Epidemiology and Control of Citrus Greasy Spot on Valencia Orange in the Humid Tropics of Costa Rica.

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An epidemiological study of Mycosphaerella citri, the cause of greasy spot of citrus, was conducted for 2 years at Finca 6 and El Parque, Costa Rica. Ascospores were the primary source of inoculum; only a few conidia were trapped during the study. Ascospores were trapped first during late April or early May. The spore discharge pattern was associated closely with seasonal rainfall distribution; in both years, the number of ascospores trapped increased rapidly through May, peaked during early June, declined rapidly through July, and were negligible during the rest of the year. Most ascospores were trapped during a 6- to 8-week period. Three principal discharge patterns were observed. One pattern, associated with rainfall, was characterized by the release of large numbers of ascospores within 1 h of the beginning of rainfall. Ascospore release often continued for several hours after the rainfall ended. A second discharge pattern was associated with dew. Large numbers of spores were trapped during periods when dew was heaviest (0200 to 0700 h). A third pattern consisted of a combination of the first two patterns (i.e., discharge during evening rain followed by a second discharge associated with dew). No relationship was found between different combinations of rainfall data and either the first or peak ascospore catch. Similarly, there was no consistent relationship between weekly percentages of leaves at different stages of decomposition and ascospore catch that could be used to predict the period of peak ascospore discharge. Weekly total number of leaves on the orchard floor was a better predictor of total ascospore catch. Two sprays of a copper fungicide reduced greasy spot incidence, severity, and defoliation compared to an unsprayed control. Oil, sprayed twice as a standard grower treatment, did not reduce disease incidence or severity compared to the unsprayed control.