TEMPORAL AND SPATIAL DYNAMICS OF POSTBLOOM FRUIT DROP OF CITRUS IN FLORIDA

Temporal progress of postbloom fruit drop of citrus in Florida was studied in four experimental plots of 25-81 3- to 4-yr-old trees of Valencia sweet orange in Gainesville and Lake Alfred and of Pineapple sweet orange and Ruby Red grapefruit in Hastings. Blossoms of central focal trees in each plot were inoculated with the slow-growing orange strain of Colletotrichum gloeosporioides. Incidence of trees and of open blossoms with postbloom fruit drop was monitored over time. Directionality of spread from a point source was measured in four contiguous quadrants of each plot. Disease gradients were estimated on four transects radiating from a central focal tree. Disease increased in intensity and spread in space following rain events in all four plots. The temporal increase of postbloom fruit drop expressed as proportion of affected trees or blossoms was fit better by a Gompertz model than by linear, monomolecular, logistic, or exponential models. Rate parameters of 0.05, 0.11, 0.05, and 0.04 gompits/day for disease incidence on trees and 0.07, 0.05, 0.04, and 0.02 gompits/day for disease incidence on flowers were found for the Gainesville, Lake Alfred, Hastings-sweet orange, and Hastings-grapefruit, respectively. Directional spread was apparent in Lake Alfred and spread was associated with a single rain event accompanied by northerly winds. Little indication of directionality in spread of disease was noted in the other three plots. There was some evidence of development of secondary foci in some plots. Spread appeared to be primarily by windblown rain.