Garden vegetables can be attacked by a wide range of fungi, bacteria, viruses, and nematodes. Since no single control measure is effective against all diseases caused by these microscopic pests, gardeners must rely on a well-balanced defense to keep diseases in check.
One of the first steps in setting up a disease control program is correct identification of disease problems—the earlier the better. With quick action, you can control leaf spots, blights, and mildews within the same season. Other disease problems may not be treatable this season, but correct disease identification allows you to take preventive disease control measures next year.
Many garden vegetable diseases are easy to identify. Others may need the advice of someone more experienced, such as an Extension county agent, home economist, or plant pathologist. Disease is best identified on plants that are less than 50 percent damaged. Do not use dead plants.
Common vegetable disease symptoms and recommended control measures are provided below. If you cannot identify a disease problem from these descriptions, call, write, or visit your county Extension staff for assistance.
To have a plant pathologist diagnose a diseased specimen, wrap the specimen in newspaper, paper towel, or (preferably) waxed paper. Pack it in a box or carton and ship it immediately to Extension Plant Pathology Lab, P. O. Box 9655, Mississippi State, MS 39762.
Additional information on diseases and nematodes that attack garden vegetables is available at your county Extension office.
Diseases take their toll in Mississippi gardens every year, but adequate planning and following recommended disease controls will keep losses to a minimum.
Specific Diseases and Control Measures
Damping-Off (seedling disease)—Seeds of many vegetables are susceptible to damping-off fungi when planted in infested soils. The seeds may fail to germinate, or seedlings may be attacked before or after emergence and turn brown, shrink, and finally die. Control measures include these: plant fungicide-treated seeds in well-drained areas; do not apply excessive amounts of nitrate forms of nitrogen fertilizers; and rotate the location of the vegetables.
Root Rot of Beans and Southern Peas—Root rot is severe on green beans, lima beans, and southern peas. The disease first appears as reddish or reddish-brown areas on stems and roots. As the disease advances, discolored areas spread until the entire root and lower stem are affected. Above-ground symptoms include stunting, yellowing, drooping of leaves, failure to produce normal pods, and death.
These control practices reduce losses from root rot:
- Use high-quality seeds treated with a fungicide like Arasan.
- In-furrow fungicides (Terraclor) help control root rot. Apply one- fourth of the material in the open furrow and the remainder in the covering soil during planting.
- During cultivation, do not throw soil against plant stems.
- Plant in a 4- or 5-year rotation with other vegetables.
- Plant in well-prepared soils with a pH of about 6.5, fertilized according to a soil test and treated for nematodes if recommended. Plant seeds 1 inch deep only during favorable weather, in warm soils, and on top of a bed to avoid “drowning.”
Early Blight of Tomatoes—Early blight is a major disease of tomatoes in Mississippi. Symptoms first appear on lower, older leaves as circular, dark brown to black spots that often contain rings, giving a “target board” effect. As the disease progresses, leaves turn yellow, wither, and drop off. Frequently, only the upper half of the plant has green leaves, and in severe cases, the plant becomes completely defoliated.
Early blight also occurs on plant stems and sometimes on fruit. On seedlings, the disease may girdle the stem and give the appearance of damping-off.
Reduce losses to early blight by providing good ventilation in plant beds and watering when leaves have time to dry. Seed treatment with Thiram aids in controlling the damping-off stage. Do not set tomato plants where early blight occurred the year before, and remove and destroy all diseased plant debris in the garden after harvest.
Applications of chlorothalonil or mancozeb effectively control this disease. There is no waiting period after application until harvest for Bordeaux mixture or chlorothalonil, but there is a 5-day waiting period for mancozeb.
Begin applying when plants are 8 to 10 inches tall, and continue at 7-day intervals through the growing season. Applications of these fungicides also control some of the other leaf, stem, and fruit diseases of tomatoes.
