Fungicide Application Decisions System
for Postbloom Fruit Drop in Citrus
In citrus areas with a single bloom, fungicide applications are justifiable when PFD-FAD indicates that applications are needed. Fruit losses under those conditions will be incurred. Even if prices are low, the yield increases obtained from a fungicide spray will help pay costs already incurred such as irrigation, fertilization, equipment, and infrastructure. Thus, applications are justifiable even if there is a net loss for the grove. Sprays are not justified only when it is likely that the crop will not be harvested.

Even in areas where a single bloom predominates, flowering may be cyclic and prolonged. At times, it may be necessary to make a decision as to whether sufficient bloom is present at the moment to justify an application. This will depend on the value of the variety or the potential price at harvest time.

Areas with multiple blooms often have relatively few flowers at any given time, but fruit set on a higher percentage of the flowers. Thus, a judgment must be made prior to applying the PFD-FAD system as to whether fruit production will be increased sufficiently to at least pay the cost of the application.

When no fungicide has been applied in the last 14 days, a risk value of greater than 11 triggers a spray. If a spray has been applied from 11 to 14 days ago, then a risk value greater than 13 is required to trigger an application. If the last spray was applied 8 to 10 days ago, then a risk value of greater than 15 is required. No sprays are recommended if an application has been made during the last week.

The PFD-FAD system is designed to be slightly conservative. That is, occasionally an application will be recommended that would not be required. Growers may wish to introduce some additional judgment into the system based on economics and other factors. For example, if the risk value is marginal, growers may want to make an application for high value crops and accept more risk for low value crops. It is probably better to accept less risk in the earlier stages of the bloom and take more chances late, especially if a large crop has already been set.

Copyright 1994-2002  Institute of Food and Agricultural Sciences
University of Florida, Gainesville, FL 32611