VIEW ARTICLE

Vector Relations

Soil Factors Affecting the Reproduction and Survival of Olpidium brassicae and its Transmission of Big Vein Agent to Lettuce. F. V. Westerlund, Research Plant Pathologist, Moran Seeds, Inc., Salinas, CA 93901; R. N. Campbell(2), R. G. Grogan(3), and J. M. Duniway(4). (2)(3)(4)Professors, Department of Plant Pathology, University of California, Davis, CA 95616. Phytopathology 68:927-935. Accepted for publication 11 November 1977. Copyright © 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-927.

A semi-quantitative method was developed to estimate the relative numbers of resting sporangia of Olpidium brassicae in the soil. There were more propagules in a big-vein-prone (BV-P) soil than in a big-vein-intermediate (BV-I) soil and virtually none in a big-vein-suppressive (BV-S) soil. Similar numbers of resting sporangia were produced in roots of inoculated lettuces in BV-P or BV-S soils at matric potentials (φ_m) fluctuating between 0 and –150 millibars (mb), whether the soils were fumigated with methyl bromide or not. Zoospores of O. brassicae remained motile for 12-24 hr in sand-soil mixtures of either BV-P or BV-S soil and they remained infective for 48 hr. Thus, no biological or chemical factors were detected to account for the differences in big-vein occurrence between BV-P and BV-S soils. Resting sporangia in naturally infested, air-dried soils required wetting in nearly saturated soil for about 6 days before they were capable of germination. Once rewetted the resting sporangia germinated whether host roots were present or not; they germinated rapidly at 18 and 22 C and slowly at 10 C. Germination of resting sporangia in BV-P soil only occurred at φ_m≥–60 mb and the number of lettuce plants infected was reduced as φ_m decreased from 0 to –60 mb. Zoospores were released from vegetative sporangia only when the water content of the sand was equivalent to that at saturation (φ_m = 0). From a point source zoospores moved at least 40 mm in sand at φ_m ≥ –20 mb and 30-40 mm in BV-P and BV-S soils at φ_m = 0. At lower φ_m the distance zoospores moved was reduced especially in BV-P soil in which they moved only 10 mm at φ_m = –40. Thus, the most important factor to account for the big-vein proneness of soils seemed to be the soil-water relations and the rapidity with which water drained from the soils.