Crop Rotations on Organic Farms

families might include Brassicaceae (colecrops), followed by Asteraceae (lettuce, cutflowers), followed by Solanaceae (tomatoes,potatoes, peppers, eggplants), followed byCurbitaceae (squashes, cucumbers, melons).Length of Rotation. The length of timesoilborne pathogens remain viable in thefield is critical to any decision about thelength of the rotation before replanting thesame vegetable crop. Although there aresome exceptions, a four-year rotation thatincludes a succession of crops not susceptibleto the same pathogens will generallyminimize problems from soilborne diseases.For this strategy to be effective, however, itis important that pathogen-susceptibleweeds and volunteer plants be excludedfrom the field, too.Table 2. Rotation periods to reduce soilbornediseasesYears without aVegetable Disease Susceptible <strong>Crop</strong>Asparagus Fusarium rot 8Cabbage Clubroot 7Cabbage Blackleg 3 - 4Cabbage Black rot 2 - 3Muskmelon Fusarium wilt 5Parsnip Root canker 2Peas Root rots 3 - 4Peas Fusarium wilt 5Pumpkin Black rot 2Radish Clubroot 7Source: S.A. Johnson & P.J. Nitzche, USDASome exceptions to this rule include clubroot of crucifers, sclerotinia white mold onlettuce, and fusarium wilt (a diseaseaffecting many vegetables). Long rotationsof four years and more are desirable foravoiding these diseases. Where there is ahistory of problems with long-livedpathogens, these practices have provenbeneficial:• soil solarization (covering a field areawith clear plastic so the sun can raisesoil temperatures enough to destroypathogens),• compost additions,• use of resistant varieties, and• long rotations.As a general rule, a two-year rotation willreduce the incidence of foliar (leaf) diseasesbecause a primary source of inoculum forinfection is often the infected tissue fromprevious crops. Generally, that tissue will bewell-decomposed in two years, and anyinoculum will disappear along with cropresidues. For example, most of theinoculum for early blight in tomatoes,cercospora of cucurbits, and most foliarbacterial diseases can be eliminated bydestruction and incorporation of residuesand a one-year waiting period beforereplanting.Tillage Practices. Various tillagepractices can make an impact on diseasecontrol or crop rotations. For instance,southern blight pathogen survival from oneyear to the next is generally restricted tothe upper 2 or 3 inches of soil, and burialbelow these depths is an effective diseasecontrol strategy. Farmers must also considerhow different types of tillage systems caninfluence a rotation. For example, ifmanagement plans call for a period of notill,incorporation of crop residues will bedelayed and the selection of crops for arotation in this instance may be morelimited. Tillage practices that enhance soildrainage generally reduce the incidence ofseedling diseases. Including rotational cropsthat are planted on high ridges or in plantbeds in the rotation often reduces theincidence of damping-off disease.Organic Production—<strong>Crop</strong> <strong>Rotations</strong> on Organic Farms 10