Evaluation of a Model for Prediction of Postbloom Fruit Drop of Citrus. L W Timmer, Professor, University of Florida, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850. S. E. Zitko, Senior Biologist, University of Florida, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850. Plant Dis. 80:380. Accepted for publication 20 December 1995. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0380.

Postbloom fruit drop, caused by Colletotrichum acutatum, produces orange-brown lesions on flower petals, abscission of fruitlets, and formation of persistent calyces (buttons). A previously developed model that predicts the percentage of flower infection 3 to 4 days in advance based on the current number of infected flowers and rainfall for the previous 5 days was evaluated in three navel and Valencia orange groves in 1993 and in five groves each in 1994 and 1995. There was a significant (P ≤ 0.05) relationship between the predicted and observed percentages of affected flowers in seven of the nine cases in which sufficient disease developed to warrant fungicide applications {R² = 0.38 to 0.86). From one to three applications were made in each of these cases based on the model predictions. In many of the cases, the fungicide applications reduced the area under the curve for disease incidence on flowers and the number of buttons formed compared to the unsprayed controls. Fungicide applications increased fruit counts (P ≤ 0.10) in eight of the nine cases with increases over the unsprayed controls ranging from 25 to 523%. Model predictions were accurate except when rain events were of short duration and tree canopies dried quickly. Model-based decisions on fungicide applications resulted in reduced disease, large increases in fruit production, and elimination of unnecessary sprays.