Diseases of vegetables grown in the home garden may reduce both the
yield and quality of vegetables. Controlling such diseases often determines
success or failure and adds immeasurably to the pleasure derived from the
garden. Diseases of vegetables are caused by microorganisms, including fungi,
bacteria (including mycoplasmas and spiroplasmas), viruses, and nematodes.
These organisms are spread by wind-blown or water-splashed spores, insects,
infected seeds or transplants, by the movement of infested soil, and by
humans handling wet plants.
Vegetable diseases can be successfully controlled using an integrated
disease control program. Such a program involves the use of crop rotation,
sanitation, disease-resistant or tolerant varieties, disease-free seeds
or transplants, sound horticultural practices, balanced soil fertility
(based on soil tests), and proper and timely applications of pesticides.
There is no single control practice that can control all of the diseases
of any vegetable crop. Therefore, several measures must be used to achieve
satisfactory control.
DISEASE CONTROL BEFORE THE GARDEN IS PLANTED
Choose and prepare the site. The site for the vegetable garden should
be well-drained, have adequate sunlight (8 or 10 hours of direct sunshine
per day), and good air circulation. The soil should have adequate soil
fertility and the proper pH (5.5 to 7.0). Information on garden location,
soil fertility, and soil pH can be found in University of Illinois Extension
Circular 1331, Vegetable Gardening in the
Midwest. If the area has produced garden vegetables for several years,
sanitation and rotation practices will reduce the risk of disease-causing
organisms that have survived from preceding crops. Removing or turning
under dead and diseased plant material in the fall is important for disease
control. Crop residues from healthy plants can be composted and returned
to the garden. Disease-causing fungi and bacteria can survive composting
unless the decomposition of the crop residue is complete, a situation
that seldom occurs. Therefore, we suggest that no diseased plant parts
be placed in the compost pile. Some disease organisms will even survive
complete composting, including corn smut; clubroot of cabbage, broccoli,
cauliflower and other crucifers; Verticillium wilt of potato, tomato,
pepper, and eggplant; and root-knot nematodes. Turning (plowing or tilling)
under crop residues in the fall helps prevent the over-wintering of many
disease-causing organisms. Besides destroying the “winter home”
of various organisms, this practice also helps control the insects that
transmit certain diseases. Also, crop debris that is turned under improves
soil tilth and makes spring garden work easier.
Crop rotation. Crop rotation is an important control
measure since many disease-causing organisms only attack related plants
in the same family. Unrelated plants do not serve as hosts on which these
organisms can multiply. Therefore, if possible, avoid planting any vegetables
within each of the following groups in the same location more than once
every three or four years.
Cole crops–(cabbage family) broccoli, Brussels
sprout, cabbage, cauliflower, Chinese cabbage, mustard greens, kohlrabi,
radish, rutabaga, and turnip. Cucurbits–(cucumber family) cucumber,
gourds, muskmelon, pumpkin, squash (all types), and watermelon.
Solanaceous crops–(tomato family) eggplant, husk
tomato (ground cherry), potato, pepper, tomato. Chenopodiaceous crops–beet
(red and sugar) and spinach.
Amaryllidaceae–(onion family) chives, leeks,
onions, shallots, and garlic.
Legumes–pea, beans (all types). Umbelliferous
crops–carrot, parsnip, celery. For example, cabbage, broccoli, radish,
or turnips should not be planted in the same location for two successive
years. Cabbage could be followed with beans, a vine crop such as cucumber,
or with sweet corn.
Diseases affecting beans, peas, sweet corn, carrot, lettuce, spinach,
rhubarb, okra, edible soybeans, and similar plants are usually specific
to only one of these crops, and generally will not infect others or members
of the major vegetable crop groups.
Choose disease-resistant varieties. The use of well-adapted, disease-resistant
varieties is the simplest and most efficient method of controlling many
diseases. Lists of disease-resistant vegetables recommended for use by
Illinois gardeners are given in Circular 1331 (Vegetable Gardening in
the Midwest. Additional information on disease resistance is also given
in the following paragraphs. A listing of varieties and their resistance
can be found in Circular 1363, Midwest Vegetable Production Guide for
Commercial Growers.
