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N.A. Peres, and M.M. Dewdney2

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 (Equation 1). 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.

LyraEDISServletT
Equation 1.

A fungicide application is indicated if these three criteria are met: 1) the model predicts a disease incidence of greater than 20%; 2) sufficient bloom is present or developing to represent a significant portion of the total crop; and 3) no fungicide application has been made in the last 10-14 days.

Groves with persistent calyxes (buttons) from the previous year should be closely examined once the bloom begins. If infected flowers are present on scattered early bloom, model recommendations should be followed once sufficient bloom is present. Groves with a history of PFD should be checked twice weekly during the bloom period. Ground and aerial applications are effective for control of PFD. The removal of declining trees, where off-season blooms may provide a site for fungal spore buildup, and a reduction in overhead irrigation during bloom should reduce disease severity.

An alternative to the PFD model, called the PFD-FAD system, has been developed. It is more complete in that it takes into consideration the disease history in the grove, the cultivar susceptibility, and the time of the last fungicide application. The PFD-FAD system is easy to use and requires less precise information than the PFD model. It can be found at: http://pfd.ifas.ufl.edu/ and is also available in Spanish and Portuguese.

Of the products recommended for control of PFD, Abound, Gem, and Headline are effective but do not have a long residual effect, and Ferbam is less effective. Ferbam is not sufficiently effective to be used alone but can be combined with low rates of other products to maximize protection and reduce the risk of resistance development. No resistance has been detected to date. Neither Abound, Gem, nor Headline should be used alone more than once per season, but can be used more than once if combined with Ferbam.

Recommended Chemical Controls

READ THE LABEL.

Rates for pesticides in Table 1 are given as the maximum amount required to treat mature citrus trees unless otherwise noted. To treat smaller trees with commercial application equipment including handguns, mix the per acre rate for mature trees in 125 gallons of water. Calibrate and arrange nozzles to deliver thorough distribution and treat as many acres as this volume of spray allows.

Table 1.

Recommended Chemical Controls for Postbloom Fruit Drop.

Pesticide

FRAC

MOA1

Mature Trees

Rate/Acre2

Abound 2.08 F

11

12.4-15.4 fl oz. Do not apply more than 92.3 fl oz/acre/season for all uses.

Abound 2.08 F + Ferbam

11, M3

12.4 fl oz + 5 lb

Gem 500 SC

11

1.9-3.8 fl oz. Do not apply more than 15.4 fl oz/acre/season.

Gem + Ferbam

11, M3

4.0 oz + 5 lb

Headline

11

9.0-12.0 fl oz. Do not apply more than 49 fl oz/acre/season for all uses.

Headline + Ferbam

11, M3

9.0 fl oz + 5 lb

1Mode of action class for citrus pesticides from the Fungicide Resistance Action Committee (FRAC) 2009. Refer to ENY-624, Pesticide Resistance and Resistance Management, in the 2010 Florida Citrus Pest Management Guide for more details.

2Lower rates can be used on smaller trees. Do not use less than the minimum label rate.

Footnotes

1. This document is PP-45, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Date printed: December 1995. Date revised: November 2009. This publication is included in SP-43, 2010 Florida Citrus Pest Management Guide. For a copy of this guide, request information on its purchase at your county extension office. Please visit the EDIS Web site at http://edis.ifas.ufl.edu.

2. N.A. Peres, assistant professor, Plant Pathology Department, Gulf Coast REC, Wimauma, Florida; and M.M. Dewdney, assistant professor, Plant Pathology Department, Citrus REC, Lake Alfred, Florida; Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.

The use of trade names in this publication is solely for the purpose of providing specific information. It is not a guarantee or warranty of the products named, and does not signify that they are approved to the exclusion of others of suitable composition.

Download IFAS Educational PDF
  • Full credit is given to the UF/IFAS for the information on this page. For the publication, its source, and date of the informations publication please see footnotes above.
 
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