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Lesion Expansion as an Epidemic Component

A simulator for the enlargement of cohorts of circular lesions on cohorts of host tissue was used to examine five epidemiological parameters: radial rate (mm day^-1) of lesion expansion, k _exp; maximum basic infection rate, R _m; proportion of lesion area as infectious, f; initial lesion size (mm²), z; and proportion of susceptible host sites, s. Based on the proportion of disease severity at day 50 and the proportion of the total disease that originated solely from lesion expansion, k_exp was the most sensitive of the five parameters. A radial rate of only 0.1 mm day^-1 resulted in a proportion of >0.7 of the diseased area that came from lesion expansion. In an extensive survey of phytopathological literature, many plant pathogens had radial rates greater than 0.1 mm day^-1, which would result in a proportion of >0.95 of the total disease that comes from lesion expansion. Susceptible host sites, s, was a sensitive parameter, as this determined the host area into which lesions could expand. Naturally, R_m was a sensitive parameter for the proportion of disease on day 50, as it controlled the overall speed of the epidemic. Initial lesion size was a relatively insensitive parameter, although z interacted significantly with s. The greatest proportion of disease that originated from lesion expansion occurred with fast k_exp, small z, and low values of s, R_m, and f. The model was validated with lesion numbers and severities obtained in natural epidemics of Cercospora medicaginis on alfalfa and Exserohilum turcicum on maize. We recommend that the ‘epidemic quintuplet’ used to describe polycyclic epidemics be expanded to the ‘epidemic sextuplet’ with the inclusion of k_exp, since lesion expansion is a major component of many polycyclic epidemics.