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Monthly Changes of Pathogenic Races of Pyricularia Oryzae in a Blast Nursery. Md. Quamaruzzaman, Former Research Scholar (now Research Assistant, Department of Mycology and Plant Pathology, East Pakistan Agricultural Rice Research Institute, Dacca-15, East Pakistan) The International Rice Research Institute, Los Baños, Laguna, The Philippines; S. H. Ou, Plant Pathologist, The International Rice Research Institute, Los Baños, Laguna, The Philippines. Phytopathology 60:1266-1269. Accepted for publication 22 March 1970. DOI: 10.1094/Phyto-60-1266.

A total of 363 monoconidial cultures of Pyricularia oryzae were isolated from diseased leaf samples collected monthly for 21 consecutive months from the blast nursery at The International Rice Research Institute (IRRI). These isolates, when inoculated individually on the 8 international and the 12 Philippine differential rice cultivars, were differentiated into 37 and 60 pathogenic races, respectively. The number, composition, and frequency of races varied greatly between months as determined by the isolates. A few prevalent races appeared in most of the months, some intermittently, and others only once during the 21-month period. The qualitative and quantitative changes of the races which occur in a blast nursery suggest the need for repeated tests to screen cultivars resistant to blast and explain why some cultivars resistant in the early stages of growth become susceptible at the flowering stage in fields.