Cover Crops for Organic Farms

For instance, some green manure or covercrops placed in a rotation can reducedamage by one nematode species but notothers. In a study in Florida, the warmseasonlegumes, which included pigeonpea,crotalaria, hairy indigo, velvetbean, andjoint vetch, reduced root-knot nematodedamage in a subsequent snapbean cropwhen the crop was compared to oneproduced in fallow. These same cover crops,however, were no more effective thanfallow in reducing damage from sting(Belonolaimus longicaudatus) and lesion(Pratylenchus brachyurus) nematodes.In some cases, a cover crop can reducepopulations of one parasitic nematode butserve as a host that increases populations ofother nematodes. While two researchers(McSorley and Gallaher) reported in 1991that sorghum-sudangrass cover cropsreduced levels of root- knot nematodes,Rhoades and Forbes (1986) found that asorghum-sudangrass cover crop increasedpopulations of B. longicaudatus and M.incognita nematodes. Farmers attempting touse crop rotations for controlling onenematode species must be aware that theserotations could benefit other damagingnematodes present in the field (McSorleyand Dickson, 1995). Potential rotationcrops should be evaluated for their effectson as many different damaging nematodesas possible.Cover Crop TipOrganic growers commonly plant rapeseed,mustard, and other brassicas as rotation cropsto “clean-up” soil during winter months.These plants have been shown to suppress awide range of parasitic nematodes.(Bending and Lincoln, 1999)Insect ManagementCover crops can be both a blessing and adrawback because they attract bothbeneficial and harmful insects to farm fields(Altieri and Letourneau, 1982; Andow,1988). When a cover crop matures or dies,both the beneficial and pest insects maymove to cash crops. The resulting effect oninsect pest populations on the farm (aneffect that also depends on severalenvironmental factors) can presentfrustrating dilemmas for a farmer. Forexample, in a study in 1991, researchersfound that a rye cover crop helped toreduce fruitworm populations in thetomato crop that followed it. But the ryecover also led to increased stinkbug damage(Roberts and Cartwright, 1991).To create the best situation, a farmer growsa cover crop to attract beneficial insectsbefore the damaging insects arrive. Thebeneficial insects are attracted by themoisture, shelter, pollen, honeydew, nectar,and potential insect prey associated withthe cover crop. These beneficial insectssubsist in the cover crop and then moveinto the vegetable crop to attack arrivingpest insects. Several studies show that thisapproach is often successful. Researchers inGeorgia reported high densities of big-eyedbugs, lady beetles, and other beneficialinsects in vetches and clovers that movedinto ensuing tomato crops (Bugg et al.,1990). In a more recent study, a researcherreported that assassin bugs destroyedColorado potato beetles feeding oneggplant that had been planted into striptilledcrimson clover (Phatak, 1998).Organic Production—Cover Crops for Organic Farms 6