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Open Access Publications from the University of California

Where have all the flowers gone? Postbloom fruit drop of citrus in the Americas.


Postbloom fruit drop (PFD), caused primarily by Colletotrichum acutatum, is a serious disease annually in the humid tropical citrus areas of the Americas and occurs more sporadically in the humid subtropics. The fungus infects flowers of all citrus species producing orange-brown lesions on petals that result in abscission of the fruitlets leaving the persistent calyx and floral disk attached to the twigs. C. acutatum also causes Key lime anthracnose and is morphologically identical to PFD, but the strains can be differentiated by molecular means and pathogenicity tests. C. acutatum produces abundant conidia on infected petals that are dispersed primarily by rain splash. After the bloom season, the fungus persists as appressoria on persistent calyces and other vegetative plant parts. Those appressoria are stimulated to germinate by flower extracts and produce secondary conidia to initiate a new disease cycle. Some cultural measures are useful in reducing disease severity, but control is based principally on application of fungicides. Benzimidazole fungicides and captafol were the best materials in the past but are no longer available. The QoI fungicides, folpet and tebuconazole + trifloxystrobin are the most effective among currently available materials. Forecasting systems based primarily on the availability of inoculum and recent rainfall were developed to aid growers on timing fungicide applications. The expert system, PFD-FAD, based on environmental conditions, inoculum, disease history, and varietal susceptibility is the most effective means currently available for scheduling fungicide applications.

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