VIEW ARTICLE

Ecology and Epidemiology

Relationship of Infection and Damping-Off of Soybean to Inoculum Density of Pythium ultimum. R. S. Ferriss, Assistant professor, Department of Plant Pathology, University of Kentucky, Lexington 40546; Phytopathology 72:1397-1403. Accepted for publication 11 December 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1397.

Soybean seeds were planted in pasteurized soil artificially infested with 0.1 to 600 sporangia of Pythium ultimum per gram of soil (spg). Five variables were evaluated as functions of inoculum density (ID); disease incidence (DI), infection of seeds 2 days after planting (seed infection), infection of seed coats 2 days after planting (coat infection), colonization of heat-killed seeds 1 day after planting (dead seed colonization), and colonization of selective medium in soil microbiological sampling tubes 1 day after insertion into soil (tube colonization). Five types of regression models were fitted to each data set after transformation of data by the multiple infection transformation, –In (1–Z), in which Z = the proportion of affected units. DI was not significantly different at IDs of from 10 to 600 spg. Data for DI at lower IDs and seed infection were fitted to equations that indicated the existence of a spermoplane effect (sensu R. Baker), the existence of limiting sites (sensu J. E. Vanderplank), and a nonlinear relationship between the variables and ID. Data for coat infection, dead seed colonization, and tube colonization were fitted to equations that indicated the existence of a spermosphere effect (sensu Baker), the existence of no limiting sites (sensu Vanderplank), and a linear relationship between the variables and ID. Calculated spermosphere widths were approximately 8, 130, and 60 times the average radius of P. ultimum sporangia for seed infection, coat infection, and dead seed colonization, respectively. The results indicate that the relationships of DI and seed infection to ID are probably not true spermoplane effects (in which only propagules that actually touch the seed surface can initiate infections), but are situations in which some other factor affects the relationship of the variable and ID. This other factor seems to be associated with host viability, and may be related to unequal susceptibility within the host population.