Influence of Initial Density and Distribution of Inoculum on the Epidemiology of Tobacco Black Shank

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Abstract
Residual inoculum of Phytophthora parasitica var. nicotianae was aggregated in field soil at the time tobacco transplants were set. The negative binomial dispersion parameter (k) ranged from 0.02 to 2.03 in the upper 10 cm of soil among individual plots. Aggregation of inoculum increased with depth in the soil profile. Mean inoculum densities were generally greatest in the upper 10 cm of soil and ranged from 0.01 to 1.31 propagules per gram of soil among individual plots. Mortality in the resistant cultivar, Speight G-28, was related directly to initial mean inoculum density throughout the epidemic but not to the aggregation of inoculum. Mortality in the susceptible cultivar, Hicks, was not related to initial mean inoculum density, but late in the epidemic it was related to the aggregation of inoculum (measured by Lloyd''s index of patchiness, 1 + 1/k) and to the interaction between inoculum density and aggregation. For equivalent initial mean inoculum densities, more extreme aggregation of inoculum delayed disease development in Hicks. The combination of both low initial mean density and extreme aggregation of inoculum further delayed disease development and resulted in a reduction in the final percentage of mortality attained.