Blossom-End Rot of Tomatoes—Blossom-end rot occurs on the tomato fruit. It may also be a problem on peppers, squash, and watermelons. It is more common on fruit that is one-third to one-half grown and occurs on the blossom end of the fruit. It begins as a small, water-soaked spot that develops into a dark brown, leathery spot that may involve half the fruit. The surface of the spot shrinks and becomes flat or sunken.
Blossom-end rot is caused by a lack of calcium in the developing fruit. The uptake of calcium from the soil by the tomato plant can be reduced by fluctuations in soil moisture—either excessively wet soil or excessively dry soil. The disease commonly occurs when plants are grown rapidly and luxuriantly early in the season and are then subjected to prolonged dry weather. Because of drying conditions, the disease may be more serious on the windward side of a garden and on staked tomatoes than on unstaked or bushy plants.
Prevent blossom-end rot by maintaining a soil pH around 6.5, irrigating and mulching to maintain uniform soil moisture, and avoiding heavy applications of nitrogen. Control blossom-end rot by spraying with 4 tablespoons of 96 percent calcium chloride per gallon of water at 7- to 10-day intervals for four applications. Begin spraying with first appearance of symptoms. Overdosing plants with calcium chloride may result in leaf burn. Calcium chloride is suggested only for tomatoes.
Spotted Wilt of Tomatoes and Peppers—This viral disease is transmitted by several species of thrips and may kill plants or drastically reduce fruit-set. Fruits from diseased plants are generally small and distorted. Tomatoes develop irregular yellowish blotches.
Initial symptoms appear as thickening of veins on younger foliage. Younger foliage generally exhibits a pronounced downward curling. Internodes become shortened, and immature fruit does not ripen. Dark purple streaks can occur on leaves, stems, and fruits. Other symptoms are blighting and blackening of young shoots. On individual leaflets, small, dark, circular dead spots may appear. Badly spotted leaves may turn dark and wither.
Some varieties are now being released with resistance. Check with your seed source. It is not clear how effective or long-lived these resistant cultivars will be.
Try these control practices:
- Remove and destroy diseased plants.
- Keep weed populations down in and around gardens to reduce movement of virus-carrying thrips from weeds to garden plants.
- Suppress thrips by applying approved insecticides (Malathion 50 or Diazinon 25 EC at 2 teaspoons per gallon).
- Further suppress thrips with shiny mulch materials around tomatoes and other susceptible vegetables. Apparently, light reflection from the mulch surface repels thrips and reduces the chances of virus transmission. Conventional black plastic may be sprayed or hand painted with aluminum-colored paint. Oil-based paints adhere to plastic surfaces and may be easily applied. This technique gives best results when mulch is laid down at the time of planting and used in combination with other recommended control procedures.
Southern Blight—Southern blight affects most garden vegetables. The fungus that causes southern blight attacks plant parts (roots, stems, leaves, or fruit) that are in contact with or just under the soil surface.
The first visible symptoms are usually an advancing yellowing and wilting of the foliage, beginning with the lower leaves. During warm, moist weather, a white fungus growth may appear on the lower stem near the soil surface and on organic debris in the soil. Later, light tan to dark brown mustard seed-like bodies called sclerotia develop in the mold. As the disease advances, several plants next to one another in the row die.
Southern blight is difficult to control, but you can reduce losses with these practices:
- Plow 6 inches deep in the fall to bury organic debris and the sclerotia.
- Avoid throwing soil on the plants when cultivating.
- Where a few scattered plants are affected, remove them from the garden along with the soil 6 inches deep and 6 inches from the stem.
- Control foliar diseases, since dead leaves on the ground may trigger infection. Also control weeds early in the season for the same reason.
- Wrap transplant stems with a 4- by 4-inch strip of aluminum foil and plant so that 2 inches of wrapped stems are below and 2 inches are above the soil.
Stem Anthracnose of Lima Beans—Stem anthracnose is the most common disease of lima beans. The first stages of infection appear on pods as small, brick-red blotches. These blotches may spread over the entire surface of the pods. Later, the diseased areas become brownish to grayish and may have many tiny black specks which are fruiting bodies of the fungus. Occasionally, diseased pods fall from the plant.
