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 foliage wetting 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 spread to healthy tissues by water splash.
If leaves from the previous season are heavily infected by citrus scab, 3 applications of fungicide are needed to control the disease: the first at about 1/4 expansion of the spring flush, a second at petal fall, and a third about three weeks later. Timing of the fungicide applications is critical for citrus scab management, but if there is little carryover of disease from the previous season, the first spray can be omitted. Ferbam, Enable 2F, 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 protect foliage. Any of these products can be used in the petal fall spray, but do not use strobilurin products (Abound, Gem, or Headline) twice in a row. Copper fungicides, Abound, Gem, or Headline are good choices for the third spray because they will protect fruit from early melanose as well as from scab. However, 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 because Ferbam and Enable 2F are less effective for Alternaria control. If canker is a concern, the only product that can manage both diseases is copper. 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 frequently rotated with alternate modes of action. Resistance has been documented in Florida for citrus scab. These products are all strobilurin fungicides, and only one should be selected for scab control each season.
Fruit usually becomes resistant to scab by sometime 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.
Recommended chemical controls for citrus scab.