Parasitic nematode populations in natural fallows and improved cover crops and their effects on subsequent crops in Kenya

Abstract

A field study was conducted at two sites in western Kenya during 1995–1997, to evaluate the effects of 6- and 12-month sesbania ( Sesbania sesban) ‘cover crops’ in comparison with 6- and 12-month natural ‘weed fallows’, a 6-month crotalaria ( Crotalaria agatiflora) cover crop and continuous maize on the dynamics of parasitic nematode populations and their effects on subsequent maize and bean crops. Following 12-month sesbania, the plots were split for nematicide treatment during the cropping phase. Infestations of the root-knot ( Meloidogyne spp.), root-lesion ( Pratylenchus zeae), reniform ( Rotylenchulus variabilis) and spiral ( Helicotylenchus spp. and Scutellonema spp.) nematodes were monitored. Although a number of plant species in natural fallows were hosts to both M. incognita and M. javanica, even natural fallows 12 months long did not increase populations of these nematodes to cause any significant yield reduction in the subsequent nematode-susceptible bean. Crotalaria cover crop and continuous maize decreased M. incognita and M. javanica populations but increased P. zeae populations to levels that could limit their growth. Only the 12-month sesbania cover crop increased Meloidogyne populations greatly in soil and roots at the site of light-textured soil. Sesbania itself appeared to be fairly tolerant to the nematode over a 12-month growing period. Maize was not damaged by the root-knot nematodes after sesbania, but bean yield reduced by 52–87% following 12-month sesbania. A one-season interval with maize after sesbania cover was adequate to limit root-knot nematode damage on susceptible bean. Crotalaria can be considered as an alternative cover crop to sesbania where crops susceptible to root-knot nematode are an integral part of the cropping system after fallows. However, there is a need to screen Crotalaria species to identify species that produce large amounts of biomass without increasing Pratylenchus population. Neither cover crops nor natural fallows had any influence on reniform and spiral nematodes, and none of these nematodes affected subsequent crops.