STABILITY OF PARTIAL AND COMPLETE RESISTANCE IN RICE TO PYRICULARIA-GRISEA UNDER RAIN-FED UPLAND CONDITIONS IN EASTERN COLOMBIA

Two sets of experiments were conducted to compare the relative stabili ty of partial and complete resistance in rice to Pyricularia grisea (c ausal agent of the rice blast disease) in a disease ''hot spot'' in ea stern Colombia. In the first set of experiments, 186 breeding lines se lected at the site over the F-2 to F-4 generations, ranging from compl etely to partially resistant in the F-4, were evaluated under heavy di sease pressure at the site every season from 1987 to 1990 (two seasons per year). A set of 92 lines from the International Rice Blast Nurser y (IRBN) with initial evaluations of ''resistant'' (standard evaluatio n system [SES] less than or equal to 5), also ranging from completely to partially resistant, were evaluated under the same conditions. Loca lly developed materials were rated as susceptible less frequently than lines from the IRBN set. In both sets, significantly fewer lines init ially scored as highly resistant (SES = 1 to 3) were later scored as s usceptible (SES > 5) or showed any increase in susceptibility to local pathogen populations compared to those initially rated as partially r esistant (SES = 4 to 5). In the second set of experiments, locally dev eloped and exotic lines were grouped according to partial (few suscept ible-type lesions [SES type 5]) and high resistance (few resistant typ e lesions ISES type 1 to 3]). After three seasons of evaluation, lines with high levels of resistance had a significantly lower rate of chan ge to susceptible levels. The rice cultivar Oryzica Llanos 5 was selec ted at this site from a line with complete blast resistance and was re leased for commercial cultivation in blast-prone eastern Colombia. It is grown on approximately 5 x 10(4) ha/year and has remained resistant to blast over 10 consecutive growing seasons, both in experimental bl ast nurseries and in farmers' fields where blast normally is a severe problem. This cultivar also showed uniformly high levels of blast resi stance when evaluated at seven high blast level sites in four Asian co untries in blast nurseries with 72 other cultivars with differing leve ls and origins of blast resistance. The results suggest that selecting high levels of resistance when diverse sources are combined can be us ed to develop cultivars with stable resistance to rice blast.