Diseases can occur at any stage during the course of plant growth. The rapid, accurate diagnosis of the cause of a disease, along with the implementation of a rapid treatment, is essential to ensure the protection of the crop. Certain infectious diseases caused by living, microscopic organisms have the potential to rapidly ruin a crop. However, for any particular vegetable, these diseases are not that numerous and, so, it would not be difficult for a grower to become familiar with them and take proper preventative action. Diseases caused by nonliving things (i.e. not infectious) can be much more difficult to diagnose. Usually, it is easier to rule out an infectious agent as the cause of a disease before investigating possible nonliving (abiotic) causes. This stresses the need for the grower to become familiar with the more common infectious diseases that can occur on the crop. This chapter provides an overview of the science of diagnosis and treatment of vegetable diseases. The grower is advised to consult other references listed at the end of the chapter for more detailed information related to a particular crop.

Why Is There Disease?

Disease is the outcome of an interaction between the host, the disease agent, and their environment. If the cause of infectious disease, the pathogen, is next to the host, nothing will happen unless environmental factors are favorable for its infection and development within the plant. With foliar pathogens, there is usually a minimal period of leaf wetness required to stimulate spore germination and infection. For some soilborne pathogens, infection occurs in combination with high soil moisture and certain critical soil temperatures. Knowledge of conducive environmental factors for the more important vegetable diseases presents an opportunity for more effective management: the disease can prevented by altering some of the environmental factors, or, when such factors cannot be altered, steps can be taken to minimize the impact (e.g. fungicides could be applied in advance of a period of sustained rain which would favor foliar diseases.)

Diagnosing Diseases of Vegetables

Not all diseases are caused by pathogenic organisms. Determining whether a disease is caused by a pathogen, or has nonliving (abiotic) causes requires not only the examination of individual plants, but also, noting the pattern of symptom occurrence in a field. Examine individual plants for unusual symptoms, such as leaf spots, wilts, stunting, fruit rots, misshapen leaves, cankers and stem blight. Roots should be examined for galls, root rot and necrosis (dead areas). Fields should be observed to determine if the problem is widespread and whether different plants species in and around the field are affected, which could indicate an abiotic cause. Symptoms with a nutritional or physiological cause have a more widespread occurrence within a field than infectious diseases. Initially, most disease causing pathogens will be isolated in areas and spread outward from those areas. Also, weeds or nonrelated crops are not typically affected. Soilborne pathogens are even more restricted within a field than foliar pathogens. Table VII-1 lists characteristics to consider when diagnosing the cause of a plant disease.

Diagnosis of Diseases Caused by Different Types of Pathogens

Fungi

Fungi are multicellular microscopic organisms that can grow to their food, usually in the form of filamentous strands. Their growth pattern is radial, so on surfaces such as plant leaves, the effects of their growth may be seen as circular spots. However, fungal infections of other plant parts, such as roots, may produce no visible structures. Some symptoms can indicate these infections. For example, browning of the water-conducting tissues of the stem, in combination with wilt, can indicate infection by the Fusarium wilt fungus. Other disease symptoms, such as blight (a general death of tissue), which can have a variety of causes, may require laboratory testing to confirm fungi as a cause. Fungi can produce specialized structures, such as spores, which are used for reproduction, dissemination through space and time, and survival. Sclerotia are structures that function in the long term survival of many soilborne pathogens. The southern blight fungus which infects many vegetables forms sclerotia resembling mustard seeds.

Most fungi that infect leaves require free moisture to initiate infection, with the exception of powdery mildew fungi, which need only high humidity to initiate infection.

Bacteria

Bacteria are single celled microscopic organisms, which survive by becoming dormant. The most notable exception to this is the pathogen that causes common scab on potato: this is a filamentous, multicelled bacterium that produces spores. They can be transported by wind driven rain, by insects, or the movement of infected plant parts, including seed. Bacteria that infect leaves may cause circular spots, but irregular shaped lesions that don’t extend beyond veins are more characteristic for bacterial infections. They can also cause soft rots of vegetative parts that are usually characterized by a foul smell. Other bacterial diseases that produce symptoms such as wilt require laboratory analysis for diagnosis.

Mycoplasmas are bacteria-like, only structurally simpler and smaller than bacteria. They are transmitted by leafhoppers. The most notable disease of vegetables caused by a mycloplasma is aster yellows, which affects carrots, celery and related plants. The symptoms of aster yellows are distinctive: leaves have a bronzed appearance and flowers are abnormal (leaf-like tissue grows from them, instead of petals.)

