Multivariate Comparison of Papaya Ringspot Epidemics

Abstract

Nine epidemiological parameters, estimated to characterize each of 60 disease progress curves of papaya ringspot (papaya ringspot virus type P [PRSV-P]) incidence on papaya (Carica papaya), were analyzed using a combination of principal component and cluster analysis techniques to select the best subset of variables with the highest explanatory capacity and to classify epidemics according to their degree of similarity. The effects of cultural methods, transplanting date, planting density, and field location on epidemic classification and control of papaya ringspot also were examined. Standardized area under disease progress curve (AUDPC s ), shape parameter (c) of the Weibull distribution function, and time between transplant date and first symptoms (X 0 ) were selected, using principal component analysis, as the most important variables and represented 83.5% of the overall variance. The remaining variability was explained by the apparent infection rate (r G -Gompertz) standardized by the Richard's method, the scale parameter (b) of the Weibull distribution function, initial and final disease incidence, time to reach 50% incidence, and time of epidemic duration. Five epidemic categories were defined through a direct linkage of the principal component scores associated with AUDPC s , c, and X 0 , via cluster analysis. Experimental site and transplanting date had more influence on the definition of epidemic categories than did planting density. No category indicated complete suppression of papaya ringspot. Epidemics associated with early transplanting dates (February, April, and June) were in the category representing the smallest AUDPC s ( 25%-days day -1 ), the longest time to first symptoms (≥ 120 days), and low values of c (1 to 3). AUDPC s was more sensitive to experimental site than the other factors. Transplanting dates in February and June were suggested as additional alternatives to April and May transplanting dates practiced by growers, to delay the epidemic onset and to reduce the incidence of papaya ringspot of Central Veracruz, Mexico.