Cropping Effects of Marigolds, Corn, and Okra on Population Levels of Meloidogyne javanica and on Carrot Yields.

Root-knot nematodes, particularly Meloidogynejavanica (Treub.) Chitwood, 1949 and M. incognita (Kofoid and White) Chitwood, 1949, are among the most important pathogens of carrots (Daucus carota L.) in Brazil. They damage tap roots by galling, digitation, and constriction, resulting in lower market value (3). Even low soil densities of root-knot nematodes have been reported to reduce marketable root yields (3,11). Root-knot nematode control is important to increase the value of carrot production. Crop rotation is an effective method to control M. incognita (8). Huang et al. (4) controlled the nematodes by growing Crotalaria spectabilis Roth for 8 months before planting carrot. Another cover crop used in rotation are marigolds (Tagetes spp.) (7,10) which contain a substance toxic to nematodes (9). Corn, a poor host to rootknot nematodes (1), is also used in rotation to suppress nematode populations (2,8). Because of the high economic value, corn would be valuable to use in rotation with carrots. The effect of marigold and corn on suppressing M. javanica populations in soil, when used in rotation with carrot, and subsequent carrot yield were studied. Okra, highly susceptible to M. javanica, served as a control crop in this study. Experimental plots were established at the National Vegetable Research Cen te r / EMBRAPA, Brasilia, D.F., in soil (12.3% sand, 27.1% silt, 60.6% clay, and pH 4.9) infested with M. javanica. Plots (each 4 x 5 m 2) were arranged in a randomized com-