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Mating and Pseudothecial Development in Mycosphaerella citri, the Cause of Citrus Greasy Spot

Greasy spot, caused by Mycosphaerella citri, is a serious disease of citrus in the Caribbean basin. M. citri is a loculoascomycete and produces pseudothecia in decomposing leaves after intermittent wetting and drying. A new in vitro mating technique was developed for production of pseudothecia on sterilized leaf disks in petri dishes. Of the single-ascospore cultures that were recovered from individual asci, four were one mating type and four were a second mating type (tentatively designated mat+ and mat-), indicating that M. citri probably is heterothallic and bipolar like most other loculoascomycetes. Most populations of ascospores recovered from individual leaves or from leaves from groves of different citrus species and various locations had a 1:1 ratio of mating types consistent with random mating. Cytological studies demonstrated that the ontogeny of pseudothecial development was similar to other loculoascomycetes. The formation of mature pseudothecia required 30 to 45 cycles of wetting and drying of infected, dead leaves which required ≈60 to 90 days. The in vitro system for pseudothecial production and the knowledge of the mating system in M. citri will facilitate genetic studies of this important pathogen.