Table VII-1 Characteristics to evaluate when Diagnosing Diseases

Viruses

Viruses are submicroscopic entities that can only replicate inside living plant cells. They require agents such as insects to transmit them to plants. A typical virus symptom is a mosaic pattern on leaves, but viruses may cause other symptoms, such as necrotic lesions and stunting, which can have other causes. Insect vectors of viruses include by aphids, leafhoppers, white flies and thrips. Viruses often have wide host ranges, including weeds that are not botanically related to the crop, and symptoms are not always produced in these plants.

Although some viruses can tentatively be identified by symptoms produced on plants (particularly mosaic or ringspots), accurate diagnosis requires laboratory testing.

Nematodes

Nematodes are microscopic roundworms that feed on the roots of plants. The root knot nematode produces root galls and deformed roots on a wide range of crops. Heavy infestation can lead to wilting and death of plants. Nematodes can also cause stubbiness, necrosis and stunting of roots, but these symptoms are not distinctive. Soil or root analysis is required to confirm diagnosis.

Environmental Factors Affecting Plant Pathogens

Temperature

Soil temperatures that are suboptimal for seed germination and seedling growth can favor the development of damping off pathogens, such as Pythium and Rhizoctonia. Often, the impact of these pathogens can be reduced by delaying planting, until the soil temperature increases. Foliar pathogens have different optimal temperatures for development. Symptoms of bacterial diseases on leaves are produced at high temperatures, at which a fungal disease like downy mildew might be halted. These temperature requirements are known for many diseases and with a disease like late blight of potato, can be used to accurately apply fungicides when they are needed.

Moisture

Moisture is the driving force for foliar diseases. Free moisture on the plant surface is necessary for growth and infection of many bacteria and fungi. There is usually a minimum, critical time requirement. Moisture can also infect spore production by fungi. This characteristic is the basis of a predictive system for fungicide choice for management of purple blotch of onion.

Saturated soil moisture conditions can create an environment conducive for root-rotting pathogens such as Pythium or other fungi that cause fruit rot.

Soil pH

Soil pH affects the activity of some soilborne pathogens. Diseases caused by the cotton root rot fungus tend to occur in alkaline soils, while Fusarium wilt diseases are more prevalent in acidic soils. The growth of the common scab pathogen of potato is reduced in acidic soils. However, it is generally not feasible to alter soil pH to manage disease.

Disease Control Methods

Cultivar Selection

In some cases, vegetable cultivars may be resistant or tolerant to important diseases. Resistant cultivars have no or low severity of disease, while tolerant cultivars may become diseased without incurring yield loss. The use of these cultivars does not necessarily eliminate the need for other disease management approaches, such as the use of fungicides.

Disease-Free Planting Material

Several pathogens can be spread by contaminated seed, for example, the bacteria causing leaf spot of pepper and tomato, black rot of crucifiers and fruit blotch of watermelon; and fungi causing gummy stem blight and anthracnose on watermelon. Some seed companies indicate whether seed lots have been tested for these pathogens. However, low levels of seed contamination could increase if transplants are produced from the seed. Infected transplants may have slight or no symptoms. The use of a soilless mix and good sanitation practices during the production of the transplants will greatly reduce the risk of infection by soilborne pathogens. The risk of introducing a foliar epidemic, such as gummy stem blight, can be minimized by inspecting transplants and making preventative applications of fungicides or bactericides (copper-based) early in the season.

Rotation

Many pathogens which cause disease survive in the soil, on crop residue. Rotation with nonsusceptible crops, or fallowing, can reduce the pathogen population. Rotation is not a practical alternative for some soilborne fungi, such as Fusarium wilt fungi and the southern blight fungus, because their survival structures persist for years in soil. White rust, an important fungus disease of spinach, mustard, turnip and swiss chard, forms survival structures in the leaves of diseased plants that require 2 to 3 years to die out.

Burial of crop residue can augment the beneficial effects of crop rotation. Many foliar pathogens can survive in crop residue, but only for one or two years. Burial of crop residue decreases pathogen survival and physically removes their access to the new crop. Shredding of crop residue can speed up the decomposition of certain pathogens. For example, the bacterium that causes black rot of cabbage can survive in plant residue for nearly a year, but if the residue is shredded, the survival time decreases to less than 2 months.

