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Ecology and Epidemiology

Estimated Distances for Infection of Wheat Roots by Gaeumannomyces graminis var. tritici in Soils Suppressive and Conducive to Take-All. H. T. Wilkinson, Postdoctoral research associate, Department of Plant Pathology, Pullman, WA 99164, Assistant professor, Department of Plant Pathology, University of Illinois, Urbana 61801; J. R. Alldredge(2), and R. J. Cook(3). (2)Assistant professor, Statistical Services, Washington State University, Pullman, WA 99164; (3)Research plant pathologist, U.S. Department of Agriculture, Agricultural Research Service, Pullman, WA 99164. Phytopathology 75:557-559. Accepted for publication 27 November 1984. 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, 1985. DOI: 10.1094/Phyto-75-557.

A Shano silt loam (SSL) cropped consecutively to irrigated wheat for 22 yr (suppressive to take-all of wheat caused by Gaeumannomyces graminis var. tritici) and a Ritzville silt loam (RSL) never cropped to wheat (conducive to take-all) were compared to determine if suppressiveness is expressed as a stricture on the distance an inoculum source can be from the root and still cause infection. Data for the number of infections per plant produced by three sizes of inoculum particles (0.25- 5.0, 5.0- 1.0, and 1.0- 2.0 mm in diameter), each at nine particle densities (0.1 to 10.0 mg/g soil), were used in a statistical model to obtain estimated mean distance for infection (EDI). For any given particle size, the mean EDI value was about the same in SSL and RSL even though these soils are suppressive and conducive, respectively, to take-all. The EDI values were about 1.7 mm in both soils with inoculum particles 0.25- 0.50 mm and 2.8 and 3.9 mm in diameter in SSL and RSL, respectively, with particles 0.50- 1.0 mm. Following an adjustment for multiple infections per particle, the EDI values for particles 1.0- 2.0 mm were estimated at 11.3 and 10.8 mm in SSL and RSL, respectively. Fumigation (methyl bromide) and treatment with moist heat (60 C for 30 min) resulted in significantly greater EDI values for the two smaller particle sizes in both soils, but not for the larger (1.0- 2.0 mm) particles. The distance the fungus can grow from an inoculum source to a host root is thus affected by the associated microbiota, to which the fungus is most vulnerable when its food base is near the minimum threshold size. However, this form of antagonism was independent of the antagonism responsible for take-all decline.