Geographic and climatic discontinuity in production of cleistothecia in Podosphaera aphanis
K. A. BEKOSCKE (1), B. Asalf (2), A. Stensvand (2), A. M. Tronsmo (2), R. Seem (1), N. Peres (3), L. Cadle-Davidson (4), M. T. Brewer (5), D. Gadoury (1). (1) Cornell University, Geneva, NY, U.S.A.; (2) Bioforsk, Aas, Norway; (3) University of Florida, Wimauma, FL, U.S.A.; (4) USDA-ARS GGRU, Geneva, NY, U.S.A.; (5) University of Georgia, Athens, GA, U.S.A.
Although <i>Podosphaera aphanis</i> occurs commonly throughout the range of strawberry production, cleistothecia are irregularly reported - particularly from relatively warm climates. Strawberry leaves with mildew colonies representing populations of <i>P. aphanis</i> were collected across a climatic temperature gradient of the continental U.S. Extracted DNA was then subjected to a PCR-based assay to detect the MAT1-1 and MAT1-2 mating types. Isolates representing each mating type were detected among nearly all samples. In parallel work, temperatures above 13C to 15C strongly suppressed ascocarp initiation when compatible isolates were paired on strawberry leaves. Absence of cleistothecia across the range of strawberry production does not appear to be due to the absence or unequal distribution of mating types of the pathogen, but to suppression of ascocarp initiation by high temperatures in warm climates, in glasshouses, or in high-tunnel production systems. Ascocarps can be expected to form rapidly in such environments if and when temperatures fall below 15C.
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