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Histopathology of Citrus Greasy Spot and Identification of the Causal Fungus. J. O. Whiteside, Associate Plant Pathologist, University of Florida, Institute of Food and Agricultural Sciences, Agricultural Research and Education Center, Lake Alfred 33850; Phytopathology 62:260-263. Accepted for publication 13 September 1971. DOI: 10.1094/Phyto-62-260.

The fungus causing greasy spot of citrus in Florida is designated as Mycosphaerella citri sp. nov., and the imperfect state, which produces long cylindrical conidia on simple conidiophores arising from extramatricular hyphae, is classified as a Stenella. Hyphae penetrate citrus leaves and rind through stomata, and commonly cause death of guard cells. Fungal growth within the host is intercellular. In orange rind, the hyphae reach only a few cells beneath the substomatal chamber, and therefore cause little necrosis. After penetrating stomata on leaves, the hyphae grow very slowly through the adjacent compacted layer of mesophyll, but grow more rapidly through the large air spaces of the spongy mesophyll. Cells of the spongy mesophyll become hypertrophic and eventually necrotic. The extent of injury resulting from each stomatal penetration is very limited, and a concentration of many penetrations is required to produce macroscopic symptoms.