Controlling Diseases in the Home Vegetable Garden

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Establishing and maintaining a healthy, disease-free vegetable garden requires good cultural practices and preventive measures as well as specific disease control methods. The “general practices” presented here are those generally necessary to prevent or control commonly occurring diseases. Not all treatments are needed in all locations every year. Past experience and the degree of perfection desired help determine a disease control program.

General Practices

Disease control in the home vegetable garden must start long before seeds and plants are placed in the ground. Plan a long-range program and keep records from one year to the next, especially noting the location of plant types within the garden. This will facilitate effective crop rotation, which is one method of minimizing root rot diseases caused by soilborne pathogens.

Bacterial speck of tomatoes. Bacteria can come from seed, plants, or from contaminated tools or containers.

Establish good cultural and sanitation practices. This includes proper soil preparation, fertilizing and watering, as well as early detection and removal of infected plants. The latter is especially true if you want to exclude the use of fungicides in vegetable production.

An equally important, but frequently overlooked, preventive method of disease control is the use of disease-resistant vegetable varieties. There are many disease-resistant varieties available to the home gardener that produce abundant, high quality fruit under Minnesota growing conditions. See Disease-Resistant Vegetable Varieties, FO-2412, or seed catalogs.

The gardener must be aware that disease can be introduced via infected seed or through the purchase of diseased plants; therefore, careful inspection of transplants is recommended. It is desirable to purchase seed and transplants from a local, reliable source.

Since splashing water is the principle method of spreading fungal and bacterial foliar diseases, it is important to water plants at the ground level and early in the day. This allows the foliage to dry prior to the evening hours and reduces splashing of spores from infected tissue or soil onto the healthy tissue. Proper spacing of plants increases air flow and will also help reduce disease severity. Control of foliar diseases during periods of frequent rainfall can be difficult even with the use of fungicides. Generally fungicide applications should begin at the first detection of disease.

Aster yellows on carrot. Note multiple crowns and excessive side branches on main root. Control aster leafhopper to prevent infection.

Also be aware of viral diseases that infect vegetable crops. Viral pathogens usually are transmitted from plant to plant by insects or by handling infected plants. The use of cigarettes or tobacco in any form by the home gardener should be avoided. Tobacco mosaic virus, often a contaminant of tobacco, can be transmitted from plant to plant simply through handling; therefore, do not smoke while handling or transplanting plants. (See Tomato-Tobacco Mosaic Virus Disease, FS-1168.) Generally plants infected with a virus are stunted, develop a mosaic pattern on the leaves or distorted foliage, and produce poor quality fruit. Some control of virus can be obtained by early detection and removal of infected plants, although complete control often is difficult.

A disease called aster yellows occurs on carrots, lettuce, potatoes, tomatoes and many other plants. This phytoplasma (MLO) disease is spread by the leafhopper insect, and control of the disease requires control of the insects. Treatment during the first half of the growing season is most important. Complete control is difficult, especially when leafhopper populations are high.

Disease Control Tips

Septoria leaf spot of tomato. Caused by a fungus. Anthracnose of muskmelon. Caused by a fungus.

1. Provide adequate space between plants. This allows good air movement and will be helpful in controlling diseases.
   
   
2. Plant disease-resistant varieties when available. See Disease-Resistant Vegetable Varieties, FO-2412.
   
   
3. Avoid planting tomatoes, eggplant, beans and potatoes on the same ground year after year. Also avoid planting successive crops of these vegetables side-by-side during the same growing season because some of them are susceptible to the same diseases.
   
   
4. Water plants early in the morning hours so that the foliage can dry during the day. Watering plants in the evening creates ideal conditions for development and spread of disease.
   
   
5. Control insects early in the season since many of them transmit viral, phytoplasma, and bacterial diseases to healthy plants.
   
   
6. Control weeds around the garden because many disease organisms survive on weeds and can be transferred to garden crops by insects, wind, etc.
   
   
7. Apply mulch to reduce splashing spores from infected soil or debris to healthy tissue. Mulch also helps control weeds and retain soil moisture.
   
   
8. Disease control in the garden may require the use of a fungicide. (See Table 1.) Keep a sprayer on hand to apply the fungicides used for disease control.
   
   

Individual Crop Treatments

Beans—Rotate crops to help prevent root rot disease; plant disease-free seed; do not handle plants or work in the garden while plants are wet.

Carrots—Rotate crops; thin plants properly.

Crucifers (cabbage, broccoli, cauliflower)—Rotate crops; purchase disease-free transplants; buy seed varieties with disease resistance such as cabbage varieties resistant to Fusarium yellows.

Cucurbits (cucumbers, melons, pumpkins, squash)—Rotate crops; purchase disease-free transplants with resistance to specific diseases (consult your seed catalog or extension bulletins for specific varieties with disease resistance); apply fungicide when needed for foliar disease control; space plants properly; control cucumber beetle to prevent bacterial wilt infection.

Onions—Rotate crops; space plants properly.

Peas—Rotate crops; purchase disease-resistant varieties.

Peppers—Rotate crops; purchase disease-free transplants.

Potatoes—Rotate crops; plant certified seed; space plants properly; apply a fungicide at first detection of disease.

Tomatoes—Rotate crops; space plants properly; plant disease-resistant varieties. Apply a fungicide at first sign of disease.

Fungicide Seed Treatment for Damping Off Control

When planted, seeds are subject to attack by various soil-borne organisms. Fungicide seed treatments are applied to help prevent both seed rot during germination and damping off after plant emergence. Captan is an example of a fungicide used as seed treatment.

Root rot of melon.
Caused by a soil-borne fungus.

Fungicides for Spray or Dusting

The following fungicides provide effective control of many vegetable plant diseases. Carefully read the label to determine specific crop usage and limitation of these fungicides.

Precautions

Fungicides should be handled with care and respect. Always read the label directions carefully prior to purchase and use. Wash treated vegetables before use.

Purchase only enough fungicide for one season’s use because chemicals sometimes deteriorate when held through the winter. Eliminating storage of chemicals between growing seasons also avoids any possibility of youngsters and pets getting into them.

Fungicides should be stored in a locked cabinet. If a fungicide is accidently swallowed, call a physician immediately, giving the name of the fungicide. Take along the container or package label.

F.L. Pfleger, S.L. Gould, and W.J. White-McDougall
Department of Plant Pathology

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