REGULATION OF POPULATION-DENSITIES OF HETERODERA-CAJANI AND OTHER PLANT-PARASITIC NEMATODES BY CROP ROTATIONS ON VERTISOLS IN SEMIARID TROPICAL PRODUCTION SYSTEMS IN INDIA

The significance of double crop (intercrop and sequential crop), singl e crop (rainy season crop fallow from June to September), and rotation s on densities of Heterodera cajani, Helicotylenchus retusus, and Roty lenchulus reniformis was studied on Vertisol (Typic Pellusterts) betwe en 1987 and 1993. Cowpea (Vigna sinensis), mungbean (Phaseolus aureus) , and pigeonpea (Cajanus cajan) greatly increased the population densi ties of H. cajani and suppressed the population densities of other pla nt-parasitic nematodes. Mean population densities of H, cajani were ab out 8 times lower in single crop systems than in double crop systems, with pigeonpea as a component intercrop. Plots planted to sorghum, saf flower, and chickpea in the preceding year contained fewer H. cajani e ggs and juveniles than did plots previously planted to pigeonpea, cowp ea, or mungbean. Continuous cropping of sorghum in the rainy season an d safflower in the post rainy season markedly reduced the population d ensity of H. cajani. Sorghum, safflower, and chickpea favored increase d population densities of H. retusus. Adding cowpea to the system resu lted in a significant increase in the densities of R. reniformis. Mean densities of total plant-parasitic nematodes were three times greater in double crop systems, with pigeonpea as a component intercrop than in single crop systems with rainy season fallow component. Cropping sy stems had a regulatory effect on the nematode populations and could be an effective nematode management tactic. Intercropping of sorghum wit h H. cajani tolerant pigeonpea could be effective in increasing the pr oductivity of traditional production systems in H. cajani infested reg ions.