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2026–2027 Florida Citrus Production Guide: Alternaria Brown Spot

Megan Dewdney

Application Timing Recommendations

  • Alternaria brown spot (ABS) is a disease of tangerines and tangerine hybrids.
    • Grapefruit is very rarely affected.
  • Start applications before disease development when spring flush is ¼–½ of full expansion.
    • If ABS is severe, a second application may be needed near full expansion.
  • Plan a petal fall application.
    • Strobilurin-containing fungicides may be a good choice here because they are effective against citrus scab as well.
  • In April and May, the goal of applications is to maintain a protective coat on the fruit.
  • Because fruit growth slows in June, usually only 1 to 2 applications are needed. The fruit are considered resistant by mid-July.

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.

Table 2. Suggested threshold scores to be used in different situations with the ALTER-RATER.

Fungicide Resistance

Alternaria alternata has been documented to be resistant to strobilurins (FRAC MOA 11) in most parts of the Florida tangerine production areas. The level of resistance varies from location to location and strobilurin use history. Amistar Top, Gem, Headline, Pristine, and Quadris are all strobilurin-containing fungicides. All single-site fungicides are at risk for resistance development. Strobilurin (FRAC MOA 11), DMI (FRAC MOA 3), or SDHI (FRAC MOA 7) fungicides should not be used for Alternaria control more than four times in a season; never use more than two applications of the same mode of action in a row. Gem is slightly less effective for control of this disease and should be used at the high rate where disease is moderate to severe. As of 2025, Ferbam is no longer registered for use. The only multisite fungicide remaining for use in citrus is copper, which is effective against ABS at higher rates. Keep this in mind when designing your rotation plans for ABS management.

DO NOT APPLY AMISTAR TOP, GEM, HEADLINE, PRISTINE, OR QUADRIS IN NURSERIES. Application of these fungicides in nurseries can result in selection of resistant strains, which are then distributed on nursery stock to groves.

Citrus Advisory System: http://agroclimate.org/tools/cas/

Citrus Copper Application Scheduler: http://agroclimate.org/tools/citrus-copper-application-scheduler/

UF/IFAS publication PP289, "A Web-Based Tool for Timing Copper Applications in Florida Citrus": https://doi.org/10.32473/edis-pp289-2012

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 gal of water. Calibrate and arrange nozzles to deliver thorough distribution, and treat as many acres as this volume of spray allows.

Table 3. Recommended chemical controls for Alternaria brown spot.

Publication #CG117

Commercial

Disclaimers

The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the products named, and references to them in this publication do not signify our approval to the exclusion of other products of suitable composition.

Use pesticides safely. Read and follow directions on the manufacturer's label.