VIEW ARTICLE

Ecology and Epidemiology

Relationships Among Soil Depth, Soil Texture, and Inoculum Placement in Infection of Carrot Roots by Eruptively Germinating Sclerotia of Sclerotium rolfsii. Zamir K. Punja, Formerly adjunct assistant professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695, and visiting scientist, Campbell Soup Company, Currently research scientist and manager, Campbell Institute for Research and Technology, Route 1, Box 1314, Davis, CA 95616; Phytopathology 76:976-980. Accepted for publication 24 March 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-976.

The maximum lateral distance (competence distance) at the surface of pasteurized or nonsterile soil from which mycelium from an eruptively germinating (competent) sclerotium of Sclerotium rolfsii grew to infect a carrot root was 3 cm. With increasing depth in either soil, the competence distance was reduced. At depths of 7-8 cm, sclerotia had to be in contact with the root surface to infect. The volume of soil (competence volume) surrounding the root within which a competent sclerotium had a probability (P ≥ 0.1) of initiating an infection was shaped like the inverted frustum of a cone. Within this volume, the probability of infection was reduced from a value of 1.0 with each increment in distance from the root surface and with increasing depth in soil. The average infection efficiency (obtained by averaging all probability values) for a sclerotium located within the competence volume was 0.45 in a pasteurized sandy loam soil and 0.34 in nonsterile field soil; the values were slightly higher when the proportion of sand was increased. Infection efficiencies were always greater at the soil surface and close to the root surface. An average inoculum density of 2.5 sclerotia per 100 cm³ of soil resulted in 100% infection of carrot root slices under greenhouse conditions.