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Factors in Loss of Pathogenicity in Gaeumannomyces graminis var. tritici. T. Naiki, Research plant pathologist, Gifu University, Gifu-Shi 501-11, Japan; R. J. Cook, research plant pathologist, Agricultural Research Service, U.S. Department of Agriculture, Pullman, WA 99164. Phytopathology 73:1652-1656. Accepted for publication 12 July 1983. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1983. DOI: 10.1094/Phyto-73-1652.

Fewer than half of 111 pathogenic cultures of Gaeumannomyces graminis var. tritici were still pathogenic after maintenance for 9 mo on dilute (one-fifth strength) homemade potato-dextrose agar. Cultures lost ability to cause disease at about the same rate whether kept continuously growing (transferred every 10 days) at 24 C or stored (without transfer) at 12 or 24 C. Cultures were more likely to remain pathogenic if passed through the host (reisolated from infected roots of inoculated seedlings) at approximately 1-mo intervals beginning as soon as possible after the fungus was isolated from nature. Monoascosporic cultures also lost ability to produce disease during maintenance in culture, but less often than did their parents. The proportion of monoascosporic cultures rated as pathogenic was 36/38 for those obtained from three pathogenic parents but only 106/198 for those obtained from the same three parents after the parents had lost ability to produce disease. Results were similar for cultures started from single cells from fragmented hyphae; 35/50 of single-celled cultures from pathogenic parents, but only 2/150 of those from nonpathogenic parents, were rated pathogenic. The results are evidence against mycovirus infections being responsible for loss of pathogenicity and suggest that the fungus either is commonly heterokaryotic, with a low frequency of nuclei lacking genes for virulence and/or carrying genes that can suppress expression of virulence, or has a cytoplasmic determinant (plasmid) that affects expression of virulence. Possibly, selection pressure during maintenance in agar culture favors a shift in genetic determinants toward inability to cause disease.