Expression of Tolerance for Meloidogyne graminicola in Rice Cultivars as Affected by Soil Type and Flooding.

The effects of different water regimes on the pathogenicity of Meloidogyne graminicola on six rice cultivars were determined in two soil types in three greenhouse experiments. Two water regimes, simulating continuous flooding and intermittent flooding, were used with five of the cultivars. All cultivars were susceptible to the nematode, but IR72 and IR74 were more tolerant than IR20 and IR29 under intermittent flooding. All were tolerant under continuous flooding. UPLRi-5 was grown under multiple water regimes: no flooding; continuous flooding; flooding starting at maximum tillering, panicle initiation, or booting stage; and flooding from sowing until maximum tillering or booting. In sandy loam soil, M. graminicola reduced stem and leaf dry weight, root dry weight, and grain weight under all water regimes. In clay loam soil, the nematode reduced root weight when the soil was not flooded or flooded only for a short time, from panicle initiation, or booting to maturity, and from sowing to maximum tillering. In clay loam soil, stem and leaf dry weight, as well as grain weight, were reduced by the nematode under all water regimes except continuous flooding or when the soil was flooded from sowing to booting stage. These results indicate that rice cultivar tolerance of M. graminicola varies with water regime and that yield losses due to M. graminicola may be prevented or minimized when the rice crop is flooded early and kept flooded until a late stage of development.