Studies on Partial Resistance to Rice Blast in the Tropics

Abstract

Partial resistance to blast (Magnaporthe grisea) has not been clearly distinguished from major gene resistance in the tropics. Several major genes for resistance to blast were identified recently in IR cultivars at the International Rice Research Institute (IRRI) and the reactions of these genes to the Philippine blast isolates were studied. Consequently, suitable blast isolates could be selected to eliminate the effect of these major genes for evaluating partial resistance to blast under field conditions. Seventy-two cultivars and breeding lines were inoculated with 3 blast isolates during 2 seasons in field experiments conducted in 1999 in the Philippines. Serious blast infection was induced by partial shading, continuous irrigation and the application of large amount of fertilizer. Disease damage was evaluated using the Standard Evaluation System for rice blast. Partial resistance was clearly distinguished from major gene resistance and the differences between moderate levels of resistance conferred by major genes and partial resistance were clarified. Among the materials used, IR64 showed high level of partial resistance. Partial resistance was moderate in IR60 and IR36 and low in IR50 and CO39. These results were consistent with the results of sequential planting carried out at IRRI earlier. Partial resistance levels of several Japanese cultivars at IRRI were also consistent with their evaluations in Japan. Significant positive correlations observed among partial resistance to the three isolates indicate that partial resistance is horizontal in the tropics. Distinction between major gene and partial resistance to blast is necessary for identifying donors for developing durably resistant cultivars.