of leaves as well. Fruit may exhibit uneven ripening and reduction in size/number. Control measures include avoiding use of tobacco products (no smoking!) and crop rotation.

Emily Gatch
Greenhouse Coordinator and Assistant Seed Cleaner


Pleasant, Barbara. 1995. Plant diseases: the gardener's guide to earth-safe remedies. Pownal, VT: Storey Books.

Jones, J.B., J.P. Jones, R.E. Stall, and T.A., Zitter (eds.). 1991. Compendium of tomato diseases. APS Press. (The American Phytopathological Society publishes excellent crop-specific compendia of plant diseases, which can be purchased from at the online bookstore.)

Vegetable MD Online from Cornell University This website offers excellent disease diagnosis and management information for a wide variety of vegetables.

Photo captions: (1) Tomato hornworm image courtesy of the National Gardening Association (2) Graphic representation of the disease triangle (3) Fusarium wilt in a row of tomato plants courtesy of Dr. Randy Gardner, NC State University (4) Septoria Leaf Spot image courtesy of University of Illinois Dept. of Crop Sciences collection (5) Verticillium wilt image courtesy of Lewis Jett Assistant Professor University of Missouri (6) White mold photo courtesy of T.A. Zitter, Cornell University (7)Bacterial canker photo courtesy of T.A. Zitter, Cornell University

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