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Predicting emergence of hop shoots systemically infected by Pseudoperonospora humuli in Wisconsin using a simple degree-day model

Michelle Marks: University of Wisconsin-Madison

<div>Hop (<em>Humulus lupulus</em>) downy mildew, caused by the oomycete <em>Pseudoperonospora humuli</em>, is the most important disease infecting hops in Wisconsin (WI) and many other temperate growing regions. The pathogen overwinters in the perennial hop rootstock and emerges in spring from systemically infected shoots, or ‘basal spikes’. A predictive model based on growing degree-days (GDDs) calculated from air temperatures was developed in the Pacific Northwest (PNW). A threshold of 111.3 GDDs was determined to accurately predict basal spike emergence within 5 days. This threshold was calculated with a base temperature of 6.5°C and initiated on a date after which significant cold periods no longer persisted. Our goal was to validate this threshold for use in WI and, if necessary, develop a new threshold. Air temperatures and observations of basal spikes (weekly from late March-early May) were collected at 4 commercial hop yards during 2015-17. GDD accumulations were calculated for each location, and the day of year (DOY) at which the threshold was met for each location/year combination was calculated and compared to the date of actual observed spike emergence. For all years and locations, the DOY at which 111.3 GDDs was met occurred within 6 days of spike emergence (dates ranged from 18 April to 7 May). We determined that the PNW model using a 111.3 GDD threshold for basal spike emergence is valid for use in WI and can help growers enhance early season disease management.</div>