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Publication #PP-147

2014 Florida Citrus Pest Management Guide: Alternaria Brown Spot1

M.M. Dewdney and L.W. Timmer2

Alternaria brown spot, caused by the fungus Alternaria alternata affects Minneola tangelos, Dancy tangerines, Murcotts, and less frequently Orlando tangelos, Novas, Lees, and Sunburst. In rare cases, it may also infect grapefruit. Where severe, the disease results in extensive fruit drop and must be controlled on processing and fresh market fruit.

Spores of Alternaria are airborne. Most spores are produced by lesions on mature leaves on the tree, or recently fallen infected leaves on the grove floor. Many management practices are helpful in reducing the severity of Alternaria brown spot. When new groves of susceptible varieties are planted, they should be established with disease-free nursery stock. Trees grown in greenhouses without overhead irrigation are usually free of Alternaria, but should be inspeted carefully to insure that there are no unexpected trees with lesions. Even though spores are airborne, plantings of healthy trees will remain disease-free for long periods. If is present from the outset, it builds to high populations during the period of vegetative growth on young trees and subsequently is difficult to control on fruit. When establishing new plantings, it is best to locate susceptible varieties in higher areas, where air drainage and ventilation are good and leaves dry more rapidly. Less vigorous rootstocks such as Cleopatra mandarin should be selected, rather than vigorous stocks such as Carrizo citrange. Minneola tangelo groves in low, wet areas have conditions so favorable for alternaria brown spot that may be virtually uncontrollable. Susceptible trees should be spaced more widely than oranges to promote rapid canopy drying. In existing plantings, it is important not to promote excessive vegetative growth. Overwatering and excessive fertilization should be avoided. Light hedging should be done regularly rather than hedging severely but less frequently.

Copper fungicides, Ferbam, Abound, Gem, Headline, Pristine, and Quadris Top are the registered products that are effective for disease control. Disease favorability varies considerably according to the susceptibility of the variety, the grove disease history, and the environmental conditions each year. Generally, the first spray should be applied when the spring flush is about 1/4–1/2 of full expansion and before disease development. In severe cases, another spray may be needed when the flush is near full expansion, because if high levels of infection occur on the spring flush, brown spot becomes difficult to control on fruit. Another spray should be applied shortly after petal fall. Ferbam, Abound, Gem, Headline, Pristine or Quadris Top may be the best choice for one or both of these two applications, especially if the grove has problems with both citrus scab and Alternaria brown spot. Thereafter, all sprays should be applied solely to maintain a protective coating on the fruit. During April and May, applications may be needed as often as every 10 days or as infrequently as once per month. Spray intervals should be determined based on the frequency of rainfall and grove disease history. By June, high rainfall and humid nights usually assure an abundant supply of inoculum and favorable conditions for infection. In most cases, two applications will be needed during this month. Copper fungicides may produce fruit blemishes if applied during hot weather. Thus, Abound, Gem, Ferbam, Headline, Pristine or Quadris Top may be substituted for copper applications as needed from May to July. The fruit generally becomes resistant by early to mid-July although affected fruit may continue to drop for some time afterward. The scenario described is for difficult cases and it is sometimes possible to use many fewer sprays.

The preferred method to time fungicide sprays is the ALTER-RATER, a weather-based model. That model is available as a Fact Sheet on the EDIS website (http://www.edis.ifas.ufl.edu/ch183) or on the CREC website (http://www.crec.ifas.ufl.edu/crec_websites/fungal/Alter-Rater.shtml). Table 1 indicates the points assigned in the ALTER-RATER model. Brown spot is most severe when rainfall is greater than 0.1 inch, daily leaf wetness duration exceeds 10 h and average daily temperature is between 68 and 83°F. Table 2 indicates the suggested thresholds to be applied with the ALTER-RATER. A fungicide application is made when the threshold is reached.

With such frequent sprays, low rates of copper may be used. With average quality products, usually about 2 lb of metallic copper per acre are needed for each 3 weeks, or 1 lb if sprays are more frequent. Even lower rates of metallic copper can be used if high quality products are employed. The copper residue levels over time can be monitored with the use of the Citrus Copper Application Scheduler which can be found at http://www.agroclimate.org/tools/cudecay/. More details are available in EDIS PP289 "A Web-Based Tool for Timing Copper Applications in Florida Citrus".

Abound, Gem, Headline, Pristine and Quadris Top are all strobilurin containing fungicides and Alternaria has been documented to be resistant to strobilurins in most parts of the Florida tangerine production areas. Strobilurins (FRAC 11), DMI (FRAC 3), or SDHI (FRAC 7) fungicides should not be used for Alternaria control more than four times in a season and never more than two applications of the same mode of action in a row. Gem is slightly less effective for control of this disease and it should be used at the high rate where disease is moderate to severe. Ferbam is less effective for Alternaria control than copper, Abound, Gem, Headline, Pristine or Quadris Top.

DO NOT APPLY ABOUND, GEM, HEADLINE, PRISTINE OR QUADRIS TOP 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 3.

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. 

The number of points assigned to each day with ALTER-RATER according to the environmental conditions on that day. Daily point scores are added until the selected spray threshold is reached.

Rainfall > 0.1 inch

Leaf Wetness > 10 h

Avg DailyTemp (°F)

Daily Points Assigned

+

+

68-83

11

+

+

> 83

8

+

+

< 68

6

+

68-83

6

+

> 83

4

+

< 68

3

+

68-83

6

+

> 83

6

+

< 68

4

68-83

3

> 83

0

< 68

0

Table 2. 

Suggested threshold scores to be used in different situations with the ALTER-RATER.

Suggested Threshold Scores

Situation

50

Heavily infested Minneola, Dancy, Orlando, Sunburst; many flatwoods groves, east coast and SW Florida

100

Moderately infested Minneola or Dancy, many Murcotts; Ridge and north Florida groves

150

Light infestations, any variety, mostly Ridge and north Florida groves

Table 3. 

Recommended Chemical Controls for Alternaria Brown Spot

Pesticide

FRAC MOA1

Mature Trees Rate/Acre2

copper fungicide

M1

Use label rate.

Ferbam Granuflo

M3

5-6 lb. Maximum 3 applications a year and do not apply more than 6 lb ai/acre in a single application.

Abound3

11

12.0-15.5 fl oz. Do not apply more than 92.3 fl oz (1.5 lb ai)/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. Do not apply within 7 days of harvest.

Headline SC3

11

12-15 fl oz. Do not apply more than 54 fl oz (0.88 lb ai)/acre/season for all uses.

Pristine4

7/11

16-18.5 oz Do not apply more than 74 oz/acre/season for all uses.

Quadris Top4

11/3

15.4 fl oz. Do not apply more than 61.5 fl oz/acre/season for all uses. Do not apply more than 0.5 lb ai/acre/season of difenoconazole. Do not apply more than 1.5 lb ai/acre/season of azoxystrobin.

1Mode of action class for citrus pesticides from the Fungicide Resistance Action Committee (FRAC) 2013. Refer to ENY624, Pesticide Resistance and Resistance Management, in the 2014 Florida Citrus Pest Management Guide for more details.

2Lower rates can be used on smaller trees. Do not use less than 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. Do not use in citrus propagation nurseries.

4Do not make more than 4 applications of Pristine or Quadris Top/season. Do not make more than 2 sequential applications of Pristine or Quadris Top before alternating to a non-strobilurin, SDHI or DMI fungicide.

Footnotes

1.

This document is PP-147, one of a series of the Plant Pathology Department, UF/IFAS Extension. Original publication date December 1995. Revised September 2013. 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; UF/IFAS Extension, 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.


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U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.