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Temperature Regulates the Initiation of Chasmothecia in Powdery Mildew of Strawberry

First and third authors: Department of Plant and Environmental Sciences, Norwegian University of Life Sciences, 1432 Ås, Norway; first and seventh authors: Norwegian Institute for Agricultural and Environmental Research (Bioforsk), Plant Health and Plant Protection Division, 1432 Ås, Norway; second and fourth authors: Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva, New York 14456; fifth author: United States Department of Agriculture–Agricultural Research Service, Grape Genetics Research Unit, Geneva, NY 14456; sixth author: Department of Plant Pathology, University of Georgia, Athens 30602.

The formation of chasmothecia by the strawberry powdery mildew pathogen (Podosphaera aphanis) is widespread but often sporadic throughout the range of strawberry cultivation. In some production regions, notably in warmer climates, chasmothecia are reportedly rare. We confirmed that the pathogen is heterothallic, and that initiation of chasmothecia is not only dependent upon the presence of isolates of both mating types but also largely suppressed at temperatures >13°C. Compared with incubation at a constant temperature of 25°C, progressively more chasmothecia were initiated when temperatures were decreased to 13°C for progressively longer times. At lower temperatures, production of chasmothecia was associated with a decline in but not total cessation of conidial formation, and pairings of compatible isolates sporulated abundantly at 25°C. We developed mating-type markers specific to P. aphanis and used these to confirm the presence of both mating types in populations that had not yet initiated chasmothecia. The geographic discontinuity of chasmothecia production and the sporadic and seemingly unpredictable appearance of chasmothecia in P. aphanis are possibly due to the combined influence of heterothallism and suppression of chasmothecia formation by high temperatures.

Additional keywords: Fragaria × ananassa, Sphaerotheca macularis.