Soil Factors Affecting the Reproduction and Survival of Olpidium brassicae and its Transmission of Big Vein Agent to Lettuce

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Abstract
A semi-quantitative method was developed to estimate the relative numbers of resting sporangia of O. 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 [Lactuca sativa] in BV-P or BV-S soils at matric potentials (.psi.m) fluctuating between 0 and -150 millibars (mb), whether the soil were fumigated with methyl bromide or not. Zoospores of O. brassicae remained motile for 12-24 h in sand-soil mixtures of either BV-P or BV-S soil, and they remained infective for 48 h. 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.degree. C and slowly at 10.degree. C. Germination of resting sporangia in BV-P soil only occurred at .psi.m .gtoreq. -60 mb, and the number of lettuce plants infected was reduced as .psi.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 (.psi.m = 0). From a point source zoospores moved at least 40 mm in sand at .psi. .gtoreq. -20 mb and 30-40 mm in BV-P and BV-S soils at .psi.m = 0. At lower .psi.m the distance zoospores moved was reduced especially in BV-P soil in which they moved only 10 mm at .psi.m = -40. 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.