Crop rotation can also break the cycles of pest and disease problems that build up in soils planted repeatedly to the same crop. For disease prevention, the idea is to plan your rotation so that no two crops subject to similar diseases follow one another within the incubation period period for that disease. For example, try not to plant cabbage or its relatives in the same plot more than once every four years, as the various fungal and bacterial diseases that plague that botanical family are able to overwinter in the soil. Without a host the following season, though—because you moved the cabbage family members out, to a different plot—these diseases are not able to survive. By the time the plot is growing cabbages again the pathogen is long since dead. The same holds true for plants belonging to the squash family, carrot family, onion family, and the rest. If you can avoid planting the same family of plants in the same place in successive years you’ll go a long way toward preventing the buildup of disease organisms in your garden’s soil.
The same principle holds for insect pests: crop rotation makes it harder for emerging insects to find their preferred food each spring. By the time the larvae of the cucumber beetle locate the new spot to which the cucumbers have been moved, the plants have gotten a good head start and are better able to resist their attack. A related tactic—called trap cropping—works for a number of different insect pests and plant families: set a few plants of the affected species in the old spot as a trap for the emerging pests. For example, take a few extra-early eggplants and set them out a week or so ahead of your normal schedule, right where you had potatoes the year before (not normally a good idea). Colorado potato beetles love eggplants above all else, including potatoes. As soon as they appear, they’ll find the eggplants you set there and chow down. All you have to do is stop by once a day and pick them off the plants. After you’ve had a week or two to trap the beetles, plant your main crops of potatoes and eggplants elsewhere, and your pest problems with both crops will be greatly reduced.