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Action of Oil in the Control of Citrus Greasy Spot. J. O. Whiteside, Associate Plant Pathologist, University of Florida, Institute of Food and Agricultural Sciences, Agricultural Research and Education Center, Lake Alfred, Florida 33850; Phytopathology 63:262-266. Accepted for publication 25 August 1972. DOI: 10.1094/Phyto-63-262.

Sprays of 1% oil-water emulsion applied to sweet orange (Citrus sinensis) leaves did not reduce germination and germ tube growth by ascospores of Mycosphaerella citri deposited 4 hr later on the treated lower leaf surface. However, oil sprays applied even as early as 56 days before inoculation reduced appressoria formation and the number of subsequent hyphal penetrations into the mesophyll. Hyphal penetration was also reduced by oil sprays applied 14 days after inoculation. The level of disease control, however, was always much greater than could be attributed solely to reduced hyphal penetration. Furthermore, oil sprays still controlled greasy spot even when applied after the fungus had invaded the spongy mesophyll. This suggests that the main action of oil in controlling greasy spot is by its inducing greater host resistance, thereby preventing or delaying hypertrophy of the spongy mesophyll.

Additional keywords: perithecial development, benomyl, copper fungicide.