VIEW ARTICLE

Effect of Inoculum Density, Planting Depth, and Soil Temperature on Urocystis colchici Infection of Onion. W. C. Stienstra, Former Graduate Assistant, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48823, Present address: Department of Plant Pathology, University of Minnesota, St. Paul 55101; M. L. Lacy, Associate Professor, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48823. Phytopathology 62:282-286. Accepted for publication 23 September 1971. DOI: 10.1094/Phyto-62-282.

Curves drawn using the multiple infection, log-probit, or log-log transformations tended to flatten at upper inoculum concentrations, but percentage of disease was linearly related to the distance between spores (D) over a wide inoculum range. The linearity using D and the nonlinearity using other transformations were explained by the nonlinear relationship between D and number of spores per gram of soil. Soil was artificially infested in layers at 0-5, 5-10, and 10-15 mm deep, and in all possible combinations. Onion seeds were planted 5, 10, or 15 mm deep. Deeper planting resulted in more disease at a given inoculum density due to (i) increased amount of susceptible tissue exposed; and (ii) longer exposure of susceptible tissue to inoculum. Greatest amount of disease resulted when seeds were planted at 15 mm with all layers of soil infested. Least amount of disease resulted when inoculum was below the seed. At inoculum densities of 1,000 (D = 2.0 mm) or 2,000 (D = 1.7 mm) spores/g soil, amounts of disease were similar at each of four temperatures (12, 20, 24, and 28 C), but significantly more disease occurred at the higher inoculum density (16 C). Highest infection at both inoculum densities occurred at 20 C.