Start with disease-free seeds, plants, and planting materials. It is
important to start with disease-free seeds or planting materials (bulbs,
tubers, transplants, sets, and so on) to keep from introducing serious
diseases into the garden. In general, gardeners are encouraged not to
save their own seed but to purchase seed from reputable seed dealers.
Hot-water soaks and fungicide seed treatments may be used to control the
seedborne disease organisms that cause seed rots and damping-off. Since
temperature controls and timing must be precise, home gardeners usually
buy seed that has already been treated with hot water by the seed processor.
Hot-water treatment of cabbage, broccoli, Brussels sprout, and cauliflower
seed is particularly important.
Many vegetable seeds are coated with a fungicide at the time of purchase,
as evidenced by the colored coating on the seed. Untreated seed can be
treated by the home gardener by placing a small quantity (the size of
one or two match heads) of captan or thiram in the seed packet and shaking
the packet for a minute or two to thoroughly cover the seed. Excess protectant
may be sifted out before planting.
Do not plant diseased material (for example, transplants, sets, bulbs,
or tubers). All planting material should be healthy and free of yellowing
and brown or black spots, and should not be stunted or show poor development.
Only certified, disease-free potato tubers or sweet potato slips should
be used. Examine transplants thoroughly for signs of leaf or stem disease.
It never pays to buy and plant diseased transplants, no matter what the
price.
Damping-off diseases affecting home-grown vegetable transplants can be
controlled by the use of diseasefree soil and fungicides.
CULTURAL PRACTICES
Cultural practices such as controlling weeds, planting at the right
time, planting at the proper depth and spacing, employing cages or stakes,
and watering when the temperature is rising to promote a rapid drying
of the foliage can help control many foliar diseases. Mulches help to
control fruit rots and aid in control of blossom-end rot of tomato, pepper,
and vine crops by maintaining a uniform supply of moisture in the soil.
Weed control in and around the garden reduces the risk of attacks by viruses
that overwinter in these
plants. Do not work or harvest wet plants, since this can spread many
diseases, particularly those caused by bacterial and fungal pathogens.
DISEASE CONTROL DURING THE GROWING SEASON
Many diseases, such as the early blight disease of potato and tomato,
occur each year despite all preplanting precautions. For such diseases,
applications of fungicides and bactericides to the growing plants may
be needed. The best way to apply these materials is as sprays before the
disease occurs. Often a wetting agent such as liquid detergent or soap
(1/2 teaspoon in 1 gallon) is added to obtain more thorough wetting and
coverage of the foliage. Apply sprays to the point of run-off on a 7-
to 10-day schedule. This maintains a fresh or effective covering of fungicide
and protects the new growth. Fungicides and bactericides currently recommended
for use on vegetables is given in the Illinois Pest Control Handbook which
is updated each year.
PRINCIPAL DISEASES OF VEGETABLES AND CONTROL MEASURES
Asparagus
Rust. Grow resistant varieties Start spray applications
after harvest and continue until mid-August on a 10-day schedule with
an approved fungicide.
Crown and root rots. Maintain proper fertility and good
soil drainage. Avoid over-cutting.
Beans
Seed decay and damping-off. Use captan- or thiram-treated
seed. Plant in a warm, moist, well-prepared seedbed.
Bacterial blights. Do not save seed from infected plants.
Purchase seed from a reputable seed dealer. Spray a copper-based bactericide
such as copper sulfate at the first sign of disease, and continue on a
7- day scheduled until harvest. Do not work among wet plants.
Rust. Grow rust-resistant varieties. Apply fungicides
at the first sign of disease and continue on a 7-day schedule.
Mosaics. Grow resistant varieties..
Beets
Seed decay and damping-off. Plant seed treated with
thiram or captan.
Leaf spots. Spray with an approved fungicide on a 7-day
schedule.
Carrots
Seed rot and damping-off. Plant seed treated with captan
or thiram.
Leaf spots. Practice a 2- to 3-year crop rotation.
Spray weekly with an approved fungicide, starting about mid-June.