A pathogen that attacks one member of a plant family frequently will infect other members of that family or group. Table VII-2 lists vegetables which are susceptible to similar diseases. A standard rotation program should use only one member of each group on a site in a 2 to 3 year period. An exception to this recommendation is the Fusarium wilt fungus. There are variants of this fungus that infect one crop only. Thus, the Fusarium wilt fungus that infects watermelon will not infect cantaloupe or other cucurbits and these crops can be grown in soils where Fusarium wilt of watermelon was a problem.

Rotation is important in the control of root knot nematodes. If susceptible crops are grown repeatedly on nematode infested land, production levels decrease until susceptible crops are no longer economical. Onions, shallots, garlic, leek or sweet corn should be grown on sites where root knot nematodes are a known problem. Fallowing can also reduce populations, but the fallow land must be kept weed free, since many weeds are also hosts for this nematode.

Table VII-2. Grouping of Vegetables based on Susceptibly to Similar Diseases

Cultural Management Practices during the Growing Season

Planting when the soil temperature is optimum for seed germination and seedling growth will reduce seed decay and damping off.

In areas normally experiencing heavy rainfall, vegetables should be planted on raised beds to allow for drainage of excess water from the root zone. Saturated soil favors the development of some root rot fungi. Raised beds also expose more of the soil surface to sunlight, which increases soil temperature, leading to faster seed germination, which reduces the length of time that plants are susceptible to damping off organisms.

The use of mulches to provide a physical barrier between saturated soil and mature fruit will reduce fruit rot.

The method of water application can have a significant effect on disease development. Foliage disease levels can increase if water is applied by overhead sprinklers because this creates a more humid microclimate, and the splashing water will physically move pathogens within the field. Furrow irrigation can move spores of the Phytophthora blight fungus down rows of peppers. Drip irrigation minimizes pathogen spread within a field.

Most viruses that infect vegetables are transmitted by insects. Aphids migrating into fields can transmit viruses. These aphids may not become a pest problem and may, in fact, disappear before the first virus symptom is noticed. Thus, the use of insecticides is usually ineffective for virus control. Stylet oil applied to foliage, which prevents feeding of aphids and whiteflies, has met with some success in reducing virus infection.

Perennial weeds can harbor many plant viruses. Often, these weeds show no symptoms. Aphids or other insect vectors feed on infected weeds and spread the pathogen to the crop. Thus, weed control around the edges of the field can help control certain viruses.

Postharvest Management of Diseases

Care should be practiced during harvesting and grading of vegetables to prevent wounding that allows the entry of decay causing pathogens. Wooden or metal picking containers should be sanitized with a chlorine solution. When washing field soil from vegetables, the water should be changed regularly to prevent the spread of pathogens. Chlorinated water (70 ppm; prepared by adding 1.33 gallons of a 5.25% sodium hypochlorite solution to 1,000 gallons of water) is recommended for washing. The level of chlorine in the wash water should never drop below 25 ppm and a fresh solution should be prepared at least every day.

Vegetables should be quickly cooled after harvest to slow development of postharvest decay. The temperature at which most vegetables should be stored is between slightly above freezing and 40°F.

Chemical Control

Chemicals can be classified based on the targeted pathogens: fungicides act against fungi, bactericides against bacteria, and nematicides against nematodes. These categories are not mutually exclusive, e.g. copper-based bactericides also have activity against fungi, and soil fumigants such as metam sodium, which control nematodes, also control some soilborne fungi. Disease control chemicals can be applied at different stages of the cropping cycle: preplant (e.g. a broad spectrum soil fumigant); at planting, as a seed treatment or as an in-furrow application to control damping off fungi or nematode; during the growing season to control fungal and bacterial pathogens of foliage; or as a postharvest treatment to fruits. Most chemicals on the market act directly on the pathogen, but there are newer materials that act indirectly, by inducing an increased level of disease resistance in the plant. These resistance activators must be applied before the onset of disease to be effective.

