• Topics: Plant Pathology | Alternaria Diseases in Citrus | SP 043 Florida Citrus Pest Management Guide | Timmer, Lavern W | Dewdney, Megan M

2012 Florida Citrus Pest Management Guide: Alternaria Brown Spot¹

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 the 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. Even though spores are airborne, plantings of healthy trees will remain disease-free for long periods. If Alternaria 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 Alternaria 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. This is 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 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’ (http://edis.ifas.ufl.edu/pp289).

Abound, Gem, Headline, Pristine and Quadris Top are all strobilurin containing fungicides and Alternaria has been documented to be resistant to strobilurins in isolated cases in Florida. Strobilurins, DMI or SDHI 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

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.

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