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Title: Temperature regulates the initiation of cleistothecia in powdery mildew of strawberry

Author
item ASALF, BELACHEW - Norwegian University Of Life Sciences
item GADOURY, DAVID - Cornell University
item TRONSMO, A - Norwegian University Of Life Sciences
item SEEM, ROBERT - Cornell University
item Cadle-Davidson, Lance
item BREWER, MARIN - University Of Georgia
item STENSVAND, ARNE - Norwegian Institute For Food Research

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/29/2013
Publication Date: 7/1/2013
Citation: Asalf, B., Gadoury, D., Tronsmo, A.M., Seem, R., Cadle Davidson, L.E., Brewer, M., Stensvand, A. 2013. Temperature regulates the initiation of cleistothecia in powdery mildew of strawberry. Phytopathology. 103:717-724.

Interpretive Summary: Sexual reproduction by the strawberry powdery mildew fungus (Podosphaera aphanis) is widespread and provides a means for genetic recombination and formation of overwintering structures. However, in some production regions, particularly in warmer climates, sexual reproduction is reportedly rare. We confirmed that two mating types are required for sexual reproduction, and we identified DNA markers associated with each mating type. Strawberry powdery mildew primarily reproduces asexually during the growing season, producing hordes of identical clones. However, when both mating types were present at low temperatures (13 deg C or lower), we observed a preferential shift to sexual reproduction. This strategy may represent a selective advantage for strawberry powdery mildew: the fungus continues clonal reproduction until an environmental cue (decreasing temperature) triggers development of the overwintering stage.

Technical Abstract: The formation of cleistothecia 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, they are reportedly rare. We confirmed that the pathogen is heterothallic, and that initiation of cleistothecia is not only dependent upon the presence of isolates of both mating types, but that initiation is largely suppressed at temperatures above 13 deg C. Compared to incubation at a constant temperature of 25 deg C, progressively more cleistothecia were initiated when temperatures were decreased to 13 deg C for progressively longer times. At lower temperatures, production of cleistothecia was associated with a decline, but not total cessation of conidial formation, and pairings of compatible 24 isolates sporulated abundantly at 25 deg 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 cleistothecia. The geographic discontinuity of cleistothecia production and the sporadic and seemingly unpredictable appearance of cleistothecia in P. aphanis are possibly due to the combined influence of heterothallism and suppression of cleistothecia formation by high temperatures.