Estimated Distances for Infection of Wheat Roots byGaeumannomyces graminisvar.triticiin Soils Suppressive and Conducive to Take-All

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Abstract
A Shano silt loam (SSL) cropped consecutively to irrigated wheat for 22 yr (suppressive to take-all of wheat caused by G. 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 3 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.degree. C for 30 min) resulted in significantly greater EDI values for the 2 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.