MN Fruit & Vegetable IPM News - Vol. 1, No. 3

Traditionally, most growers have used a calendar or a phenological schedule to determine pest events and time pest control in their orchards. The calendar method means spraying on a weekly schedule without monitoring fruit or pest development. The phenological schedule takes into account the developmental stage of the plant, such as pink or petal fall. The calendar method is grossly inaccurate since insect development may not occur at the same calendar dates each year. The phenological method, while more accurate, assumes incorrectly that pest populations peak at the same stage of host development every year. A more scientific and accurate method of forecasting pest development and orchard events based on degree-day accumulations is required in an IPM program. Pest forecasting allows the grower to predict when specific pests will most likely occur in the orchard and cause economic losses. The degree-day concept remains the best way to predict pest development and other biological events in the orchard. The basis of the degree-day concept lies in the fact that biological events are regulated by environmental factors such as temperature, humidity, and rain. For instance, there are a minimum number of heat units (degree days) that must accumulate before the spring emergence of redbanded leafroller.

Information for calculating degree days can be obtained from the nearest weather station, local climate and weather web sites, by recording with a thermometer the daily minimum and maximum temperatures in a shaded area and calculating the degree-day units by hand, or by using computerized degree-day equipment. Adding up degree-day (DD) accumulations by hand is a little time consuming, but very easy to do. First, take the day's maximum temperature (Tmax) and add the day's minimum temperature (Tmin), then divide the result by two and subtract the base temperature. The example below uses base 50°F to calculate DD:

DD = (Tmax + Tmin) divide by 2, minus 50

Example: The lowest and highest temperatures recorded at Jane’s Orchard in Brooklyn Park, MN on 5/14/04 were 60°F and 85°F, respectively. What was the DD accumulation for that day at base 50°F?

DD = (85 + 60) divided by 2, minus 50

= 72.5 -50

= 22.5 DD

Make this calculation every day, continuing to add the degree day totals.

A computerized method of calculating accumulated degree-days can be done with a portable weather data logger, such as the "Leaf Wetness / Temperature Logger" manufactured by Spectrum Technologies Inc. (see photo). This logger has the ability to collect site-specific weather data (leaf wetness and temperature). The equipment has a long life battery and can be placed in the orchard on a stand or directly in a tree. Data accumulated by the logger can be periodically downloaded (once or twice per week) directly to a disc or to a computer hard drive. A computer software program makes the calculation of degree days and disease forecasting. Spectrum Technologies has developed software programs that will run several disease models including: apple scab, fire blight, and powdery mildew. Both the apple scab and fire blight models have been evaluated in Minnesota. The portable electronic weather data logger is a vast improvement over the old mechanical leaf wetness, temperature, and humidity recorders. The logger uses two methods to calculate degree days. The High-Low-Average method considers only the high and low temperatures for the day and applies a sine wave approximation to compute the total number of degree days acquired. The Integrated method computes the actual number of degree days acquired based on an hourly average temperature.

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ACKNOWLEDGEMENTS

Co-Editors: Bill Hutchison, Department of Entomology, University of Minnesota, hutch002@umn.edu
Jeanne Ciborowski, Minnesota Department of Agriculture, Ag. Resources Division,jeanne.ciborowski@state.mn.us
Suzanne Wold-Burkness, Department of Entomology, University of Minnesota, woldx018@umn.edu

The Newsletter is published weekly from May through August, cooperatively, by the Minnesota Department of Agriculture (MDA) and the University of Minnesota (U of MN). Reports are posted on the U of MN and MDA web sites on Fridays. If you have suggestions and/or comments, please send your contributions by 4 p.m., Tuesday to Jeanne Ciborowski, 651-297-3217, jeanne.ciborowski@state.mn.us , MDA, 90 W. Plato Blvd., St. Paul, MN 55107-2094. You can access the Newsletter at the U of MN web site in htm. format at: www.vegedge.umn.edu/MNFruit&VegNews/mnindex.htm and at the MDA web site in pdf. format at: www.mda.state.mn.us/biocon/fruitreports/default.htm

Partial funding for this publication is provided through partnership agreements with the Minnesota Fruit and Vegetable Growers Association and the United States Department of Agriculture – Risk Management Agency (RMA) and the RMA Community Outreach and Assistance Partnership Program. These institutions are equal opportunity providers.

DISCLAIMER

References to products in this publication is not intended to be an endorsement to the exclusion of others which may have similar uses. Any person using products listed in this publication assumes full responsibility for their use in accordance with current manufacturer directions.

Last Revised May 21, 2004.
The University, including the Minnesota Extension Service, is an equal opportunity educator and employer. ©1999-2004 Minnesota Extension Service, University of Minnesota. All rights reserved. Contact copyright@extension.umn.edu for information on reproduction or use of this material.