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Incubation of excised apothecia enhances ascus maturation of Sclerotinia sclerotiorum

     Department of Plant Pathology, University of California at Davis, Salinas, California 93905

Y.-L Peng ¹

     The State Key Laboratory for AgroBiotechnology, China Agricultural University, Beijing 10094, China

Q.-M Qin
K.V. Subbarao ²

     Department of Plant Pathology, University of California at Davis, Salinas, California 93905

Synchronized maturation of ascospores of Sclerotinia sclerotiorum is desirable for establishing a transformation system, conducting genetic analyses of the pathogen, defining the precise epidemiological roles of ascospores and screening plant germplasm for resistance. In general, fresh apothecia collected from germinated sclerotia contained primarily immature or discharged asci. This study was undertaken to investigate whether maturation of asci and ascospores could be enhanced by incubation of excised apothecia and to determine the effects of factors such as temperature, excision time, light and ventilation on maturation of asci and ascospores in excised apothecia. Maturation of asci was compared between intact and excised apothecia that were incubated under similar conditions. Results demonstrated that temperature was an important factor affecting ascus maturation of S. sclerotiorum during incubation of excised apothecia, and the optimum temperature was around 21 C. After incubation at 21 C for 30 h, the percentage of undischarged mature asci in excised apothecia increased up to 70–80%. This increase was accompanied by a significant increase in ascospore production of up to 5 x 10⁵ ascospores per apothecium. Detailed time course studies indicated that mature asci peaked at 30–36 h of postexcision incubation. Mature asci and the number of ascospores were higher in open incubation than in closed incubation, suggesting that accumulation of volatile substances was not required for ascus/ascospore maturation during postexcision incubation and ventilation could enhance the maturation process. Light also did not affect the maturation of asci during the incubation of excised apothecia. Germination rates for ascospores from excised apothecia under various treatments were similar to those from untreated apothecia but declined slightly with time postexcision. The incubation of excised apothecia promoted ascus maturation compared with intact apothecia.

Key words: ascospore delineation, ascospore germination, epidemiology, fungal biology, light, temperature, ventilation