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ARS Home » Plains Area » Kerrville, Texas » Knipling-Bushland U.S. Livestock Insects Research Laboratory » LAPRU » Research » Publications at this Location » Publication #336047

Research Project: Cattle Fever Tick Control and Eradication

Location: Livestock Arthropod Pests Research

Title: Suppression of greasy spot disease caused by Mycosphaerella citri Whiteside on grapefruit trees in an organic orchard using an aqueous organic mixture of composted cornmeal, humic acid, molasses, and fish oil vs vegetable oil

Author
item Showler, Allan

Submitted to: Crop Protection Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/13/2017
Publication Date: 5/30/2017
Citation: Showler, A. 2017. Suppression of greasy spot disease caused by Mycosphaerella citri Whiteside on grapefruit trees in an organic orchard using an aqueous organic mixture of composted cornmeal, humic acid, molasses, and fish oil vs vegetable oil. Crop Protection Journal. 99:137-143. https://doi.org/10.1016/j.cropro.2017.05.016.
DOI: https://doi.org/10.1016/j.cropro.2017.05.016

Interpretive Summary: Greasy spot fungal disease of citrus afflicts citrus trees in all citrus-growing areas of the United States and elsewhere in the Western Hemisphere, causing premature defoliation, blemished fruit, and reduced tree vigor, yield, and fruit size. We investigated the effects of compost extract and vegetable oil on greasy spot suppression in an organic grapefruit orchard in South Texas. The extent of greasy spot infection on each tree was consistently reduced by each of the oil and by compost tea throughout much of each of three growing seasons. Compost extract had a more consistent, though lesser, effect on greasy spot reduction when precipitation was relatively high in the spring than when spring rains were relatively light. Heavier spring rains were associated with spread of infection, but light spring rain failed to trigger spread until the arrival of late summer rains. In four of five field experiments, late season bare patches on the tree canopies were more extensive on the control trees than on treated trees. While our fruit yield data was not consistent in terms of treatment effects, compost extract and vegetable oil each protected foliage to a limited extent during years with wet springs, but the greatest protection was observed during a year with a relatively dry spring. Possible mechanisms for reported effects are discussed.

Technical Abstract: Greasy spot disease of citrus, caused by the fungus Mycosphaerella citri Whiteside, afflicts citrus trees in all citrus-growing areas of the United States, eastern Mexico, Central America, and the Caribbean islands, causing premature defoliation, blemished fruit, and reduced tree vigor, yield, and fruit size. This three-year study investigated the effects of organic, nonconventional tactics using compost extract and vegetable oil, separately and in combination, for greasy spot suppression in an organic grapefruit, Citrus paradisi Macfad., orchard in the Lower Rio Grande Valley of Texas. The extent of greasy spot infection on each tree was consistently reduced by each of the treatments throughout much of every growing season. Compost extract had a more consistent, though lesser, effect on greasy spot reduction when precipitation was relatively high in the spring than when spring rains were relatively light. Heavier spring rains were associated with concurrent spread of infection, but light spring rain failed to trigger spread until the arrival of late summer rains. In four of five field experiments conducted during this study, late season bare patches on the tree canopies were more extensive on the control trees than on treated trees. While our fruit yield data was not consistent in terms of treatment effects, compost extract and vegetable oil each protected foliage to a limited extent during years with wet springs, but the greatest protection was observed during a year with a relatively dry spring. Possible mechanisms for treatment effects are discussed.