GENETIC-CONTROL OF BLAST IN RELATION TO NITROGEN-FERTILIZATION IN UPLAND RICE

Six upland rice cultivars were evaluated under two nitrogen levels (10 kg and 60 kg N/ha) for blast control based on slow leaf and panicle b last characteristics in field experiments conducted during three conse cutive years. The criteria for measuring slow blasting resistance were area under disease progress curve (AUDPC), maximum severity during di sease progress (Ymax) and the rate of increase until the disease reach es maximum ((r)ymax). There was a high positive correlation among the criteria utilized. The control of leaf blast in improved rice cultivar s (Cuiabana, Centro America, Guarani, Rio Paranaiba) ranged from 36 to 56% for 10 kg/ha of N and from 19 to 49% for 60 kg/ha of N over the s usceptible checks (IAC 47, IAC 165). The effect of slow blasting resis tance on panicle blast control was less than on leaf blast in most of the cultivars. Increasing the nitrogen level from 10 to 60 kg/ha resul ted in a mean yield increase by only 12% corresponding to 258 kg/ha. T he data suggest that the existing level of slow blasting resistance in the improved rice cultivars is efficient only under low nitrogen leve l.