LESION EXPANSION AS AN EPIDEMIC COMPONENT

A simulator for the enlargement of cohorts of circular lesions on coho rts 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; initi al lesion size (mm(2)), z; and proportion of susceptible host sites, s . Based on the proportion of disease severity at day 50 and the propor tion 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 disease d area that came from lesion expansion. In an extensive survey of phyt opathological literature, many plant pathogens had radial rates greate r 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 s ites, s, was a sensitive parameter, as this determined the host area i nto which lesions could expand. Naturally, R-m was a sensitive paramet er for the proportion of disease on day 50, as it controlled the overa ll speed of the epidemic. Initial lesion size was a relatively insensi tive parameter, although z interacted significantly with s. The greate st proportion of disease that originated from lesion expansion occurre d with fast k(exp), small z, and low values of s, R-m, and f. The mode l was validated with lesion numbers and severities obtained in natural epidemics of Cercospora medicaginis on alfalfa and Exserohilum turcic um on maize. We recommend that the 'epidemic quintuplet' used to descr ibe polycyclic epidemics be expanded to the 'epidemic sextuplet' with the inclusion of k(exp), since lesion expansion is a major component o f many polycyclic epidemics.