Epidemiology and Control of Citrus Greasy Spot in Different Citrus-Growing Areas in Florida

Greasy spot, caused by Mycosphaerella citri, causes defoliation and yield losses on grapefruit in all areas of Florida, but is more severe in southwest Florida and the east coast than in central Florida. The amount of leaf litter, numbers of ascospores produced, and severity of greasy spot on trap plants were monitored throughout 1997 and 1998 in Immokalee (southwest Florida) and Lake Alfred (central Florida). Leaf litter and ascospore production were greatest from March to July in both locations, with little litter and few ascospores thereafter. Ascospore production occurred earlier in Immokalee than in Lake Alfred in both years. Disease on trap plants was moderate to severe throughout the year except from November to February. Large numbers of ascospores produced early in the year when conditions were less favorable resulted in the same disease severity levels as low numbers of ascospores produced later in the year when environmental conditions were favorable. Greater greasy spot severity in southwest Florida, compared with central Florida, is more likely due to higher rainfall and warmer winter temperatures than to differences in time of infection. Single annual copper fungicide applications were made each month from April to August in 1998 and 1999 in LaBelle (southwest Florida), Ft. Pierce (east coast), and Lake Alfred to determine the most effective time of application. Two two-spray treatments, May + July and June + August, were also evaluated in 1999. A single copper fungicide application in June provided the most consistently effective control across all locations. The June + August two-spray treatment was very effective in disease control, but usually no better than a well-timed single application.