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

L W Timmer,N A Peres

doi:10.5070/c421028302

Abstract

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.

Highlights

Postbloom fruit drop (PFD) was first formally described in Belize by Fagan (1979)
Agostini et al (1992) differentiated the strains of Colletotrichum occurring in Florida and designated them as slow-growing orange (SGO) for the pathogen of PFD, KLA for isolates causing anthracnose on Key lime, and fast-growing gray for the saprophyte and postharvest pathogen
They found that the KLA isolates produced all the symptoms of PFD when inoculated on sweet orange flowers, but SGO did not produce typical lime anthracnose symptoms on Key lime leaves

Summary

Introduction

Postbloom fruit drop (PFD) was first formally described in Belize by Fagan (1979). there are previous reports of a disease that had all of the symptoms of PFD in various areas of Central and South America in the 1950s and 60s (Timmer et al 1994). The disease is known to occur in all humid citrus areas from Florida (USA) in the north to Uruguay in the south It is endemic in many humid tropical areas such as southern Mexico, Belize, Costa Rica, and many islands of the Caribbean. In those areas, it occurs every year and it is one of the main diseases that affect yield. The calyces and floral disks which normally abscise if no fruit is set, remain attached to the twig (Fig. 2) These persistent calyces, commonly called buttons, survive for the life of the twig. Leaves surrounding infected flowers are often distorted with twisted laminae and enlarged veins

Causal Agents

Findings

Disease Cycle and Epidemiology