Timing of Fungicide Applications for Control of Postbloom Fruit Drop of Citrus in Florida. L. W. Timmer, Professor, Citrus Research and Education Center, University of Florida, Institute of Food and Agricultural Sciences, 700 Experiment Station Road, Lake Alfred 33850. S. E. Zitko, Senior Biologist, Citrus Research and Education Center, University of Florida, Institute of Food and Agricultural Sciences, 700 Experiment Station Road, Lake Alfred 33850. Plant Dis. 76:820-823. Accepted for publication 24 March 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0820.

Fungicide application schedules of benomyl or captafol for control of citrus postbloom fruit drop caused by Colletotrichum gloeosporioides were evaluated in three orchards during the 1989–1991 bloom periods. In seasons and locations where disease incidence was low, a single application of either fungicide at midbloom or two applications, one at early bloom and one at midbloom, reduced blossom blight incidence and the formation of persistent buttons (floral disk and calyx). Where disease incidence was great, only weekly or 10-day schedules provided a high degree of control of blossom blight and button formation. The number, rather than timing, of applications during the bloom period appeared to be the primary determinant of the degree of disease control. Blossom blight incidence and button formation decreased exponentially as the number of applications increased in many cases. When disease incidence was low or moderate, fruit counts or total yields were not increased by any application schedule. In one case, when disease incidence was high, fruit counts were increased about sevenfold and total yield was increased threefold when applications were made every 10 days, but other application schedules in the same experiment did not increase fruit counts or yield.