2010 Florida Citrus Pest Management Guide: Citrus Scab
Citrus scab caused by the fungus Elsinoe 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. 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.