Influence of Soil Water Status on the Epidemiology of Tobacco Black Shank

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Abstract
The relationship of soil water status to the rate of increase in mortality of tabacco in a field infested with Phytophthora parasitica var. nicotianae was examined. Multiple cycles in the increase in mortality occurred in the susceptible cultivar, Hicks. Generally only a single cycle in the increase in mortality occurred in the resistant cultivar, Speight G-28; this was attributed to the delay of appreciable mortality until after flowering, which limited time for secondary cycles. Increases in the average rates of change in mortality were negatively cross correlated with increases in the average rates of change in soil water status for both cultivars. These increases differed between cultivars in both their magnitudes and the times of the initial and maximum responses. Ordinary runs analysis was conducted over time to ascertain the temporal pattern of the random or nonrandom occurrence of plant mortality. Increases in the percentages of plots with a nonrandom occurrence of plant mortality coincided with periods of increased soil moisture. Evidence for the increase or spread of inoculum was more pronounced in the susceptible than in the resident cultivar as demonstrated by the eventual near uniformly of mortality throughout plots of Hicks compared with plots of Speight G-28.
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