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Publication #PP-146

 

2012 Florida Citrus Pest Management Guide: Citrus Scab1

M.M. Dewdney and L.W. Timmer2

Citrus scab caused by the fungus Elsinoë 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 that occur on leaves and fruit. One to 2 hours of wetting are sufficient for spore production and only 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 of fungicide 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. With citrus scab, the timing of the spray applications is critical but 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 in a row. 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 can 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 is usually sufficient for scab control. The scab fungus may develop resistance to Abound, Gem, or Headline if these products are not rotated with alternate modes of action frequently. 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.

See Table 1.

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.

Tables

Table 1. 

Recommended Chemical Controls for Citrus Scab.

Pesticide

FRAC

MOA1

Mature Trees

Rate/Acre2

copper fungicide

M1

Use label rate.

Enable 2F

3

8 fl oz. Do not apply more than 3 times per year; no more than 0.38 lb a.i./acre. Minimum retreatment interval is 21 days.

Ferbam Granuflo

M3

5-7.5 lbs (15-26 lbs for nursery stock)

Abound 2.083

11

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

Gem 25WG3

11

4.0-8.0 oz. Do not apply more than 32 oz/acre/season for all uses.

Gem 500 SC3

11

1.9-3.8 fl oz. Do not apply more than 15.2 fl oz/acre/season for all uses.

Headline3

11

9-12 fl oz. Do not apply more than 54 fl oz/acre/season for all uses.

1Mode of action class for citrus pesticides from the Fungicide Resistance Action Committee (FRAC) 2011. Refer to ENY-624, Pesticide Resistance and Resistance Management, in the 2012 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.

3Do not use more than 4 applications of strobilurin fungicides/season. Do not make more than 2 sequential applications of strobilurin fungicides.

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. Original publication date May 2000. Revised February 2012. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

M.M. Dewdney, assistant professor, and L.W. Timmer, professor emeritus, 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.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

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. Nick T. Place, Dean.