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 versus vegetable oil

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 an aqueous organic mixture, steeped for 10 h, of composted cornmeal, humic acid, molasses, and fish oil and an aqueous suspension of 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. The organic mixture 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. 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 fruit yield was not consistent in terms of treatment effects, the organic mixture and vegetable oil treatments each protected foliage from greasy spot infection to limited extents during years with wet springs. The greatest protection was observed during a year with a relatively dry spring.