Is There a Doctor in the Garden?
Plant pathogens belong to one of four primary groups:
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, a 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:
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):
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):
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):
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):
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 Canker (Clavibacter michiganensis subsp. michiganensis):
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.
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.
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 www.apsnet.org at the online bookstore.)
Vegetable MD Online from Cornell University http://vegetablemdonline.ppath.cornell.edu/cropindex.htm 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