TEMPORAL AND SPATIAL DEVELOPMENT OF PAPAYA RINGSPOT IN VERACRUZ, MEXICO

The temporal progress and spatial distribution of papaya ringspot virus (PRV) and populations of aphid vectors were monitored in two papaya (Carica papaya) plantations in the state of Veracruz, Mexico. The incidence of PRV had a typical sigmoidal curve and the logistic model was more acceptable for describing the disease progress than the Gompertz model. The rate (r(L)) of increase in disease incidence (0.034 and 0.023/unit/day in Plot A and B, respectively) differed (P = 0.05) between the plots; differences in number of aphids trapped could account for the difference. The initial determination of an apparent regular spatial pattern early in the epidemic, obtained with a quadrat size determined by Greig-Smith's method (n = 8 plants/quadrat) at 50 % disease incidence, was inconsistent with our biological observations in the field. An arbitrarily selected, square quadrat size (n = 9 plants/quadrat), was more consistent with visual observation for describing the spatial pattern in the field. With 9 plants/quadrat a random pattern was found. Aphid populations had a bimodal distribution at both sites with the highest population peak in December-February and a secondary peak in August-September and change in disease incidence was generally related to the aphid population level in the previous month. None of the five potential aphid vectors (Myzus persicae, Aphis gossypii, A. nerii, A. citricola and Macrosiphon euphorbiae) of PRV in Mexico colonized the papaya plants, however, which may explain the absence of clustering of disease.