The annual cycle for many disease organisms is to attack the crop in the summer, survive the winter as spores in the soil or in plant litter, then attack the new planting the following year. So if you plant the same crop in the same spot year after year, the pathogen populations can continue to build and the disease will become an increasingly serious problem. You can break the cycle by moving the susceptible plant to another spot. Over time, many pathogens die back, and then it is safe to plant the original crop in that spot again. This is the idea behind rotating crops to prevent plant diseases.
The first step in planning crop rotation is to make a list of the crops you intend to plant. Most diseases tend to attack plants within in the same botanical family, so group your crops together in these botanical groups. Our common vegetables usually belong to one of ten families. Tomatoes, potatoes, peppers, and eggplants are in the Nightshade Family (Solanaceae); cabbage, radishes, turnips, broccoli, cauliflower, and brussels sprouts are in the cabbage family (Cruciferae), and are also called cole crops. Spinach is in the family Chenopodiaceae, and lettuce and other salad greens are in the Lettuce Family (Compositae). Peas, beans, and lentils are in the Legume Family (Leguminosae), and carrots, parsley, and dill are in the Umbelliferae Family. Cucumbers, the squashes, and melons are in the Melon Family (Cucurbitaceae), and onions, leeks, garlic, and chives are in the Lily Family (Liliaceae). Corn is in the family Gramineae, and strawberries are in the Rose Family.
The second step is to divide your garden into the same number of sections as the number of plant families you will have. For example, if you have six plant families, divide your garden into six sections. It works best if all the sections are the same size to ensure each section of soil is rotated from year to year.
Step three is to assign a plant family to each section, making notes of the assignment on a permanent map of your garden. (It is difficult to remember the order of rotation after two or three years without permanent records.) In each subsequent year, move each plant family to the next section in the garden, always following the same order. If you have 6 groups of plants, 5 years will pass before the same crop is planted in the same section of ground. A cycle of at least 4 years is most effective.
Crop rotation works best for controlling soilborne diseases that attack only a few species of plants. Most of the diseases caused by soilborne fungi are particularly easy to control this way. Added bonuses of crop rotation include control of certain insect pests and prevention nutrient depletion in the soil in particular spots in your garden.
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