2010 Florida Citrus Pest Management Guide: Greasy Spot
Greasy spot is caused by the fungus Mycosphaerella citri. Management must be considered in groves intended for processing and fresh market fruit. Greasy spot is usually more severe on leaves of grapefruit, pineapples, Hamlins, and tangelos than on Valencias, Temples, Murcotts, and most tangerines and their hybrids.
Airborne ascospores produced in decomposing leaf litter on the grove floor are the primary source of inoculum for greasy spot. These spores germinate on the underside of the leaves and the fungus grows for a time on the surface before penetrating through the stomates (natural openings on lower leaf surface). Internal growth is slow and symptoms do not appear for several months. Warm humid nights and high rainfall, typical of Florida summers, favor infection and disease development. Major ascospore release usually occurs from April to early June with favorable conditions for infection occurring from June through September. Young leaves are susceptible and older leaves remain susceptible throughout their life.