A brick-red streaking may occur along the veins on the under side of leaves and on young stems.
Reddish spots occur on the lower leaf surface and enlarge and become noticeable on the upper leaf surface. Occasionally, leaves are killed and fall from the plant. Severely diseased plants are yellow and stunted.
Reduce damage from stem anthracnose by following these practices:
- Because stem anthracnose can be carried over on seed to the next free, western-grown seed.
- Never plant lima beans in the same location more than once in 3 years.
- Avoid fall planting of lima beans in an area of the garden where stem anthracnose was a problem the previous spring.
- Apply Bordeaux mixture or another copper-based fungicide on a 7-day schedule, beginning at full bloom.
Mosaic—This virus disease commonly infects beans, sweet corn, squash, melons, cucumbers, peas, peppers, and tomatoes. Symptoms include the following:
- Misshapen leaves with light and dark green areas.
- Fruit with green specks, yellow and green mottling, or bumps.
- Distorted fruit.
- Overall stunted plants. Control of virus diseases is difficult. Reduce chances of mosaic in these ways:
-Plant resistant varieties when available.
-Remove diseased plants as they appear.
-Purchase certified transplants or buy western-grown seed.
-Do not use tobacco products when handling plants.
Phenoxy herbicide damage (such as 2,4-D) resembles symptoms of mosaic disease. Leaves and stems are typically twisted, deformed, curled, leathery, and excessively long and narrow. Apply herbicides carefully and correctly in and around the garden.
Powdery Mildew—Powdery mildew is caused by a fungus that commonly occurs as a white, powdery growth on leaves of cucumbers, squash, melons, beans, and English peas. Benomyl and chlorothalonil effectively control powdery mildew on vine crops, and sulfur provides control on beans and peas.
Fusarium Wilt—This fungal disease often infects watermelons, cabbage, tomatoes, sweet potatoes, beans, and peas. Lower leaves often turn yellow on one side of the plant only. Two brownish streaks that originate from the roots are exposed when the stems are split lengthwise. Infected plants are usually stunted and wilted. The best way to control fusarium wilt is using resistant varieties.
Fruit Rot—Bacteria and fungi often infect fruit, resulting in soft, slimy fruit with an offensive odor. You can reduce the occurrence of fruit rot by staking, mulching, avoiding mechanical injury to fruits, controlling insects, following a regular fungicide program, and removing mature fruit from the garden.
Rust—This fungus disease occurs commonly on beans and sweet corn as reddish-brown spots on leaves that rub off when touched. Apply fungicides like chlorothalonil or sprayable sulfur at the first sign of disease and at weekly intervals thereafter until the disease is under control.
Nematode Diseases—Nematodes are slender, tiny, worm-like animals that feed on plant roots, stems, and leaves. Nematodes cannot ordinarily be seen with the naked eye and go unnoticed until plants become unthrifty and stunted. They seldom kill plants; however, they can reduce quality and yields of many vegetables, such as beans, beets, carrots, cucumbers, lima beans, okra, peas, squash, tomatoes, and watermelons.
Nematode injury to roots reduces uptake of water and nutrients from the soil. Typical aboveground symptoms are general stunting, yellowing, loss of vigor, and overall decline. The most common underground symptom in gardens is root-knot galling.
Nematodes other than the root-knot nematode also can cause severe plant damage. Some of the less obvious symptoms are stubby roots, tiny lesions, excessively branched roots, or complete loss of secondary roots.
The best time to determine if you have a nematode problem is during the fall when nematodes are most active. To detect root-knot nematodes in the growing season, observe the galled roots. Have your soil tested for nematodes by sending a soil sample to the Extension Plant Pathology Lab, P. O. Box 9655, Mississippi State, MS 39762.