There are a limited number of nematicides available and their use on vegetables is more restricted than fungicides or bactericides. Nemacur (fenamiphos), Vydate (oxamyl) and Mocap (ethoprop) are applied at the time of planting, or shortly thereafter. Soil fumigants are applied prior to planting, with a time interval to allow dissipation of the chemical prior to planting. These fumigants include: Telone C-17 or Telone C-35 (1,3-dichloropropene + chloropicrin), Telone II (1,3-diclhloropropene), Vapam (metam sodium), methyl bromide, and chloropicrin. Methyl bromide will soon lose its registration, but another fumigant with similar properties, methyl iodide, may eventually replace it.

Copper based chemicals (e.g. Kocide) are used as bactericides. These chemicals work to prevent new infections; they do not cure established infections. Some isolates of the bacterium that causes leaf spot of pepper and tomato are resistant to copper based chemicals and such resistance could also develop for other bacterial pathogens. Although these chemicals also have activity against fungi, there is no likelihood of a resistance problem with fungal pathogens.

Fungicides can be applied to the seed to protect from organisms on the surface of the seed and from damping off pathogens. Fungicides used for seed treatment include captan (Captan), fludioxonil (Maxim), thiram (Thiram), mefenoxam (Apron), and metalaxyl (Allegiance). Biological seed treatments are also commercially available, e.g. Kodiak and T-22 (These are living organisms applied in the same manner as chemicals.) Seed treatment fungicides are usually applied by the seed company and the fungicides used are indicated on the label.

Growers can supplement the activity of seed treatment fungicides by applying additional fungicides at the time of planting. This is recommended when soilborne disease pressure is higher than usual. During the growing season, fungicide or bactericide sprays may be required to control foliar diseases. The essentials for good disease control are: preventative applications (i.e. before the onset of visible symptoms), thorough coverage of the plant, and application at an appropriate frequency and rates. If disease pressure is low (usually when there is a period of dry weather) good control can be obtained by using protectant or nonsystemic fungicides such as chlorothalonil or the EBDC chemicals (e.g. mancozeb). Systemic fungicides have higher activities against several foliar pathogens and should be used when disease pressure is higher (periods of rainy weather).

Management of Systemic Fungicides

Most systemic fungicides have a narrow mode of action. Mutants of fungal pathogens resistant to these fungicides can occur naturally. Because foliar fungal pathogens can have many reproductive cycles in one-growing season, mutants resistant to a systemic fungicide can quickly become the dominant component of the pathogen population. The outcome is a failure of the fungicide. The fungus causing late blight on potatoes and tomatoes has developed resistance to metalaxyl (Ridomil), which used to be the most effective fungicide for this disease. There is widespread resistance of benomyl (Benlate) in populations of the gummy stem blight fungus and powdery mildew fungi. In addition to these pathogens, the fungi that cause anthracnose and downy mildew are candidates for the development of resistance to systemic fungicides that are currently on the market.

With some systemic fungicides, resistance development is prevented because formulations are always sold mixed with a protectant fungicide. The presence of a protectant fungicide prevents any mutant resistant to the systemic fungicide from developing further. For example, formulations of Ridomil Gold labeled for foliar diseases of vegetables are mixed with either chlorothalonil or mancozeb.

However, other formulations of systemic fungicides are not sold mixed with protectant fungicides. Thus, it is imperative that the grower is aware of potential resistance problems and take steps to prevent this. First, systemic fungicides should not be used to treat a disease after symptoms are seen, but should be used prior to onset of symptoms. From past history in an area, the grower will know which foliar diseases are a constant problem and the approximate time that they appear. Fungicide applications should be made prior to this time. For some diseases, there are predictive formulas that can be used as a tool to decide when to initiate fungicide applications. For late blight of potatoes, there are predictive models such as Blitecast and Wisdom, which use weather information to forecast when conditions are favorable for pathogen development. Additionally, fungicides should be used at recommended rates and intervals, as indicated on the label. The whole field should be treated.

The systemic fungicide should be used in alternation with protectant fungicides, or other systemic fungicides belonging to different chemical groups. Thus, the systemic fungicide is applied and, at the next application, a protectant fungicide is applied, or, a different systemic fungicide. One systemic chemical should not constitute more than 30 to 50% of the total applications to a crop in one season. Pathogens that are resistant to one systemic fungicide tend to be resistant to related fungicides. This is known as cross-resistance. Isolates of the gummy stem blight fungus that were resistant to benomyl were also resistant to thiabendazole and thiophanate-methyl; these fungicides are all in the benzimidazole group and have a similar mode of action. Cross-resistance could be a problem with strobilurin (Quadris, Flint) and triazole (Tilt, Nova) fungicide groups. Thus, in a powdery mildew control program, both Quadris and Flint can be used, but they should be alternated with Nova, which belongs to a different fungicide group, and the combined number of applications of Quadris and Flint should constitute more than 30 to 50% of the total number of applications.

