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ISSUE 5  June 4, 1998



    Don’t be too hasty in tearing up frosted crops and reseeding. Three to four days are needed for assessment for damage, if any.

    Temperatures on May 29, 30 and 31 were reported as low as 25 to 31EF from several northern regions of the state.

    Seedlings hardened by continuous low night and day temperatures are more resistant than seedlings hardened by alternating high and low day and night temperatures. There are considerable variety differences in all crops and no research has been done on the varieties we are currently growing. Corn plants less than 6 to 8 inches tall (five-leaf stage or less) will recover from frost because the growing point is still below the soil surface and usually not damaged. 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, (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, slowly 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.

    Dicotyledonous plants such as sugarbeet, sunflower, soybean, dry bean, canola and flax are more easily killed by frost than are small grains. Sunflower in the cotyledon stage may often withstand temperatures as low as 26EF. Sunflower in the 2, 4 and 6 true leaf stage becomes more sensitive to frost and bud damage may occur.

    Sunflower in the cotyledon stages can withstand temperatures in the 25-26F range for short periods if they are just emerging from the soil. Sunflower in the 2, 4 and 6 leaf stages become more sensitive with each development stage and terminal bud damage can occur. I feel that in the 2-leaf or V2 stage then 26-27F would be the lower limit. For the 4 and 6 leaf stages then 28-29F as the lower limit. Also the tolerance also can be influenced by the hardening off process. That is if it is cool or cold for several days such as 33-34F at nights and the seedlings become somewhat accustomed to the lower temps, then perhaps a little better tolerance to lower temperatures. Its going from warm temps to extreme freezing temperatures all at once are the most injurious. Wet soils and some dew also help in reduction of freeze injury. Cold and dry conditions help add more to seedling injury.

Flax is quite susceptible when it is first coming out of the ground.  It can, in some cases, tolerate temperatures of 28EF if it has a couple of true leaves.  It depends some on growth conditions the previous couple days.  Check to see if the stem is turning black.  After the 2 leaf stage and it's hardened off it can stand temps in the low 20's.

    Alfalfa will be damaged by temperatures in the mid 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 field dries up enough for good equipment performance. Temperatures of 32 degrees F and below will kill buckwheat or drybeans, and temperatures of 29-30EF for soybeans, while alfalfa, canola, crambe, flax and sugar beet will tolerate temperatures in the mid 20’s. Crops such as field peas or lentils have good frost tolerance since several growing points remain below ground in the seedling stage.

    Broad leaf crops that have their growing point (terminal bud) at the top of the plant are more susceptible to frost damage than our cereal species. Beans for example are quite sensitive to frost. Beans may leaf out again after a light frost from axillary buds in the leaf axils. One of these branches will then become the main axis of the plant if the first growing point is killed.


    Recent heavy rains in some eastern areas of North Dakota have promoted inquiries of flooding and ponding in corn and its effects.

    The extent to which flooding injures corn is determined by several factors including plant stage of development when flooding occurs, the duration of flooding, and air/soil temperatures. Prior to the 6-leaf stage (when the growing point is near or at the soil surface), corn can survive only 2-4 days of flooded conditions. Once corn has reached the silking stage shallow depths of flooding will not cause any noticeable amounts of damage. If temperatures are warm during flooding (greater than 77 degrees F) plants may not survive 24 hours. Cooler temperatures prolong survival. Iowa studies found that flooding when corn is about 6-inches in height for 72, 48, and 24 hours reduced corn yields by 32, 22, and 18%, respectively, at a low N fertilizer level (50 lb N per acre). At a high level of N (350 lb N per acre) these yield reductions ranged from 19 to 14% in one year to less than 5% the following year.

    Research indicates that the oxygen concentration approaches zero after 24-hours in a flooded soil. Without oxygen, the plant cannot perform critical life sustaining functions, such as nutrient and water uptake is impaired, root growth is inhibited, etc. Even if flooding doesn’t kill plants outright it may have a long term negative impact on crop performance. If excess moisture in the early vegetative stages retards root development, plants may be subject to greater injury during a dry summer because root systems are not sufficiently developed to access available subsoil water.

    If flooding in corn is less than 48 hours, crop injury should be limited. To confirm plant survival, check the color of the growing point (it should be white and cream colored, while a darkening or softening usually precedes plant death) and look for new leaf growth 3 to 5 days after water drains from the field.

    Cold, wet weather conditions also favor development of seed rots and seedling blights. Seed treatments are usually effective but can provide protection only so long; if seedling development is slowed or delayed 2-3 weeks, soil-borne pathogens have a much greater opportunity to cause damage. Other disease problems which may become greater risks due to flooding and cool temperatures are corn smut and crazy top. The fungus that causes crazy top depends on saturated soil conditions to infect corn seedlings. There is limited hybrid resistance to these diseases and predicting damage is difficult because disease symptoms do not appear until later in the growing season.

Duane R. Berglund
NDSU Extension Agronomist


    Frost damage to small grains is highly dependant on the stage of the crop, the temperature and duration of the low temperature, and environmental conditions that existed before the frost occurred.

    Small grains are among the most frost tolerant of crops. From early developmental stages to jointing they can easily withstand temperatures of 25o F. After jointing the growing point moves above the soil surface and is more susceptible to frost damage. Small grains in the jointing stage have been shown to withstand 25o F temperatures with out damage to the growing point, however, injury is more likely than at earlier stages.

