Crop Rotations on Organic Farms

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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
Hard-case Pathogens. Someparticularly troublesome pathogens in thesoil can be controlled by relatively shortrotations of specific plants that areunsuitable hosts for the disease. Otherpathogens require longer rotations tocontrol.An example of a disease that requires a longrotation is Granville wilt or southernbacterial wilt. In fact, it is not possible tomanage this pathogen successfully wherethe infestation level is moderate to highwithout an appropriate crop rotation.<strong>Rotations</strong> are effective against southernbacterial wilt because these bacteria do notmultiply in the soil without susceptibleplant tissue. Thus, populations decline if asuitable plant, such as the tomato, is absentfor even one year. Planting a nonhost crop,such as soybeans, fescue, corn, cotton, andsorghum, for just one year will significantlyreduce the losses to this disease in thefollowing tomato crop.Some Pathogens Require Long-term <strong>Crop</strong><strong>Rotations</strong>An example of a pathogen that requireslonger crop rotations for control isStreptomyces soil rot or pox of sweet potato. Atypical rotation plan to control this pathogenmight look like this:• Year 1. Check soil pH, apply lime ifneeded, and plant beans.• Years 2 and 3. Plant corn and small grain.• Year 4. Plant tobacco.• Year 5. Check soil pH. If at pH 5.2 orunder, solarize soil and plant sweetpotatoes. When developing a rotationprogram, sweet potatoes should notfollow a crop requiring a high soil pH.Source: Averre and Ristaino, 1991As is true with any other soilbornepathogen, the longer the rotation, the moreefficient the control. Other managementpractices also help to control this pathogen,such as drainage improvements, avoidanceof late or deep cultivation (or both), stalkand root destruction, and soil solarization(Melton and Shew, 1998).Nematode ManagementSusceptibility to parasitic nematodes, whichare common plant pests, is anotherconsideration in planning a crop rotation.This is a complex issue because manydifferent pest nematode species occur, andtheir ability to infect vegetable crops varieswith the nematode species and the crop.Generally, rotations should separate a cropsensitive to a particular nematode, say rootknot nematode, with crops that are notsensitive or easily infected by thatnematode. Nematode populations thendecline in intervening years to levels belowthresholds for economic injury tosusceptible crops. When nematodepopulations become lower than thesethresholds, sensitive crops can be replanted.Green manure crops that do not serve ashosts to problem nematodes are sometimesused as intervention crops. There is a greatdeal of ongoing research in matchingvegetable crops and green manures tosuppress adversarial nematodes. Rapeseedand mustard have shown insensitivity toinfection by a wide range of parasiticnematodes and are commonly planted byorganic farmers to “clean up” soil duringwinter months. Some cover crops have alsobeen shown to suppress nematodepopulations, such as velvetbean, sorghumsudangrass,and sunnhemp.Organic Production—<strong>Crop</strong> <strong>Rotations</strong> on Organic Farms 11
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Crop Rotations on Organic Farms

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