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Environmental Factors Affecting Infection of Citrus Leaves by Mycosphaerella citri. J. O. Whiteside, Plant Pathologist, University of Florida, Institute of Food and Agricultural Sciences, Agricultural Research and Education Center, Lake Alfred 33850; Phytopathology 64:115-120. Accepted for publication 2 July 1973. DOI: 10.1094/Phyto-64-115.

Penetration of citrus leaves by Mycosphaerella citri sufficient to cause visible symptoms of greasy spot, requires prolonged or frequently repeated periods of near 100% relative humidity (RH), combined with high temp. At RH near the required min of ca. 92%, ascospore germination and germ tube growth was greater on sucrose- or honeydew-coated glass than on clean glass. A preinoculation spray of sucrose greatly increased disease severity because of its nutritional effects in promoting extramatrical hyphal growth and, hence, the number of stomatal penetrations. The observed association of greasy spot with honeydew-excreting pests in a greenhouse, in which the atmosphere was otherwise too dry for infection, was attributed to the hygroscopicity and sucrose content of the honeydew. Appressoria developed only in the outerstomatal chamber, and were produced in greater numbers when the ventral surface of the inoculated leaves was dried off periodically, than when it was kept continuously wet. In vitro, stomatal guttation fluid sometimes caused hyphal tips to swell as though inducing appressorium formation. If this fluid represents the actual appressorium-inducing agent, then certain environmental conditions might also influence infection indirectly by promoting stomatal guttation.