Once you know nematodes are present, you can use certain cultural practices to help reduce nematode populations. These include setting nematode-free transplants, rotating crops, fallowing, practicing good sanitation, controlling weeds, and planting resistant varieties. Vegetable varieties having resistance or tolerance to root-knot nematodes are marked in the list of recommended varieties starting on page 22. In addition, you can plant marigolds in gardens to help reduce nematode populations.
As an alternative to chemical treatment, solarization can reduce parasitic nematode populations. Solarization is the use of heat from the sun for killing nematodes in the soil prior to planting. To use solarization, place clear plastic (1 to 1.5 ml thick) on moist, tilled soil, and seal the edges with soil, bricks, or other materials. Apply the plastic in May or June. Leave it in place for at least 8 weeks. Remove the plastic in August in time to establish a fall garden, if desired. If not, remove it before cold weather begins.
Leaf Spots—Leaf spots, caused by fungi or bacteria, commonly occur on many vegetables. They appear on leaves and sometimes stems as distinct, dark-colored or tan spots one-sixteenth to 1 inch in diameter. The regular application of a fungicide generally provides acceptable control of fungal leaf spots. Applying copper fungicides helps control bacterial as well as fungal leaf spots.
Bacterial Wilt of Cucumbers—This destructive disease is caused by a bacterium that overwinters in the bodies of adult striped and spotted cucumber beetles. As these beetles feed on young plants in the spring, bacteria are introduced into the vascular system. Here they are able to multiply rapidly and produce a sticky material that stops movement of moisture through the plant. As a result, leaves on an infected runner wilt rapidly, and within a short time all runners become permanently wilted. Plants can die within a week or two after initial symptoms appear. Yellowing is not normally associated with this disease.
A symptom of bacterial wilt is a thick, white, sticky substance that oozes from the cut stem of a wilted vine. If you press your fingertip against the cut surface several minutes after cutting and then slowly remove it, the bacterial ooze frequently remains attached and strings out in thin threads.
Since bacterial wilt-resistant cucumber varieties are not commonly available, the best control is to keep cucumber beetle populations in check. A rigid spray schedule with recommended insecticides (refer to the Insect Control section on page 11) should reduce the incidence of bacterial wilt.
Black Rot of Cabbage—This disease attacks cabbage and other crucifer crops like collards, mustard, cauliflower, Brussels sprouts, kohlrabi, rutabagas, kale, and rape. Black rot may affect plants at any stage of growth but usually is most prominent close to maturity. On older plants, yellow wedge-shaped areas appear at leaf margins and expand toward the center. Blackened veins are apparent in affected areas. Vascular tissue within the stem also may become discolored.
Black rot causes head dwarfing, and soft rot frequently develops on affected heads.
Practices important for controlling black rot include these:
- Use disease-free seeds that have been hot-water treated. This is the most effective treatment for ridding seeds of the causal bacteria.
- Purchase transplants that have been certified as disease-free.
- Rotate in the field so that at least 2 years, and preferably 3, elapse between cruciferous crops.
Yeast Spot of Lima Beans—Gray-brown sunken lesions on young or nearly mature seeds is a good indication of this disease. Yeast spot is more a problem in seasons when southern green stink bug populations are high because the yeast fungus enters seeds through pod punctures this insect makes. The spots, or lesions, develop within 2 to 3 days of inoculation. Bright, sunny days allow the stink bug to move from one plant to another, spreading the disease.
Yeast spot is best controlled by following a good insect control program to discourage build-up of stink bugs.
Precaution: Because of possible changes in pesticide recommendations, you must follow all label instructions.
|Fungicides for Disease Control|
|Brussels Sprouts||0||See label|
Copper and Sulfur — Cleared for use on most vegetables; no time limitations.
Check label to determine that the fungicide is cleared for use on the intended crop.
Numbers indicate the number of days that must pass from last application to harvest.
Blank spaces indicate the fungicide is not cleared for use on that crop.
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Southern blight is a fungal disease of tomatoes commonly characterized by white, thread-like growth and brown or tan, round structures known as sclerotia at the base of the stem.