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Assessment of Blast Disease and Yield Loss in Susceptible and Partially Resistant Rice Cultivars in Two Irrigated Lowland Environments. J. M. Bonman, Plant Pathologist, International Rice Research Institute, P.O. Box 933, Manila, Philippines. B. A. Estrada, C. K. Kim, D. S. Ra, and E. J. Lee. Assistant Scientist, International Rice Research Institute, P.O. Box 933, Manila, Philippines; Plant Pathologists, Plant Pathologist and Head, Department of Plant Pathology, Agricultural Sciences Institute, Rural Development Administration, Suweon 440-707, Korea. Plant Dis. 75:462-466. Accepted for publication 17 October 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0462.

The importance of blast disease, caused by Pyricularia oryzae, varies with environment, and irrigated lowland rice in Korea is more prone to the disease than irrigated lowland rice in the Philippines. Rice cultivars with partial resistance can reduce losses from blast. Fungicide-protected and inoculated field plots were used to assess the effectiveness of locally adapted partially resistant cultivars in reducing leaf blast, neck blast, and yield losses. Over six trials conducted from 1987 to 1989, yield loss was positively correlated with the incidence of severe neck blast (r2 = 0.95). The incidence of neck blast was low and yield losses negligible in partially resistant cultivars IR36 in the Philippines and Seomjin in Korea. Philippine cultivar IR66, which has partial resistance to neck blast but not to leaf blast, also showed a low incidence of neck blast and low yield losses. Partially resistant cultivar Bonggwang showed intermediate neck blast incidence of 16.2% and yield loss of 31.2% in Korea. The maximum losses measured in the susceptible checks were 20.9% in IR50 in the Philippines and 50.2% in Daechang in Korea. Under the less conducive irrigated lowland conditions in the Philippines, partial resistance alone can be used to manage blast disease. Under the highly conducive conditions in Korea, only cultivars with high levels of partial resistance will be effective alone, and cultivars with lower resistance may require the use of additional cultural and chemical control tactics.