Ascospore Deposition and Epiphytic Growth in Relation to Fungicide Timing for Control of Greasy Spot Rind Blotch Caused by Mycosphaerella citri.

Greasy spot rind blotch is a serious problem in Florida for the production of grapefruit (Citrus paradisi) for the fresh market. In the 1970s to the early 1980s, the disease was described in detail and the cause was determined to be Mycosphaerella citri, the same species responsible for greasy spot of foliage. The most appropriate timing for fungicide sprays was determined at that time, but peak ascospore release has changed in recent years. In the present study, the relationship of ascospore deposition and fungal growth on fruit was determined in order to more accurately time fungicide applications. Infection of fruit appears to occur similarly to that of leaves: by deposition of ascospores and germination to produce epiphytic growth followed by penetration of the fungus through stomata. Ascospore deposition occurred mostly in May and June, but epiphytic growth began only after the onset of the summer rainy season in June in 2002 and 2003. Ascospore deposition was lower in 2002 than in 2003, but development of epiphytic growth was similar in both years. Timing of fenbuconazole sprays was evaluated in the 2001, 2002, and 2003 seasons. Of the single-spray applications, those in July were the most effective, sprays in June and August were moderately effective, and those made in May or September were ineffective. Two- and three-spray programs from June through August were usually more effective than single sprays, and four monthly sprays from May to August were needed for a high level of control. Fungicide applications are needed about every 3 to 4 weeks after the beginning of the rainy season in June through August for a high level of control of rind blotch.