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Idaho Hop Powdery Mildew Project


Read on for information about the Idaho Hop Powdery Mildew Project and the powdery mildew forecasting
The US Hop Industry has funded research to develop a powdery mildew forecasting model that was validated in Oregon and Washington during the 2001growing season. On average, during the 2001 field season, growers using the model made one less pesticide application and had 60% less incidence of powdery mildew in hop cones. However, no data is available on the utility of this model for Idaho hop Growers.

The Idaho Hop Commission in collaboration with Dr. Jim Barbour Research Entomologist with University of Idaho at the Parma Research and Extension Center, in Parma ID, and Dr. Walt Mahaffee, Research Plant Pathologist at the USDA/ARS Horticultural Crops Research Laboratory in Corvallis, OR have been awarded by the state of Idaho funds targeted at validating the powdery mildew forecasting model for use by Idaho hop growers. These funds were made available to Idaho through the Federal Emergency Agricultural Assistance Act of 2001 to help specialty crop producers in Idaho.

The input variables for the model are temperature and relative humidity data at 15 minute intervals, and daily rainfall totals. Collection of this data for model development and data delivery requires remotely accessible weather data. Weather stations have been installed at three locations in the hop producing region of Southern Idaho. Weather data is currently being relayed to a base station in California owned by Fieldwise (FieldWise.com) who have been contracted to to manage and archive the weather data and produce the web pages for access to model results, and archived data. The forecasting information is calculated daily and uploaded to a web page for distribution. (click here to view the webpage)

The start of the model is triggered by either 6-12 in of growth from 50% of the hills at bud break or 6-15 in of regrowth after spring pruning. The infection index increases by 20 points on days where there is:

1) a minimum of six continuous hours of temperatures between 60° F and 80° F;

2) less than 6 hours with temperatures above 86° F; and

3) less than 1 in of rainfall on that day.

On days when these three conditions are not met, 10 points are subtracted from the index. If after subtracting a day's points the index is less than zero, reset the index to zero. If after adding a day's points the index is greater than 100, reset the index to 100. On any one day the index should not decline by more than 10 points or increase by more than 20 points.

The index value is used to determine the spray interval (number of days) between applications. The spray interval is dependent on the index and fungicides being employed. An index of 30 or less indicates that disease pressure is low and that a spray interval can be stretched to the label maximum. An index of 40 to 50 indicates that a spray interval can be of intermediate length. An index of 60 to 100 indicates spray intervals should be shortened to the label minimum due high disease pressure.



View the latest forecast information at: Idaho Hop Powdery Mildew Project

Below is information provide by USDA and Wilbur-Ellis explaining how the risk index is to used in the hop growing region of Washington. Use this as a guide in interpreting the index shown above for the Idaho hop growing region. To print a pdf version of this file for your use click the printer icon in the menu bar of the window below. If your browser does not support frames (you see the frame support message below instead of the pdf file in the frame below) click here to download the pdf file: Hops Infection Risk Forecaster Information Page. You will need a copy of Adobe Acrobat reader to view the file.