Nematode numbers and crop yield in a fenamiphos-treated sweet corn-sweet potato-vetch cropping system.

Nematode population densities and yield of sweet corn and sweet potato as affected by the nematicide fenamiphos, in a sweet corn-sweet potato-vetch cropping system, were determined in a 5-year test (1981-85). Sweet potato was the best host of Meloidogyne incognita of these three crops. Fenamiphos 15G (6.7 kg a.i./ha) incorporated broadcast in the top 15 cm of the soil layer before planting of each crop increased (P </= 0.05) yields of sweet corn in 1981 and 1982 and sweet potato number 1 grade in 1982 and 1983. Yield of sweet corn and numbers of M. incognita second-stage juveniles (J2) in the soil each month were negatively correlated from planting (r = - 0.47) to harvest (r = -0.61) in 1982. Yield of number 1 sweet potato was inversely related to numbers of J2 in the soil in July-October 1982 and July-September 1983. Yield of cracked storage roots was positively related to the numbers of J2 in the soil on one or more sampling dates in all years except 1985. Some factor(s), such as microbial degradation, resistant M. incognita development, or environment, reduced the effect of fenamiphos.