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2008 Florida Citrus Pest Management Guide: Citrus Scab1
L.W. Timmer and K.R. Chung2Citrus scab caused by Elsinoe fawcettii affects grapefruit, Temples, Murcotts, tangelos, and some other tangerine hybrids. There is no need to control citrus scab on processing fruit, except possibly on Temples where severe early infection reduces fruit size. Reduction or elimination of overhead irrigation on susceptible varieties during the active growth period of the fruit will decrease disease severity.
Spores of this fungus are produced directly on scab pustules on leaves and fruit. One to 2 hours of wetting are sufficient for spore production and an additional 3-4 hours are needed for infection. Spores are dispersed to healthy tissues by water splash.
If leaves from the previous season are heavily infected by citrus scab, 3 applications may be needed to control the disease: one at about 1/4 expansion of the spring flush, a second at petal fall, and a third about three weeks later. If there is little carryover of disease from the previous season, the first spray can be omitted. Ferbam, Enable, Abound, Gem, or Headline are good choices for the first application because they are all able to kill the fungus in old lesions and thus reduce inoculum as well as protecting foliage. Any of these products may then be used in the petal fall spray but do not use a strobilurin product (Abound, Gem, or Headline) twice. Copper fungicides, Abound, Gem, or Headline are good choices for the third spray since they will protect fruit from early melanose as well as from scab, but copper products are less effective for scab and should not be selected where scab pressure is high.
On Minneola tangelos, Murcotts, and certain other varieties, Alternaria brown spot and scab occur in the same grove. In those cases, copper fungicides, Abound, Gem, or Headline may be preferred since Ferbam and Enable 2F are less effective for Alternaria control. With average quality copper products, about 2 lb of metallic copper per acre are usually sufficient for scab control. The scab fungus may develop resistance to Abound, Gem, or Headline. These products are all strobilurin fungicides and only one should be selected for scab control each season.
Fruit usually becomes resistant to scab by some time in May about 2 months after petal fall.
DO NOT APPLY ABOUND, GEM, OR HEADLINE IN NURSERIES. Application of these fungicides in nurseries can result in selection of resistant strains which are then distributed on nursery stock to groves.
Recommended Chemical Controls
READ THE LABEL.Rates for pesticides 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 Citrus Scab
Pesticide
FRAC
MOA2
Mature Trees
Rate/Acre1
copper fungicide
M9
Use label rate.
Enable 2F
3
8 fl oz. Do not apply more than 3 times per year; no more than 0.375 lb a.i./acre.
Ferbam Granuflo 76%
M2
5-10 lb
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.
Gem 500 SC 11
1.9-3.8 fl oz. Do not apply more than 15.4 fl oz per acre per season.
Headline 11
9-12 fl oz. Do not apply more than 49 fl oz/acre/season for all uses.
1Lower rates can be used on smaller trees. Do not use less than the minimum label rate.
2Mode of action class for citrus pesticides from the Fungicide Resistance Action Committee (FRAC) 2003. Refer to ENY624, Pesticide Resistance and Resistance Management, in the 2008 Florida Citrus Pest Management Guide for more details.
Footnotes
1. This document is PP-146, 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 2007. This publication is included in SP-43, 2008 Florida Citrus Pest Management Guide. A copy of this publication may be found at http://edis.ifas.ufl.edu/CG020. Please visit the EDIS Web site at http://edis.ifas.ufl.edu. For a copy of this handbook, request information on its purchase at your county extension office.2. L.W. Timmer, professor emeritus and K.R. Chung, associate professor, Plant Pathology Department, Citrus REC, Lake Alfred, Florida; Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 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.
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U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.
Copyright Information
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