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Relationship Between Production of a Self-Inhibitor and Inability of Gaeumannomyces graminis var. tritici to Cause Take-all. 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 73:1657-1660. 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-73-1657.

Cultures of Gaeumannomyces graminis var. tritici that were rated weakly pathogenic or nonpathogenic on wheat roots in a vermiculite rooting medium also showed a zone of inhibition when grown opposite to and in the same dish with a common (tester) culture of the same fungus on Difco potato-dextrose agar (PDA) at pH 4.0. Cultures rated as pathogenic showed little or no zone of inhibition when paired with the tester culture on the acidified PDA. Noninhibitory cultures grew 3.0 mm/day at 24 C on PDA at pH 4.0, whereas inhibitory cultures grew only 0.7 mm/day on the same medium, apparently because their own growth was inhibited (self-inhibition). Inhibitory and noninhibitory cultures both grew at 5.5- 6.0 mm/day on PDA at pH 7.0, where little or no inhibition occurred. The correlation coefficient between amount of disease produced and size of zone of inhibition against the tester culture of G. graminis var. tritici at pH 4.0 was - 0.75, - 0.87, and - 0.78, respectively, for three sets of 38 isolates after 9 mo of storage in culture (on unbuffered homemade PDA) and - 0.68 for 118 monoascosporic cultures after 9 mo of storage on unbuffered PDA. Agar disks from the advancing margin of an inhibitory isolate grown on unbuffered homemade PDA at pH 5.5- 6.0 (typical of disks used as a source of inoculum in pathogenicity tests) also were inhibitory. The tendency of cultures of G. graminis var. tritici to produce less disease after a period in culture on agar media may result, in part, from an increasing potential of the culture to produce inhibitor suppressive to its own growth. Ability to produce some inhibitor may be advantageous to the fungus in nature, but the tendency may be toward greater production (self-inhibition) after a period in culture.