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Cold induced disease resistance may explain unexpected stalling of foliar epidemics of hop powdery mildew (Podosphaera macularis)
Bill Weldon: Cornell University; David Gadoury: Cornell University; Lance Cadle-Davidson: USDA Grape Genetics Research Unit
<div>Epidemics of powdery mildew (<i>Podosphaera macularis</i>) on hop foliage are typified by an initial exponential increase of incidence, while severity, <i>i.e.,</i> size of mildew colonies, remains relatively constant, despite daytime temperatures optimal for pathogen growth. A similar pattern reported in grapevine powdery mildew (<i>Erysiphe necator</i>) was associated with cold-induced disease resistance (Phytopathology 100:1240-1249). We investigated the impact of both pre- and post-inoculation cold events on susceptibility of hop to <i>P. macularis</i>. Exposing young, ontogenically susceptible hop leaves to a brief (1-4 hour) cold event (4°C) 24 hours before inoculation reduced <i>P. macularis </i>hyphal density by 26.6% and colony formation success by 16.5%. A subsequent time course study involving acute cold events suggested that the magnitude of the effect of pre-inoculation cold events was maximized 36 hours after cold exposure. Sequential overnight acute cold events applied post-inoculation extended the latency period by approximately 20% and, 24 hours after initiation of sporulation, conidia production per colony was reduced by 72.8%.<i> </i>The discovery of a parallel induction of disease resistance by brief overnight cold events in both grapevine and hop powdery mildew pathosystems suggests that host responses to acute nighttime radiational cooling might not only explain stalling of powdery mildew epidemics in grapevine and hop, but also extend to powdery mildews of other crops.</div>

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