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A Type of Stable Resistance to Blast Disease of Rice. S. H. Ou, Plant Pathologist, The International Rice Research Institute, Los Baños, Laguna, The Philippines; F. L. Nuque(2), T. T. Ebron(3), and V. A. Awoderu(4). (2)(3)(4)Assistant Plant Pathologist, Research Assistant, and former Research Fellow, respectively, The International Rice Research Institute, Los Baños, Laguna, The Philippines, (4)Present address: Federal Department of Agricultural Research, Plant Pathology Division, P.M.B. 5042, Moor Plantation, Ibadan, Nigeria. Phytopathology 61:703-706. Accepted for publication 22 January 1971. DOI: 10.1094/Phyto-61-703.

Pyricularia oryzae isolated from a few lesions appearing on certain resistant cultivars under epiphytotic conditions consistently produced only a few lesions when the isolates were inoculated back to the same cultivars. Pathogenicity tests on single conidial subcultures derived from these isolates showed that they consisted of many races. From one isolate, 43 races were identified from 189 single conidial subcultures. From other isolates, 28 races from 160 subcultures, 19 races from 52 subcultures, and eight races from 45 subcultures were found. Most of these races were nonpathogenic to the cultivars from which they were isolated. Among the subcultures belonging to the same pathogenic races, many also did not infect the resistant cultivars while others produced only a few lesions. The resistance shown by these cultivars against the pathogenic isolates is the result of the constant change of the fungus into many races and the broad spectrum of resistance in the cultivars against most of the new races. The phenomenon seems to promise more stable resistance to blast.

Additional keywords: variability of Pyricularia oryzae, varietal resistance to rice blast.