Poster Session: Cropping Systems-Sustainability

470-P

Early-season cryptic development of powdery mildew (Podosphaera aphanis) in June bearing strawberries.
B. ASLAF (1), D. M. Gadoury (2), R. C. Seem (2), A. Tronsmo (3), A. Stensvand (3)
(1) Norwegian University of Life Sciences, Aas, Norway; (2) Cornell University, Geneva, NY, U.S.A.; (3) Bioforsk, Aas, Norway

Powdery mildew of strawberry (Podosphaera aphanis) is often described as a late-season disease, but ascospore discharge can occur as plants emerge from dormancy. Our hypothesis was that epidemics may enter a cryptic phase when environmental conditions do not favor sporulation, wherein the pathogen spreads but is not macroscopically visible. Shortly after snow-melt in 2010 and 2011, we inoculated emergent leaves with either ascospores or conidia, and assessed pathogen growth and sporulation starting one week after inoculation. Favorable periods for pathogen growth and sporulation, defined by both accumulated rain-free days and days with average temperatures > 10 °C, were significantly correlated with disease incidence (p < 0.05). In 2010, only 5 favorable days occurred before sporulation was detected 30 days after inoculation. In 2011, 11 favorable days occurred, and sporulation was detected on day 17 after inoculation. Macroscopically indistinct mycelial growth was confirmed 7 days after inoculation in both years. Spread of P. aphanis can precede macroscopic signs by several weeks. Two possible management options are suggested for further evaluation, and will be reported: (i) suppression of the cryptic phase from the earliest stages of crop growth, and (ii) initiation of suppression timed to precede forecasted sporulation, as contrasted with responding to macroscopically visible disease.