Effects of Soil Matric Potential on Infection of Tobacco byPhytophthora parasiticavar.nicotianae

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Abstract
Soil naturally infested with P. parasitica var. nicotianae was placed on Buechner funnel tension plates to control soil matric potential. A single 1 mo. old ''Hicks'' tobacco seedling was transplanted into each funnel and incubated for 21 days. With an initial inoculum density of 5000 propagules/kg of soil, 60, 27, and 7% of the plants were infected at constant matric potentials of -10, -20 and -50 millibars (mb), respectively. When plants were exposed to a single 24 h saturation period after 1 wk of incubation at -30 or -50 mb, infection was significantly increased in comparison to plants not exposed to the saturation period. Initial inoculum densities of 40, 80, 170, 300, 700, 1300, 2700, and 5300 propagules/kg of soil resulted in 73, 47, 80, 73, 80, 80, 100 and 93% infection, respectively, when plants were exposed to a 24 h saturation period. Saturation periods as short as 0.5 h overcame the inhibitory effect on infection of soil matric potentials of -30 to -50 mb. The mechanism of infection enhancement by soil saturation was apparently stimulation of zoospore release and dispersal.