Issue # 2, May 6, 2004

• Frost Tolerance of Corn and Soybean

• Cutworms and Wireworms

• Growing Degree Days, What are They?

• Winter Injury of Evergreens

Frost Tolerance of Corn and Soybean

       As small grain planting is finishing up, producers are getting anxious to start with corn and soybeans.  There is some concerns with frost that should be addressed.  The average last 28 degree and 32 degree frost date for Cooperstown are May 11th and May 21 respectfully.
Corn:  The minimum germination temperature for corn is 50 degrees.  Corn can handle early season frosts due to the position of the growing point.  Corn has an elongating first internode and coleoptile method of seedling emergence.  Hence, the growing point is still below the soil surface and can regrow if injured by frost.  Also, this type of emergence allows for deeper planting depths up to five inches.
Soybeans:  The minimum germination temperature for soybean is 50 degrees.  Soybeans have an active hypocotyl method of seedling emergence.  This type of emergence, the growing point is above ground upon emergence.  Thus it is succeptible to frost once emerged.  Soybeans are easily damaged in the 28 to 32 degree range.  Temperatures of 28 degrees for any extended period of time can completely kill soybean plants (stems and leaves)  Recommended planting dates are from May 10th to May 25th.  One concern for 2004 is if the crop is injured by frost, is there enough seed of desired varieties available if reseeding is necessary.  This is one of the risks for early seeding.  

Cutworms and Wireworms

    Each year we see some problems caused by cutworms or wireworms.  The first thing growers notice is irregular spots in the field where the crop has been damaged or poor emergence.  Each insect will feed in a different way.
Cutworms:  Cutworms are the larval form of moths.  The larvae are greenish-brown to greenish-gray.  The head is pale brown with darker brown freckles.  Mature worms are 1 1/2 inches long.  The eggs were laid last summer or fall in the soil where corn, soybeans or sunflower are now being planted.  The eggs have hatched and the growing larvae are hungry!  When suspected cutworm damage is found, check for live cutworms around the damaged plants by digging in the soil at the base of damaged plants.  Count and record the number of damaged plants.  Note the size of the cutworms being found.  The treatment threshold has been reached if 3 to 6% of the plants are cut and small larvae (<3/4 inch) are present.  Consult the 2004 North Dakota Field Insect Management Guide for products registered for treatment.
Wireworms:  Wireworms are one of the most difficult insects to detect and one of the hardest to control.  They will completely consume the inside of a germinating seed.  It will also consume any fresh tissue of seedlings and can significantly reduce plant stands.  If you have seedling emergence that is somewhat irregular, start digging for wireworms in the soil around the seed or seedling.  Wireworms are elongate, heavily segmented, pale yellow to brown wire-like worms that vary in length for 1/2 to 1 1/2 inches.  They tend to prefer lighter textured soils.  In the spring, after overwintering in the soil, wireworms migrate closer to the soil surface and seek germinating seeds as a food source.  Consult the 2004 North Dakota Field Insect Management Guide for products registered for treatment.

Growing Degree Days, What are They?

    Growing degree days (GDD) correlates plant development with heat units (daily temperature extremes).  Based on plant emergence and historic temperature trends, GDD can be used to predict the time when a crop will reach a certain developmental stage.  The actual stage of a crop is best determined by visual evaluation, but GDD provides a very close estimate of developmental stage.
    Using heat units to predict plant development functions on the premises that the actual number of GDD for a crop to reach maturity remains relatively constant across environments even though calendar days may change substantially.
    GDD are based on daily high and low temperatures and are calculated for one day as follows: (high temp. + lo temp.)/2 - minimum base temp. = GDD in degrees Fahrenheit.  When the temperature falls below the minimum base temperature or exceeds the maximum base temperature for the crop in question, then the minimum or maximum base temperature is used in the calculation.  The following table lists the minimum and maximum temperatures for four crops.

    When using GDD to determine crop growth stage accumulation should start the day after planting.  Small grains require about 180 GDD for germination and emergence.  A wheat plant requires about 140 GDD for each leaf whether accumulated in four days or ten days, and about 2400 GDD to reach maturity, which can range from 83 - 100 calendar days.
    The North Dakota Agricultural Weather Network provides daily GDD.  Just select the weather station and enter the date after planting and the amount of GDD will be calculated.  Growing degree days calculated for crops should not be confused with insect degree days (DD or IDD).  Insect degree days start accumulating when the minimum temperature for the insect in question is reached.

Winter Injury of Evergreens

    Every year there are question about pine, spruce, and other evergreens turning brown in the spring.  The most common cause of browning in evergreens is drying of the foliage during winter and early spring.  If the tree's ability to pull water up from the frozen soil is not great enough to meet the needs of the foliage, that foliage will die from the tip back.  Often times, greater injury will be seen on the side of the evergreen facing the prevailing winds.  These winds can dry out foliage quickly, especially on warm, sunny winter days.
    Winter injury may be increased by non-hardy stock, previous drought conditions, herbicide injury, weak root systems, stress from insects and diseased, or other forms of stress.  To reduce the potential for winter injury in the future, water trees as needed from spring thaw until the ground freezes late in the fall.  
    Trees which have suffered browning often recover.  Since the stems and buds are better protected form winter injury, new growth will often emerge in the spring.  The overall condition of the plant should not be assessed until after the spring growth has been allowed to take place.

Please Contact Our Office For Additional Information
E-mail: john.swenson@ndsu.edu
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