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The rice root knot nematode: Its distribution and impact on rice-wheat system.

Rice root knot nematode (Meloidogyne graminicola Golden and Birchfield 1965) is a new pathogen of rice in Nepal and whose information is lacking. Field survey was conducted to understand the population densities of M. graminicola in rice with diseased and healthy looking rice plants and its impact on yield attributing characters of rice. The survey revealed that the diseased root samples had the highest frequency (88), absolute frequency (88%), relative frequency (88.9%), absolute density (393.9 g-1) and prominence value (3695.7). However, root and soil population and root knot indices were not different between diseased and healthy samples. High index of root lesion in diseased samples might be due to secondary infection of other soil borne pathogens. Reduced total and effective tillers number, filled grain panicle-1, and yield were found in diseased rice. Yield reduction in diseased rice variety, 'Masuli' was 40.5% less as compared to healthy looking. M. graminicola seems to be a well established pest of sandy loam to loamy sand soil. Greenhouse experiments were also conducted to understand the impact of different level of inoculums of M. graminicola in rice variety 'Masuli’ and its development. Initial inoculum (Pi) of 0, 0.1, 0.2, 0.4, 1, 2, 5 and 10  M. graminicola juvenile second stage (J2) were inoculated in soil. Linear relationship was found between the Pi J2 levels and J2 in soil at 15 days sampling intervals. Final (150 days after inoculation) soil and root J2 population egg density and root knot index differed significantly with Pi levels. Reproduction rates (Rr) decreased with increased Pi levels. Root length and grain yield were also decreased. The highest yield reduction of 97% was found with 10 J2 cc-1 of soil. The relationship between Pi J2 levels and grain yield was described by Y=0.1+(0.9) (0.3pi) (r2=94**). M. graminicola eggs levels of 0, 0.2, 1, 2, 4 and 12 cc-1 of soil had decreased Rr values with increased levels of Pi eggs. Pi eggs level had effect on final (150 days after inoculation) root J2, egg population and grain yield. Yield reduction ranged from 46-26% as compared to control. Increasing Pi levels of M. graminicola resulted in decreasing the yield and yield attributing characters in pot experiment, which needs more varification for field level implication. Evaluation of twelve common Nepalese rice varieties against M. graminicola in greenhouse experiment indicated that 'Masuli' and 'Chaite-6' were moderately and slightly resistant, respectively. Other cultivars were susceptible. Results also showed that 'Janaki' can be used as susceptible checks instead of 'IR-29' for screening of Nepalese cultivars. Most of the tested cultivars commonly grown by Nepalese farmers were susceptible to M. graminicola. Therefore, it is necessary to explore other resistant source of rice against M. graminicola. In pot experiment, wheat varieties 'UP262' was infested by M. graminicola that indicated cubic relationships with soil, root population and reproduction rates of M. graminicola at growth stages of wheat. Root knot symptom was not observed at any growth stage of wheat. M. graminicola reduced the root length and grain yield (42%). Thus, considering the impact of M. graminicola in rice and wheat, it was concluded that nematode in rice and wheat system, could be a serious one.

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