Chapter 7

Crop Rotation on Organic Farms

Management of Diseases with Crop Rotation

Crop rotation is an important consideration in disease management, particularly effective in controlling soil- and stubble-borne diseases. The goal of crop rotation is to reduce the amount of the pest population present in the soil. To manage a successful rotation program, it is important to know the following: 1) how long the pathogen can survive in the soil, 2) range of plant hosts, 3) how it can be spread or reintroduced into a field, and 4) methods for managing other pathogen sources (Mohler et al., 2009). It is important to remember that while crop rotations with non-susceptible crops and cover crops may help reduce pathogen numbers, significant decreases in such populations are likely to take many seasons.

Crop Rotation Periods for Reducing Pathogens

Time required for rotation to be effective can vary with disease severity, environmental conditions, and survival time (See Table 8.1). When a disease has been severe, a longer rotation may be needed to reduce the pathogen’s inoculum level sufficiently to avoid economic loss.

Pathogen Suppressive Crops

Some plants suppress pathogens in addition to being unsuitable hosts. These include some cover and green manure crops, as well as cash crops. Including disease-suppressive species in a rotation sometimes reduces the time needed before a particular cash crop can again be produced successfully. Examples of pathogen suppressive crops include some legumes (e.g., alfalfa, hairy vetch, clover, lupine) and crucifers (e.g., cauliflower, cabbage, garden cress, bok choy, broccoli). These plants suppress pathogens by stimulating beneficial organisms in the soil and by producing toxic chemicals.

Crop Rotation Considerations in Managing Diseases

Maintaining adequate and balanced fertility is very important for effective disease management. Plants have defense mechanisms against pests, including diseases. Vigorously growing plants can usually withstand a certain amount of disease. Time required for rotation to be effective can vary with disease severity and environmental conditions. When a disease has been severe, a longer rotation may be needed to reduce the pathogen’s inoculum level sufficiently to avoid economic loss.

Diseases Controlled by Crop Rotation

Understanding the mechanisms that allow or prevent management of specific diseases by rotation can improve success and avoid wasted effort. This section discusses some diseases that can be successfully managed by crop rotation; the next section, Limitations of Crop Rotation in Controlling Diseases, discusses some diseases that cannot be controlled.

Bacterial Speck of Tomato

This disease is more difficult to control with rotation than bacterial spot because the pathogen (Pseudomonas syringae pv. tomato) can survive on roots and leaves of taxonomically diverse weeds. Therefore, success requires good control of weeds and volunteer tomatoes during the rotation period. This tomato disease also affect peppers, but resistant pepper varieties are available.

Bacterial Spot of Pepper and Tomato

The bacterium causing spot (Xanthomonas campestris pv. vesicatoria) can be effectively controlled with rotation because this pathogen cannot survive in the soil once diseased plant debris decomposes. A minimum of two years without a host crop is recommended.

Club Root

Clubroot is caused by the soil-borne pathogen Plasmodiophora brassicae. It infects the roots of crops in the Brassica genus, such as canola and cole crops (e.g., broccoli, Brussels sprouts, cabbage, cauliflower, collards, kale, and kohlrabi).

Ergot

Ergot, caused by the fungus Claviceps purpurea, is a disease of cereal crops and grasses. It is most prevalent on rye and triticale but also attacks durum wheat, common wheat and barley, in decreasing order of susceptibility.

Lettuce Drop

Lettuce drop is caused by the fungal pathogens Sclerotinia sclerotiorum and Sclerotinia minor.

Take-all Root Rot

Take-all root rot (Gaeumannomyces graminis) is the most widespread cereal root disease in the world. The traditional and best method for control of the soil-borne disease take-all is a crop rotation that uses wheat no more frequently than every third calendar year in areas with a cold dry climate, and every other year (with at least 12 months' break from susceptible cereals or grasses) in areas with a warm wet climate.

White Mold

Sclerotinia sclerotiorum (white mold) fungus attacks many broadleaf crops. Sunflowers are most susceptible. Corn, small grains, wheat, grasses, and sorghum are nonhost crops of S. sclerotiorum that can be planted in rotation with susceptible crops such as dry beans, mustard, canola, lentils, safflower, soybeans, and sunflower.

Limitations of Crop Rotation in Controlling Diseases

Not all plant disease pathogen populations can be reduced or managed economically through crop rotation alone. Crop rotation has a limited impact on disease management when:

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