Phytopathology (2010) 100, 246-251

David M. Gadoury, Belachew Asalf, M. Catherine Heidenreich, M.L. Herrero, M.J. Welser, Robert C. Seem, Anne Marte Tronsmo and Arne Stensvand (2010)
Initiation, development, and survival of cleistothecia of Podosphaera aphanis and their role in the epidemiology of strawberry powdery mildew
Phytopathology 100 (3), 246-251
Abstract: A collection of four clonal isolates of Podosphaera aphanis was heterothallic and was composed of two mutually exclusive mating types. Cleistothecial initials ~20 to 30 μm in diameter were observed within 7 to 14 days after pairing of compatible isolates and developed into morphologically mature ascocarps within 4 weeks after initiation on both potted plants maintained in isolation and in field plantings in New York State and southern Norway. Ascospores progressed through a lengthy maturation process over winter, during which (i) the conspicuous epiplasm of the ascus was absorbed; (ii) the osmotic potential of the ascospore cytoplasm increased, resulting in bursting of prematurely freed spores in water; and, finally, (iii) resulting in the development of physiologically mature, germinable, and infectious ascospores. Release of overwintered ascospores from field collections was coincident with renewed plant growth in spring. Overwintered cleistothecia readily dehisced when wetted and released ascospores onto glass slides, detached strawberry leaves, and leaves of potted plants. Plant material exposed to discharged ascospores developed macroscopically visible mildew colonies within 7 to 10 days while noninoculated controls remained mildew free. Scanning electron and light microscopy revealed that cleistothecia of P. aphanis were enmeshed within a dense mat of hyphae on the persistent leaves of field-grown strawberry plants and were highly resistant to removal by rain while these leaves remained alive. In contrast, morphologically mature cleistothecia on leaves of nine deciduous perennial plant species were readily detached by simulated rain and seemed adapted for passive dispersal by rain to other substrates. Contrary to many previous reports, cleistothecia appear to be a functional source of primary inoculum for strawberry powdery mildew. Furthermore, they differ substantially from cleistothecia of powdery mildews of many deciduous perennial plants in their propensity to remain attached to the persistent leaves of their host during the intercrop period.
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Database assignments for author(s): Belachew Asalf, Arne Stensvand

Research topic(s) for pests/diseases/weeds:
population dynamics/ epidemiology
general biology - morphology - evolution

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Pest and/or beneficial records:

Beneficial	Pest/Disease/Weed	Crop/Product	Country	Quarant.
	Podosphaera aphanis	Strawberry (Fragaria)	Norway
	Podosphaera aphanis	Strawberry (Fragaria)	U.S.A. (NE)