Dynamics of nematode communities as influenced by soil fertility management practices and biocontrol agents

Abstract

Two field experiments were conducted to investigate the effect of soil fertility management practices and bio-control agents on the dynamics of nematode communities in Kakamega forest and the neighboring farmlands. Efficacy of a bio-control agent, Bacillus subtilis was tested alongside Rhizobium leguminosarum biovarphaseoli strain USDA 2674 for nodule formation in common bean (Phaseolus vulgaris L.var Rosecoco). Three Bacillus subtilis strains (KlS8, K194 and K263), singularly or in combination with Rhizobium leguminosarum biovar phaseoli were tested in beans and soil fertility management practices which included inorganic N: P: K (75: 26: 46) kgha', farm yard manure (5 tons ha'), PRE-PAC (800 kg P ha') and (80 kg N ha') were investigated in maize. The experiments were established on four farms representing age sequences since conversion from forest of 1-10. 10-20, 20-40 and >40 years. Bacillus subtilis strain K 194 in combination with Rhizobium treatment on beans and PRE-PAC on _maize reduced the populations of root-knot nematodes leading to an increase in bean yields from 150 to 560 kg ha-I . Diversity index analysis showed that application of PRE-PAC reduced nematode numbers compared to farmyard manure and inorganic fertilizers. Inoculating beans with Rhizobium Leguminosarum biovar phaseoli strains USDA 2674 enhanced nodulation and biomass production. Continued land conversion leads to loss of soil fertility and land degradation that ultimately results in loss of nematode biodiversity. Use of PRE-PAC resulted in modification of soil fauna environment that led to reduction in nematode numbers. For instance, the population of Pratylenchus sp. reduced by over 30 %. On the other hand, use of inorganic fertilizers released N that promoted multiplication of nematodes as observed in this study where Pratylenchus sp. population increased by over 40 %. This study further demonstrated that Bacillus spp. is a viable component of integrated nematode management packages. Potential Bacillus subtilis strains as biocontrol agent for root-knot nematode. Meloidogyne spp. as well as growth promoting agent in beans was demonstrated in the field. Strain Kl94 gave very consistent trends over three seasons where the population of Meloidogyne sp. was reduced by over 60 %. Success of growing Bacillus sp. and Rhizobia spp. in one medium for production of a bio-inoculant was demonstrated and has been packaged. Suitability of the nematode diversity as bioindicators of land use change/ intensification gradient was demonstrated. As nematode populations change during the growth of a crop, it is desirable, to standardize sampling on a stage of crop growth, this is often the seedbed or immediately after harvest. Nematode populations increased from planting time. peaking at bean flowering then reduced at harvesting time. High nematode populations were observed in the long rains than the short rains in plots planted with maize. This information indicates that priority ought to be given to plant parasitic nematodes in the long rains when designing pest management programmes in cereals. The significant interactions among soil fertility management practices, time of nematodes sampling and farm age cluster suggest that the populations of soil nematodes is influenced by fertility level, time of sampling and land conversion periods. It is recommended that, soil P and N levels be addressed as direct influence on plant parasitic nematodes. Use of organics, where available, should be recommended to the farmers.