Population densities of rice root nematode (Hirschmanniella spp.) in long-term fertility experiments in Nepal

Abstract

Long-term soil fertility experiments at Bhairahawa (rice-wheat-rice), Tarahara (rice-wheat), and Rampur (rice-wheat-maize) were sampled to assess the densities of rice root nematodes Hirschmanniella spp. Nematode densities in roots and soil samples were determined by the blender-cum-modified Baermann trays and sieving-cum-modified Baermann trays techniques, respectively. The rice root nematodes, Hirschmanniella ol"yzae and H. mucronata, were common and observed in most of the rice samples collected, although the population varied greatly among the samples. In wheat crops, rice root nematodes were observed at low densities only in the soil samples, not in roots. Significantly higher numbers of nematodes were observed in the plots receiving Nand P fertilizers as compared to those receiving N alone. Potassium fertilizer, up to 50 kg/ha, had no effect on rice root nematode populations, but significantly lower nematode populations were observed in plots fertilized with 100 kg/ha potassium as compared to the control. Also, significantly lower nematode populations and higher rice yields were observed in fields where farmyard manure was applied annually for 16-17 years. Incorporation of rice stubble and Dhaicha before flowering had no effect on nematodes.