MN Fruit & Vegetable IPM News - Vol. 4, No. 4

Memorial Day weekend brought rainfall or high humidity to Minnesota. Although strawberry growers were delighted to see the rain, the high humidity was perfectly timed for causing gray mold outbreaks in June.

In order to infect a fruit, the gray mold or botrytis spore must move from a dead or dying leaf to a blossom. Once on the flower, the spores germinate and grow into the stamens. After germinating, the botrytis stops growing until the berry ripens. As the berry turns red, botrytis grows through the fruit and produces the gray spores which give the disease its common name. Gray spores can move to nearby ripe fruit during humid weather, creating devastating secondary infections.

The botrytis spores only infect flowers during wet, mild weather. Spores need a relative humidity above 80%, moisture on the flower and temperatures between 60º and 80ºF in order to germinate. Many parts of central Minnesota only saw light showers this past week. A light shower in the evening which causes the leaves to become wet overnight can cause a gray mold outbreak, especially if the overnight low stays above 60ºF. Dew rarely allows gray mold to germinate, because the temperature is too cold during dewfall. Irrigation does not allow gray mold to germinate if the leaves dry out before evening. Most of the state has had good weather for gray mold this past week.

If you are worried about gray mold, spray as soon as possible. Since gray mold is the most common strawberry disease worldwide, there are many different fungicides labeled for gray mold, including several organically approved fungicides.

Tarnished plant bug pressure has been quite high so far this year, with many fields above the threshold of one nymph per four clusters. Fields that were sprayed for clipper weevil in early bloom had no tarnished plant bug nymphs.

Table 1: Pests during the week of May 28 – all fields were between 5% and 60% bloom.

Fields Monitored	Fields sprayed previously	Fields with tarnished plant bug nymphs	Fields with thrips	Fields with Clipper
14	5	3 (2 above threshold)	4 (0 above threshold)	6

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ACKNOWLEDGEMENTS

Co-Editors: Bill Hutchison (hutch002@umn.edu), Department of Entomology, University of Minnesota, Jeanne Ciborowski, Minnesota Department of Agriculture, Ag. Resources Management and Development Division, and Suzanne Wold-Burkness (woldx018@umn.edu), Department of Entomology, University of Minnesota

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., Wednesday to Jeanne Ciborowski, 651-201-6217, jeanne.ciborowski@state.mn.us , MDA, 625 Robert St. North, St. Paul, MN 55155. 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/plants/pestmanagement/ipm/ipmnews.htm

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

DISCLAIMER

Reference 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, 2007 by woldx018@umn.edu
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