MN Fruit & Vegetable IPM News - Vol. 1, No. 2

Editor’s note:Although the focus of this article is primarily field crops, we believe the relationships discussed will be useful to several vegetable and fruit crops.

Temperatures are expected to drop below freezing in areas of North Dakota and NE Minnesota tonight (5/12) and later this week. What type of damage will result with some of our crops that are just emerging, and in vulnerable stages, subject to frost damage? Temperatures below 32F will cause water in plant cells to freeze and resultant ice crystals will kill cells by damaging cell membranes. How different crop species react to freezing temperatures depends on where growth is taking place, where growing points are, and if cells have built in systems to prevent ice crystal formation.

Seedlings hardened by continuous low night and day temperatures are more resistant than seedlings hardened by alternating high and low day and night temperatures. The cool nights we have had recently may have helped in the hardening process.

There are considerable variety differences in all crops and no research has been done on the varieties we are currently growing.

CORN

Corn plants less than 6 inches tall (V-5) or less will recover from frost because the growing point is still below the soil surface and usually not damaged. Lethal cold temperature is a concern since a corn plant's growing point region is relatively protected from the effects of simple frost while it remains below the soil surface. Lethal cold temperatures (28 F or less) can penetrate the upper inch or two of soil, especially dry surface soils, and kill plant tissue directly, including coleoptiles and growing points. Hopefully the rains of last night and today have wet the soils adequately to provide a buffering effect to the cold penetration.

Non-lethal injury by cold temperatures may cause deformed elongation of the mesocotyl or physical damage to the coleoptile in non-emerged seedlings, resulting in the proverbial "cork-screw" symptom and subsequent leafing out underground.

Historically very few corn fields have been destroyed by spring freezes. Very limited research has been done on this subject. Damage to seedlings could be (1) complete killing (2) injury so severe that the resulting seedling is weakened to the point where it will never develop normal reproductive systems, or (3) injury evident, but seedlings remain vigorous and complete recovery can be expected.

Environmental conditions before or immediately after a low temperature greatly influence the extent of freezing injury. If the temperature drop is gradual, plants are in better condition to resist injury and can stand surprisingly low temperatures. Similarly, rising temperatures after a frost and satisfactory soil moisture conditions are desirable to aid recovery. Drought, wind and high evaporation are likely to aggravate the frost injury and lessen the chances of recovery.

SMALL GRAINS

Many northern cereal grains will lose leaf tissue that freezes. New growth will follow at the growing point before jointing and is protected below ground. In some cases the eventual maturity date may be delayed. In some areas this has already occurred and new leaves have emerged and the crop is growing. However, several frosts and destroyed leaf tissue can and will greatly weaken the growing cereal grains and may result in yield reduction.

Broad-leaf crops that have their growing point at the top of the plant are more susceptible to frost damage than our grass species. Soybeans, for example, are quite sensitive to frost. Dicotyledonous plants such as sugar beet, sunflower, soybean, dry bean, and flax are more easily killed by frost than are small grains.

SOYBEANS

Soybeans are easily damaged by frost in the 28 to 32F range. Temperatures of 28F for any extended period of time can completely kill soybean plants (stems and leaves). During the early seedling stage (VE to VC), soybeans have some tolerance to temperatures of 29-30F for short periods of time. If the seedlings have been somewhat hardened off by cool temperatures for several days, then temperatures as cool as 28 F can be tolerated. Once true leaves emerge (V1 and V2) soybeans become more susceptible to freezing temperatures below 32F for any extended period of time. Unifoliolate leaf stage is slightly more frost tolerant than first or second trifoliolate stages.

ALFALFA

Alfalfa will be damaged by temperatures in the mid to low 20's. Growth of alfalfa is from the tip of the stem. With frost damage, the top will bend over and growth of the tip will cease. Plan to take the first harvest as soon as the field dries up enough for good equipment performance.

OTHER CROPS

Temperatures at 32F and below will kill buckwheat or drybeans, while canola and crambe will tolerate temperatures in the low 20's. Crops such as field peas or lentils have good frost tolerance since growing points remain below ground in the seedling stages.

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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 14, 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.