Damage of root-knot nematode (Meloidogyne graminicola) to rice in fields with different soil types


  • R. R. Pokharel


The rice root-knot nematode (Meloidogyne graminicola) is an important pathogen of rice in Nepal. Surveys were conducted in 65 rice fields in the Rupandehi, Chitwan and Parsa districts of Nepal by collecting rice soil and root samples from 25, 20 and 20 grower fields, respectively, in 1999-2001. Soil bioassays using these field soils were done in 1999 and 2000 to understand the factors affecting variability in population densities of the rice root-knot nematode, severity of symptoms and crop yields. Rice fields with light soils and low yields had larger nematode population densities. Similarly, larger nematode population densities were recovered from symptomatic fields/symptomatic plants (chlorotic plants in patches with reduced growth and reduced tiller numbers) as compared to asymptomatic plants/fields. Higher nematode populations were observed with severe symptomatic plants. In the surveys and the screen-house tests, all rice cultivars tested were susceptible to the nematode but acted differentially. Soil physical and chemical properties from these fields were analyzed and correlated with nematode population densities using different models. A positive relationship with sand and phosphorus, a negative relationship with nitrogen, no relationship with silt and pH and a variable relationship with clay (negative to positive relationship) to nematode population densities were observed in the absence of other predictors and interaction terms in the models. However, when these predictor variables were considered in the presence of other predictor variables, with or without interactions, the relationships were different. In addition, volunteer rice and seedling rice were also sampled from different parts in 1999 to understand their role in nematode survival and dispersal, respectively. Volunteer plants and rice seedlings supported nematode survival, growth, and multiplication in the absence of a rice crop in the field, so helping to increase the severity of the nematode problem. The effect of the rice root-knot nematode populations on rice yield reduction was studied under the influence of nitrogen plus phosphorus and compost in micro-plots. The results indicated that nematode-induced rice yield reduction was low when plots were supplied with nitrogen and phosphorus as compared to control plots (no fertilizer or compost).