Evidence that Microorganisms in Suppressive Soil Associated with Wheat Take-all Decline Do Not Limit the Number of Lesions Produced byGaeumannomyces graminisvar.tritici

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Abstract
A Shano silt loam from a field cropped consecutively for 22 yr to irrigated wheat [Triticum aestivum] and highly suppressive to wheat take-all caused by Gaeumannomyces graminis var. tritici allowed about the same number of root lesions per unit particle size and concentration of inoculum (fragmented platn debris colonized by the pathogen) as did a noncropped (virgin) Ritzville silt loam highly conducive to take-all. Infection efficiencies (the number of lesions produced per unit weight of colonized particles) and threshold particle sizes (minimum particle size required for maximum efficiency) were similar in the two soils for three inoculum sources (naturally colonized wheat crowns from the field, wheat roots infected in the greenhouse, and axenically colonized oat grains) compared at up to six particle sizes (1.0-2.0 mm, 0.5-1.0 mm, 0.25-0.50 mm, 0.15-0.25 mm, 0.10-0.15 mm, and < 0.10 mm) and ten concentrations (10, 5, 2.5, 1.5, 1.0, 0.5, 0.25, 0.15, 0.1, and 0.01 mg/g of soil), in all combinations. The infection efficiency of the axenically colonized oat-grain inoculum was greater in pasteurized (60 C moist heat treatment for 30 min) than in untreated soil, but this greater efficiency occurred with both soils in response to pasteurization. Infection efficiencies for a given inoculum particle size in pasteurized soils were greatest for colonized oat grains, least for colonized roots, and intermediate for colonized crowns. Differences in nutrients available in the inoculum particle and in the rhizosphere for prepenetration growth by the pathogen may explain the differences in infection efficiencies of the different sources of inoculum in the treated and untreated soils. The known difference in take-all suppressiveness of the two soils could not be explained by effects on the incidence of root infections.