Table VII-3 summarizes the more common diseases occurring on vegetables and general control recommendations. Specific fungicide recommendations can be found in “Vegetable and Herb Disease Control Products for Texas”, Texas Agricultural Extension Publication E-10, but the pesticide label is the final authority for allowed crops, rate and timing.

Table VII-3. Common Diseases of Specific Vegetables and their Control

Laboratory Identification of Diseased Plants

There are times when field observations are not sufficient to make an accurate diagnosis. The Texas Plant Disease Diagnostic Laboratory is available to assist producers with laboratory identification of all plant disease problems, as well as control recommendations. The accuracy of the diagnosis depends upon the quality of the sample received by the laboratory. The following guidelines should be followed when submitting samples:

Sampling Vegetables for Disease Diagnosis

• Send plants showing representative symptoms.
• Samples should include tissue in advanced stage of development and also material that is just beginning to show symptoms.
• Place the sample in a plastic bag and seal. Do not add water to sample or place a wet paper towel in bag with sample.
• Samples that have a soft watery decay should be placed in a separate bag from the other plant material. Soft fruit samples such as tomatoes should be handled in separate bags.
• Label each bag sent to the laboratory with a marker on the outside of the bag or with a tag attached to the outside.
• When collecting material from virus infected plants, select the young tender growth which is showing symptoms.
• Samples should be placed in a refrigerator while waiting for shipment or at least, removed from hot areas.
• Samples which are soft and watery should be shipped by bus in a Styrofoam cooler with a freezer block. The samples should be placed in a rigid container so that they are not crushed during shipping.
• Leaf, root or stem samples can be shipped by mail as long as they are placed in a plastic bag to prevent drying out during shipping and in a sturdy box so that they will not be crushed.
• Fill out the information sheet D-1178, a copy of which is found at the end of this chapter. This information is helpful to the laboratory in making their determination and suggesting controls.

Nematode Samples

• Survey the field to determine the location of suspected problems and randomly sample those locations. Samples can be combined and a composite sample selected. A matching sample also should be taken from apparently healthy plants and sent in as a comparison.
• Samples should be taken from the surface to 6 or 8 inches below the surface.
• Samples should be kept cool.
• Samples should be placed in plastic bags for shipping.
• Each bag should be labeled on the outside as to where the sample was collected and with a code that will relate the samples back to the site. This is necessary so that the producer can determine what areas need to be treated if treatment is required.
• The form D-827 should be filled out carefully and shipped with the sample. A copy of this form is found at the end of this chapter.
• All samples should be shipped in a sturdy box by mail or bus.

Mailing address for the plant diagnostic laboratory:
Texas Plant Disease Diagnostic Laboratory
Centeq Research Plaza, Room 130
1500 Research Parkway
College Station, Texas 77843-2119
Telephone: (979) 845-8032
Fax: (979) 845-6499

Forms to accompany plant samples sent to the Texas Plant Disease Diagnostic Laboratory can be obtained from your local county Extension office.

Additional Sources of Information

• Horne, C. W., and J. D. Johnson. 1978. Control of Plant-Parasitic Nematodes around the Home and Garden. L-781, Texas AgriLife Extension.
• Philley, G. L., and H. W. Kaufman. 1997. Non-Chemical Control of Plant Diseases in the Home Garden. L-2016, Texas AgriLife Extension.
• Texas Plant Disease Handbook, Texas AgriLife Extension. http://plantpathology.tamu.edu/texlabn/notice.html
• Atlas of Soilborne Diseases of Melons, B-1595
• Isakeit, T. 1999. Bacterial Fruit Blotch of Watermelon, L-5222, Texas AgriLife Extension. http://plantpathology.tamu.edu/Texlab/Vegetables/wmelon/pdf/l5222.pdf
• Comprehensive, illustrated technical compendia for these crops: Potato, Pea, Tomato, Cucurbits, Bean, Onion & Garlic, Corn, Sweet Potato, Lettuce, and Beet; are available from the American Phytopathological Society at www.shopapspress.org/disease-diagnostic-series.html