Disease Management Practices
Selecting locally adapted crops and cultivars that are resistant to common pathogens is an important way to reduce or prevent disease problems. The disease descriptions in the pages below include information on whether or not resistant cultivars have been developed. Resistant cultivars are listed in individual chapters in Part Three of this book. Seed companies and local cooperative extension offices are also good sources of information on disease resistances and how well particular cultivars perform locally.
Promote Vigorous Plant Growth
Cultural practices that promote vigorous but balanced plant growth are the first line of defense against disease. Using too much fertilizer may result in salt damage to roots, opening the way for secondary infections by opportunistic pathogens. Balancing watering and fertilizer is also important. The succulent growth of plants given too much water and nitrogen encourages certain pathogens. On the other hand, stressed plants, especially those low in potassium and calcium, are more vulnerable to diseases such as early dyingand to physiological disorders such as blossom end rot of tomatoes which are often mistaken for infectious diseases. In a study of the susceptibility of three tomato cultivars to bacterial wilt, the wilt tolerance of cultivars with some resistance was increased by higher calcium levels in the nutrient solution. In a susceptible cultivar, however, all infected plants died, whatever the level of calcium nutrition. Physiological disorders are described in the crop chapters in Part Three.
The most important water management practice is providing drainage to keep soil around roots from becoming waterlogged. Seeds and seedlings are likely to rot in wet soil. It is also important that foliage stay dry. Infectious material or inoculum of water-borne pathogens spreads from infected to healthy leaves by water droplets, and fungal pathogens need water to germinate and enter the leaf. Other useful water management practices include:
Depending on the disease, it is often possible to reduce the disease-spreading inoculum present in a field. Specific information on reducing inoculum of the diseases most common in the south is given at the end of this chapter but in general, inoculum can be reduced by using disease-free plant material, careful crop rotations, increased soil organic matter content, and good sanitation as described below.
Disease-free plant material. For crops propagated vegetatively, the most important consideration is starting out with disease-free planting material such as certified seed that has been inspected for pathogens at all stages of production. Sweetpotato and white potato growers can establish all or part of their acreage with certified seed. Using "clean" seed is also important in crops grown from "true" seed. Seed of some crops, such as lettuce, can be tested for disease incidence. Seed, both true seed and that of vegetatively propagated plants can also be treated. Treatments include hot water, bleach solutions and fungicides, depending on the disease.
Crop rotation. Although crop rotation, especially between very different crops, is generally considered to reduce disease, it is not clear to what extent disease prevention contributes to the benefit of rotation. Rotation is most effective against diseases that attack only one crop. However, controlling the many diseases that infect several crops in the same plant family requires rotation to an entirely different family. Unfortunately some pathogens, such as those causing wilts and root rots, attack many families and rotation is unlikely to reduce disease. Diseases common in the south and crop species susceptible to them are listed alphabetically on the previous page as a convenient guide for planning crop rotations.
Cover crops. Information on the impact of cover crops on disease damage to the subsequent cash crop is limited, but cover crops have been used in some crops to suppress disease. In cotton, a hairy vetch cover crop reduced black root rot(Thielaviopsis basicola) compared to winter fallow, but Rhizoctonia solani increased. Soil populations of Pythium were greater after all legume cover crops compared to the winter fallow treatment.
Organic matter content. Increasing organic matter of soil can increase saprophytic microbial activity, which lowers population densities of pathogenic, soilborne fungi. In California incidence of corky root and Phytophthora root rot on tomato was higher on the conventional farms studied than on the organic farms. Factors correlated with higher incidence of corky root rot were higher inorganic nitrogen concentrations in the soil and plant tissue and lower soil microbial activity. Higher incidence of Phytophthora root rot was correlated with increased clay content of the soil and decreased water-stable aggregates in the soil. See Soil Management for further examples of relationships between organic matter and disease.
Sanitation. Removing or burying diseased plant matter and culls will help prevent the carry over of foliar pathogens in plant debris. Tools can be disinfected by dipping them in a 10 percent solution of household chlorine bleach. Bleach can also be used to disinfect tires, knives, and farm implements when leaving infected fields. Tomato stakes and other large items can be disinfected by the high temperatures of solarization. These basic sanitary practices lower initial levels of inoculum for many diseases, including early and late blight in potato and tomato, and leaf blight and downy mildew in onions. If temperatures during composting rise high enough and are uniformly achieved in the pile by mixing, most fungal propagules are also destroyed. However, some fungi produce resistant, long-lived reproductive structures as well as the immediately infectious forms. For example, the black sclerotia produced by Sclerotinia can survive for years. Pythium and Phytophthora can also produce long-lived resting oospores.
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