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Two Degree-Day Models for Predicting Initial Emergence of Hop Shoots Systemically Infected with Pseudoperonospora humuli. Dennis A. Johnson, Plant Pathologist, Washington State University, Irrigated Agriculture Research and Extension Center, Prosser 99350. Plant Dis. 75:285-287. Accepted for publication 5 September 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0285.

Two degree-day models were developed to predict the appearance in the spring of the first hop (Humulus lupulus) shoots systemically infected with Pseudoperonospora humuli in commercial hopyards of the cluster cultivars in the Yakima Valley of Washington. One model was based on total degree-days above air temperatures of 6.5 C accumulated from the time when temperatures reached a threshold after 1 February until the day before the emergence of the first systemically infected shoot from perennial hop crowns. A mean of 111.3 (range 30.5) accumulated degree-days was derived from 11 yr of field data, and the model effectively predicted the first appearance of systemically infected hop shoots in the Yakima Valley. A second model developed from 5 yr of data on soil temperatures calculated degree-days by summing soil temperatures above 6.5 C and was also an effective predictor of the emergence of the first systemically infected hop shoot. The soil temperature model had a mean of 88.7 (range 12.6) accumulated degree-days. These models can be used as part of a disease forecasting system to determine when monitoring of hopyards for downy mildew should begin in order to manage the disease in Washington hopyards.