Aster yellows. Destroy infected plants as soon as they
appear. Apply insecticide sprays for leafhopper control. Spray before
removing infected plants.
Cole Crops (cabbage, broccoli, Brussels sprout, cauliflower,
Chinese cabbage, kohlrabi, mustard greens, radish, rutabaga, and turnip).
Seed decay, damping-off, black rot, and blackleg. Plant
hot-water-treated seed which has also been treated with captan or thiram.
Several black-rot-resistant cabbage varieties are now available.
Leaf spots and blights. Practice 2- to 3-year crop rotation
for broccoli, cabbage, cauliflower, and Brussels sprout. Apply an approved
fungicide on a 7-day schedule.
Clubroot. Purchase disease-free transplants. PCNB (Terraclor)
may be added to the transplant water.
Fusarium yellows. Grow only varieties that are resistant
to yellows.
Cucurbits (Cucumber, gourds, muskmelon, pumpkin, squash,
and watermelon).
Seed rot and damping-off. Plant seed treated with captan.
Plant in a warm, well-prepared seed bed.
Angular leafspot. Practice a 2- to 3-year crop rotation.
Spray with a fixed copper bactericide, such as COCS. Remove diseased plant
material. Plant resistant varieties whenever possible.
Anthracnose, Alternaria leaf blight, downy mildew, blossom blights,
fruit rots and spots, and gummy stem blight or black rot. Practice
2- to 3-year crop rotation. Apply an approved fungicide at weekly intervals,
starting when the plants are in the two-leaf stage. Adequate nitrogen
fertility is essential for the successful control of Alternaria leaf blight.
Plant resistant varieties when available.
Bacterial wilt. Spray with an insecticide to control
cucumber beetles as soon as the seedlings “crack” the soil
before they emerge. Continue weekly spraying until the plants are in bloom.
Spray in the evening to avoid killing bees.
Fusarium wilt. Grow only cantaloupe and watermelon varieties
that are resistant to Fusarium wilt.
Powdery mildew. Spray with an approved fungicide at
the first sign of disease, and weekly thereafter.Grow resistant varieties.
Mosaic and other virus diseases. Grow resistant varieties
where possible. Only cucumber-mosaicresistant cucumbers should be grown.
Eggplant
Seed rot, damping-off, and Phomopsis blight. Plant seed
treated with captan or thiram.
Phomopsis blight and other fruit rots. Practice a 2-
to 3-year crop rotation. Spray weekly with an approved fungicide when
fruits are half size, or when disease first appears.
Verticillium wilt. Mulching with black plastic may
help reduce losses.
Onion
Smut, seed rot, and damping-off. Plant seed treated
with thiram or captan.
Leaf diseases. Practice a 2- to 3-year crop rotation.
Spray weekly with an approved fungicide. Carefully follow label directions
as regards the days between the last spray and harvest.
Bulb rots. Control leaf diseases. Also follow these
recommendations: avoid storing improperly cured or injured bulbs; let
all onion tops fall over (ripen) naturally; store under cold dry conditions;
and do not allow cured bulbs to be exposed to sunscald, water, or high
humidity.
Fusarium basal rot. Grow resistant varieties.
Pea
Seed rot and damping-off. Plant seed treated with captan
or thiram.
Root rot. Plant early in well-drained soils. Use a
fungicide seed treatment.
Fusarium wilt. Plant only varieties that are resistant
to wilt.
Peppers
Seed rot and damping-off. Plant seed treated with captan
or thiram.
Bacterial spot. Use the hot-water seed treatment, or
dip seed in a solution of 1 part household bleach to 3 parts water and
rinse with water. Follow with a fungicide seed treatment. Apply weekly
sprays of a fixed copper fungicide such as COCS, through the bloom stage.
Use crop rotation outside the tomato family
Tobacco mosaic. Plant resistant varieties.
Potato
Viruses, late blight, Verticillium wilt, and ring rot. Plant
only “certified” disease-free seed.
Seedpiece rots. Treat cut seedpieces with captan as
a dust or dip. Plant in warm soil (over 50°F) after the cut surfaces
have corked over (suberized).
