2010 Florida Citrus Pest Management Guide: Postbloom Fruit Drop
Postbloom fruit drop (PFD) must be controlled on processing and fresh market fruit. PFD, caused by the fungus Colletotrichum acutatum, affects all species and cultivars of citrus, but severity on a given cultivar may vary according to the time of bloom in relation to rainfall. Navel and Valencia oranges have experienced the most severe damage in Florida.
Most spores of this fungus are produced directly on the surface of infected petals. Spores are splash-dispersed by rains to healthy flowers where they infect within 24 hours and produce symptoms in 4-5 days. The fungus survives between bloom periods as resistant structures on the surface of leaves, buttons, and twigs.
A model has been developed to assist growers in determining the need and timing of fungicide applications. The model is based on: 1) the amount of fungal inoculum present (i.e., the number of diseased flowers on a 20-tree sample [TD in the model]); 2) the total rainfall for the last 5 days; and 3) the number of hours of leaf wetness greater than 10 hours for the last 5 days. The model predicts the percentage of the flowers that will be affected 4 days in the future.