Factors Contributing to the Rare Occurrence of Scab on Sweet Orange in Florida. J. O. Whiteside, Professor, University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred, FL 33850. . Plant Dis. 72:626-628. Accepted for publication 8 February 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0626.

Despite the common occurrence in Florida of a virulent sweet orange-infecting biotype of Elsinoë fawcettii, scab rarely appears in solid plantings of sweet orange. Because sweet orange foliage is immune to scab, the pathogen can survive in sweet orange groves only if there are diseased rootsprouts present from scab-susceptible rootstocks, or if inoculum-bearing fruit remain on the tree until after the next bloom. Viable propagules of E. fawcettii were detected in the stromatic portion of scab pustules on inoculated Valencia orange fruit for the first 6–9 mo after infection, but few or none were detected after 10–12 mo. No scab appeared on fruit that set from the spring bloom near 12- to 14-mo-old diseased Valencia fruit. The disappearance of scab from previously infected sweet orange groves is attributed to a loss of inoculum-producing ability of the pustules on the previous year’s in-season fruit before the next crop becomes exposed to infection and to a paucity of out-of-season late-set fruit which, if they became infected, might provide inoculum to infect the next in-season crop of fruit.