Early blight and late blight. Practice a 2- to 3-year
crop rotation. Destory or remove cull tubers. Spray at weekly intervals,
starting when the plants are 4 to 6 inches high with an approved fungicide.
Plant late blight-resistant varieties.
Verticillium wilt and root-knot nematodes. Plant resistant
varieties.
Scab. Plant resistant varieties, or adjust the soil
pH to 5.2 to 5.5. Use fungicide seedpiece treatments.
Storage rots. Store only disease-free, unbruised tubers.
Allow tubers to cure at 60°F for 5 to 7 days before reducing temperature
to 36° to 40°F (if possible).
Sweet corn
Seed rot and damping-off. Plant
seed treated with captan or thiram.
Smut. Plant resistant varieties. Remove smut galls from
the garden before they break. Do not compost smutted corn or infected
corn residues.
Stewart’s bacterial wilt. Plant resistant
varieties. Control flea beetles with a recommended insecticide. Early
season control is particularly important.
Foliage blights and rust. Plant
varieties resistant to northern and southern corn leaf blights and rusts.
Apply an approved fungicide. However, the use of fungicides is rarely
justified. Diseased plant material should be removed from the garden.
Blighted leaf tissue is safe to compost.
Maize dwarf mosaic. Plant as far from Johnsongrass
as possible. Control wild Johnsongrass plants. Plant tolerant varieties.
Spinach
Seed rot and damping-off. Plant seed treated with captan
or thiram.
Cucumber mosaic virus (blight). Plant resistant varieties.
Downy mildew or blue mold. Plant resistant varieties.
Sweet potato
Black rot, scurf, foot rot, wilt, and soil rot. Buy
certified, disease-free slips. Plant resistant varieties. Use a 3- to
4-year rotation.
Storage diseases. Do not expose roots to temperatures
below 55°F. Cure the roots immediately after digging at 85°F and
high humidity for 10 to 14 days. Store at 55°F. Never store in airtight
bags or containers. Apply an approved fungicide as a post-harvest dip.
Follow the manufacturer’s directions carefully.
Tomato
Seed rot, damping-off, bacterial spot, bacterial canker, and
tobacco mosaic virus. Plant seed which has been treated with
hot water or acid and then treated with captan or thiram. Purchase seed
or transplants only from reputable dealers. Soaking seed in copper sulfate
solution (2 ounces per gallon) or dipping in bleach solution as described
under peppers will help prevent bacterial spot and canker if seed are
saved.
Bacterial spot. Plant certified, disease-free seed or
transplants. Practice a 2-to 3-year crop rotation. Spray plants weekly
with a fixed copper fungicide, beginning with the first sign of disease,
through the bloom stage.
Early blight, Septoria leaf blight, anthracnose, buckeye rot,
and soil rots. Spray weekly with an approved fungicide. Practice
2- to 3-year crop rotation. Remove or destroy infected plant parts. Carefully
follow label directions as regards the days between the last spray and
harvest. Staking or caging plants and providing good air circulation will
help reduce foliar diseases. Staking, caging or mulching will also help
prevent fruit rots, such as anthracnose, buckeye rot, and soil rot.
Blossom-end rot. Maintain uniform soil moisture by mulching
the plants and irrigating when needed. Avoid heavy pruning, excessive
nitrogen applications, or damage to the roots by cultivation or hoeing.
Verticillium and Fusarium wilts. Grow only VF - or VFN-resistant
varieties. There are many varieties available.
Root-knot nematodes. Grow VFN-resistant varieties.
Viruses. Avoid contact between potatoes and tomatoes
to prevent “double infections” of potato and tomato viruses.
Wash hands thoroughly with soap and hot water before handling your plants.
Do not use tobacco when working with tomatoes.
The following publications are excellent guides for gardeners:
Circular 1331 Vegetable Gardening in the Midwest, and Circular 1373
Midwest Vegetable Production Guide for Commercial Growers, available from
Information Technology and Communication Services, University of Illinois,
P345, 1917 S. Wright St., Champaign, IL 61820.
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