According to the Pennsylvania State University Extension, it is becoming increasingly difficult to combat plant diseases in the United States. One of the tried-and-true methods for reducing the populations of soil-borne diseases such as root rot is crop rotation. While crop rotation may not wipe out these pathogens altogether, it can certainly reduce plant mortality and subsequent spread of the diseases.
Most root rot diseases are fungal, such as Phytophthora, Pythium, Rhizoctonia and Fusarium. This means they prefer damp, dark conditions with temperatures in the range most suitable to their plant hosts. Damage to plants or roots, excessive rain or irrigation, and soils that drain poorly are prime conditions for disease problems, no matter the pathogen.
If a plant that is susceptible to a particular disease is planted in the same spot year after year, the concentration of the pathogen will only rise. Crop rotation means planting a crop that does not have a susceptibility to the disease in the area to allow the pathogen population to decline. Essentially, you are starving the organisms. Once the populations are low enough, the chances of a healthy plant becoming infested with the pathogen are severely limited.
Many root rot diseases can survive in moist soils for several years. Alternating crops that require high soil moisture with those that prefer dry spells can also help, but only if most of the irrigation can be controlled by the gardener. The longer you can rotate the crops, and the more variety of plants you introduce to the area, the better your chances of protecting your crops from disease.
It can be tricky to decide which crops you can plant to ensure that you're not planting a host for the root rot diseases from your previous crop. Start by finding out which root rot diseases your current crop is susceptible to, then compare that with the plants you'd like to rotate into the area. Your local agricultural extension service can usually provide such lists. Avoid planting two crops in a row that are susceptible to the same disease.
Crop rotation is used for controlling a wide variety of pests, including insects, other plant diseases, and even birds and mammals. The added bonus to reducing the threat from these pests is that a pest-free plant can withstand attacks from soil-borne pathogens much better. Crop rotation can also be used to plant cover crops that add nutrients to the soil that were lost from your last crop, thereby contributing even more to the health of your plants.