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Etiology and Epidemiology of Citrus Greasy Spot. J. O. Whiteside, Associate Plant Pathologist, University of Florida, IFAS, Citrus Experiment Station, Lake Alfred 33850. Phytopathology 60:1409-1414. Accepted for publication 3 April 1970. DOI: 10.1094/Phyto-60-1409.

A Mycosphaerella fungus found producing perithecia in abundance on decomposing fallen citrus leaves in all Florida citrus groves examined was confirmed as the cause of citrus greasy spot. This fungus also produced rough-walled Cercospora-like conidia on unbranched conidiophores developing singly from hyphae growing extramatrically on living citrus leaves. In a spore trap operated continuously from February to November 1969, ascospores were trapped after rain and on nights with heavy dews. Ascospore numbers were relatively small during the winter, increased through the spring, and reached a peak in June. In blocks of grapefruit, orange, and Milam lemon, a subsequent decline in ascospore numbers was associated with rapid decomposition of leaf litter during July and August and the absence of any substantial leaf drop after early spring to replenish the perithecial substrate. In a grove of true lemons, however, ascospore numbers were still high in September due to continued leaf drop. Conidia were trapped in relatively small numbers at all locations, and represented only a minor source of inoculum. The amount of disease that developed on container-grown rough lemon trap plants placed outdoors for 2-week periods from April to November correlated closely with the number of ascospores discharged over each exposure period.