Greenhouse evaluation of rice and wheat germplasm for resistance to Meloidogyne graminicola with comments on evaluation indices and proposal of a new one

Abstract

The root-knot nematode (Meloidogyne graminicola) is a major constraint to the productivity of rice-wheat production systems in South-East Asia. The use of host resistance is the best possible solution to this problem. However, information on host resistance in rice and wheat to this nematode is limited. Thus, 96 rice cultivars and 59 selected rice entries, with known sources of resistance to other stresses, were evaluated for resistance to Nepali isolates of the nematode. Similarly, the reaction to infection by M. graminicola of 74 wheat cultivars and promising breeding lines was also assessed. Ten seeds of rice or wheat were sown per 10-cm clay pot (replicated four times), filled with pasteurized soil (30 minutes at 60 °C) and inoculated with 10 eggs of M. graminicola/cm3 soil. The pots were maintained in a greenhouse at 25 ± 3 °C, irrigated daily, and fertilized once a week. After 60 days, the roots were washed free of soil and root-galling severity (RGS) was rated on a scale of 1 (no visible galls, healthy roots) to 9 (>75% of roots galled). Eggs were then extracted from roots by blending them in 1% sodium hypochlorite solution for 3.5 minutes and sieving the suspension through a 100-mesh sieve nested over a 500-mesh sieve. The suspension from the 500-mesh sieve was collected, nematode eggs and juveniles counted, and the reproductive factor (RF) calculated by dividing total number of eggs and juveniles by total inoculum used (Pf/Pi). The RGS ratings and RF values of the nematode were converted into a new index, the reaction index (RI), derived from the RG ratings and RF values. This reaction index was used to separate germplasm lines or cultivars into immune, highly resistant, resistant, intermediate, susceptible and highly susceptible categories. The results suggested that all the commercial rice and wheat cultivars obtained from Nepal, Bangladesh, and international centres (IRRI and CIMMYT) were susceptible to M. graminicola isolates from Nepal. Rice germplasm sources having resistance genes to other pests, diseases, and physiological stresses exhibited greater variability in their reaction as compared to commercial cultivars.