Managing Diseases in Home Vegetable Gardens
Home vegetable gardens are susceptible to many diseases. In North Carolina you can expect to have disease problems in your vegetable garden sooner or later, especially if you grow a garden on the same site year after year. However, by following good cultural practices and taking preventive measures, good yields and high-quality vegetables may be obtained even if some diseases are present. The following are guidelines that will help you obtain good yields of disease-free vegetables. It is not intended to provide the final answer to all questions about vegetable disease control. Remember that disease management starts with proper disease identification; have a knowledgeable person identify the problem before implementing any control strategy. Successful disease management in vegetable gardens will result from consistently using the all control methods discussed.
Rotating crops is one of the oldest and most economical methods of controlling plant diseases, including plant-parasitic nematodes. Rotation is the practice of not growing a certain crop on the same site for more than one year. Frequently, just by planting a susceptible crop a few feet from where it was grown the previous year, you can avoid damage from disease-producing organisms (pathogens), particularly nematodes. Also, if space is available, the entire garden site may be moved to a new location after two or three years. When you move the garden site, select a new site that has been covered by grass for several years. Be sure to consider succession planting (multiple cropping) in the rotational scheme. For example, if a short-season vegetable that is susceptible to root-knot nematodes is grown in one area of the garden, you can often produce a fall crop (such as a resistant variety of tomato or sweet corn) in the same soil without a yield loss. Plan a rotational program by dividing the garden site into thirds. With this scheme, it is easier to consider all factors that affect plant growth, such as shade, fertilization, water, and time of harvest.
Planting resistant varieties is a very economical way of controlling vegetable diseases. Use resistant varieties in areas where diseases are present or where the soil is known to be infested with disease-causing organisms. Home gardeners can often use the disease-resistant varieties that commercial growers avoid because of handling and marketing considerations, such as fruit size, shape, color, and storage characteristics. For example, Venus and Saturn tomatoes are resistant to southern bacterial wilt, but the fruit is too small for commercial use. You may still need to use rotation and chemicals to control diseases to which the selected variety is not resistant.
Disease-Free Seed and Transplants
Using disease-free planting stock is a must because many important vegetable diseases, particularly bacterial diseases, are caused by pathogens that are seed borne or brought into the garden site on infected transplants. Do not save seed from gardens where diseases are prevalent.
However, if you prefer to save seed of your favorite vegetable variety (for example, beans), select seed only from healthy, non-hybrid plants. Purchase seed from a reputable dealer because external appearances normally do not reveal whether the seed is contaminated with disease causing organisms. People in certain geographical areas are able to produce disease-free seed because of climatic conditions. For example, western grown bean seed are usually free of pathogens because of the arid climate in which they are produced. Request seed from such regions. Likewise, if you are starting your crop from transplants, insist on disease-free transplants. If a plant is infected at an early age, the disease will only get worse and the plant will not perform as expected. A good disease-control program is based on prevention, not cure.
Sanitary practices are very important in helping to prevent or control plant diseases. Many disease-causing organisms survive the winter in plant debris, cull piles, compost piles, or plant stubble that remains in the garden site. Any practice that eliminates these overwintering sites for
fungi, bacteria, viruses, and nematodes reduces the occurrence of disease problems the following year. Some disease producing organisms can also survive the off-season on contaminated equipment or containers. Equipment that has been used in disease-infested vegetable gardens and containers that have been used to harvest diseased vegetables should be disinfected before being used in vegetable gardens. Also, as soon as harvest is complete, remove the remaining plant residue from the garden site and expose the roots to the sun and wind to kill the pathogens, particularly nematodes.
Once the soil has been prepared and is ready for planting, place a clear sheet of plastic over the site, burying the edges in the soil. Over the course of several warm, sunny days, temperatures under the plastic will rise to over 125 degrees, killing many soil-home pests including plant pathogens, nematodes, insects, and weeds. However, these pests may not be affected if they are more than 4 inches below the soil surface. It normally takes four to six weeks of very sunny weather to eliminate disease-causing organisms at greater depths. Solarization also works well for preparing soil to use in producing transplants. In this case, put the soil in a sealed plastic bag and place it on the driveway, walkway, or similar location so that the bag can remain in bright sunshine for a period of several weeks. Solarization probably works best for fall-planted vegetables because the optimum time to use this technique in North Carolina is during June, July, or August.
Seed is usually treated by the seed producer or seller; if you purchase untreated seed, treat it with a proper fungicide before you plant it. Properly treated seed will produce a better stand than untreated seed and will normally produce more vigorous plants that are better able to resist attack by weak pathogens. Vegetable seed. However, be sure to check the label to determine if the vegetable that you want to treat is listed. Treat only those vegetables listed on the label. Treat seed by placing the desired amount of fungicide in the seed container and shaking vigorously. A small jar or can with a lid attached also works well. Thiram and captan are good fungicides to use on most.
Proper fertilization helps to control several diseases, such as tomato blossom-end rot and potato scab. Always have the soil in the garden site analyzed and apply fertilizer and lime according to the directions contained in the soil test results. N.C. Cooperative Extension of Macon County can supply the forms and instructions needed to take a soil sample. Also, be aware of the organic matter content of the soil in the garden site and maintain it at as high a level as possible. This helps to control some diseases and to maintain a uniform water supply, which is very important in obtaining good yields from vegetable plants. Ideally, a vegetable plant should not suffer from any stress during its life.
Time of Planting
Planting vegetables at the proper time of year helps reduce losses from several pests. Damping-off is less of a problem if the seed are planted in soil within the desirable temperature range for a particular vegetable. For example, garden peas and potatoes may be planted in relatively cool soil, whereas squash, beans, or cucumbers must be planted in fairly warm soil. Quite often, vegetables planted in the spring do not become as heavily infected with pathogens as those planted in the fall, particularly for second plantings of the same vegetable or same variety. Thus, many growers produce most of their vegetables in the spring and do not try to produce a fall crop because of the pest pressure.
Many growers prefer to produce their vegetables without the use of pesticides. However, even if you follow the above practices, you may still find that you want to use fungicides and insecticides in the garden to prevent and control diseases and insects and to obtain the maximum yield of blemish-free fruit. Malathion and Permethrin are two common insecticides used by vegetable gardeners and chlorothalonil is a common fungicide. Always be sure to read and follow all label directions on the pesticide container and use the chemical only on the vegetables listed on the label. The labeling of a pesticide can change without notice. It is your responsibility to be sure the chemical is labeled for use on the vegetable before you apply it to that crop.
Also, it is best to use a fungicide in a preventive program. Most will not perform satisfactorily if they are applied after the disease has reached damaging levels. Copper, available in several formulations, is labeled for use on several vegetables and is an excellent bactericide as well as a good fungicide. You may want to consider the use of this material, even if you are an organic gardener because it is approved by several organizations that are farming.