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Relation of Inoculum Size and Concentration to Infection of Wheat Roots by Gaeumannomyces graminis var. tritici. H. T. Wilkinson, Formerly, postdoctoral research associate, Department of Plant Pathology, Washington State University, Pullman 99164, Assistant professor, Department of Plant Pathology, University of Illinois; R. J. Cook(2), and J. R. Alldredge(3). (2)Research plant pathologist, U.S. Department of Agriculture, Agricultural Research Service, Washington State University, Pullman 99164; (3)Associate professor, Biometry and Statistical Services, Washington State University, Pullman 99164. Phytopathology 75:98-103. Accepted for publication 3 July 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-98.

In this work we examined the relationship between different sizes and concentrations of plant fragments infested with Gaeumannomyces graminis var. tritici and their efficiency in producing lesions on wheat roots. Colonized wheat crowns from the field, infected roots produced in the greenhouse, and oat grains colonized under axenic conditions each were fragmented and then separated by sieving into particles ranging from 2.0 to 0.10 mm. Each of two soils, a Ritzville silt loam (RSL) and a Shano silt loam (SSL) were fumigated with methyl bromide and then amended with 10 concentrations (between 10 and 0.01 mg/g soil) of each type of infested plant fragment in all combinations. The number of root lesions produced per wheat seedling in each treatment was assessed after 28 days at 15 C. Fragments of colonized oat grains were the most infectious treatment at a given size and concentration but fragments of colonized crowns from the field were only slightly less infectious; both were significantly more effective as sources of inoculum than equivalent sizes and concentrations of colonized root fragments. In RSL, the threshold size was 0.25- 0.50 mm, below which few lesions developed regardless of concentration, and above which the number of lesions increased proportionally with concentration for both crown and oat fragments. In SSL, the threshold size for crown tissue was 0.15- 0.25 mm, and for oat grain fragments, 0.10- 0.15 mm. For root particles in SSL, the threshold size was 0.5- 1.0 mm; in RSL it was > 1.0 mm (the largest particle size tested). An analysis of plant fragments from field soils (mainly crowns) showed that only inoculum in particles with diameters ≥0.5- 1.0 mm produced lesions.