    Small grains are most susceptible to frost from the late boot through flowering. Frost injury occurs during boot through flowering when temperatures fall below 28o F. This is the stage when reproductive tissue development is occurring. Anthers, the pollen producing structures are easily damaged by frost preventing or severely reducing pollen production. Barley is more susceptible to frost injury during the boot stage because it flowers while still in the boot.

    When temperatures are below 50o F for several days before the frost, the crop will go through a hardening process and develop more tolerance to frost. Drought stress can also cause hardening of small grains. Prior to jointing hardened small grains have been shown to withstand temperatures as low as 14o F. Slow cooling will also result in less damage.

    When the duration of the low temperatures is for several hours the potential for injury increases. Plants can avoid injury through a process call super cooling, particularly when the cooling is slow. This occurs when water from within cells moves out lowering the temperature at which water within the cells will freeze. This reduces the potential for ice crystal formation and physical damage to cell membranes.


    When evaluating for frost damage it is best to wait two or three days so any injury can be easily observed.

    Winter wheat crops are at the greatest risk for injury from frost because they are in the boot to flowering stage. Evaluating these crops for damage must be done by dissecting flowers and looking at the anthers. A watery appearance indicates frost injury and those anthers will not shed pollen. If pollen shed has already occurred florets should be evaluated for kernel development which may require several days.

    During early development the growing point of small grains is below the soil surface, making it less susceptible to injury. With this protection, plants can suffer loss of above ground foliage without dying. When this occurs, it is advisable to wait several to make an accurate determination of injury. After this period, new growth on plants with uninjured growing points can be observed. If no regrowth is observed, the stem of the plant may be split to inspect the growing point. The growing point should be white or cream colored. Darkening or softening with a watery appearance of the growing point indicates injury and usually precedes plant death.

Michael D. Peel
Extension Small Grains Agronomist


Ash Defoliation

    Partial defoliation of green ash trees in much of North Dakota generated concern over the last two weeks. Ash anthracnose and ash plant bug, both potential defoliators of ash, were present on most of the dropped leaves.

    Observations from eastern to southwestern parts of the state showed that the anthracnose fungus appeared to play a significant role in defoliating ash trees this spring. For a description of ash anthracnose, see the May 21, 1998 Crop and Pest Report. The ash anthracnose fungus can cause heavy defoliation when leaf emergence and expansion is accompanied by cool weather and prolonged moisture. Although leaves are less susceptible to the anthracnose fungus when they are fully formed, this weeks weather may result in more defoliation in the near future.

    Ash plant bug can be readily identified by the white to yellow spots created in the leaf by the feeding insect and the black shiny excrement left behind by the insect on the undersides of leaves. The nymphs are 1/16 to 1/4 inch long and turn from a pale green to a light brown as they mature. Adults are about 1/4 long, light tan to brown and are already present in parts of North Dakota. Although ash plant bug damage is usually not severe enough to warrant control, some trees are already extensively damaged with large portions of leaf area showing yellow blotches with some leaves shriveling and dropping. We normally see two generations of this insect each year. Those trees currently showing extensive injury may require intervention. See E-296 for chemicals labeled for plant bug control.

Eriophyid Mites

    There have been numerous inquiries into the control of eriophyid mite gall makers on a variety of tree and shrub leaves (Boxelder, Silver Maple, American Cranberrybush Viburnum, and Green Ash). These host-specific mites produce bladder (somewhat spherical), spindle (long), or erineum (fuzzy) galls. Early spring foliage is often affected more than later foliage. Mite activity declines as the growing season progresses. As a result, a tree will normally put on enough healthy foliage to sustain tree health. Chemical control is generally not necessary, but for those perfectionists who demand spotless leaves, dormant oil may be their best management option for future years. Since very little is known about the biology of many of these mites, control strategies are speculative and cannot be provided with a high level of confidence for those mites.

Marcus Jackson
Extension Forester


    There are situations when Federal law DOES allow you to use pesticides in ways which are not specifically mentioned on the label as long as those uses are not in violation of State or tribal laws. (North Dakota accepts the Federal rules but tribes are variable so check with local authorities.) These situations are:

1.  Applying a pesticide at any dosage, concentration, or frequency LESS than that listed on the labeling.

2.  Applying a pesticide against any target pest not listed on the labeling if the application is to a plant, animal, or site that is     listed.

3.  Using any appropriate equipment or method of application that is not prohibited by the labeling.

4.  Mixing a pesticide or pesticides with a fertilizer if the mixture is not prohibited by the labeling.

5.  Mixing two or more pesticides, if all the dosages are at or below the recommended rate.

    If the applicator uses pesticides under the situations listed above, he is generally not entitled to satisfaction from the manufacturer if the results are less than desirable. The liability for nonperformance rests solely with the applicator.

    Finally, the above situations are in NO WAY inconsistent with the label’s "Directions for Use". It is still illegal to apply pesticides in any way not permitted by the labeling. The law is very specific:

1.  A pesticide may be used ONLY on the plant(s), animal(s), or site(s) named in the "Directions for Use".

2.  You may NEVER use higher dosages, higher concentrations, or more frequent applications than listed on the label.

3.  You must follow ALL directions for use, including directions concerning safety, mixing, diluting, storage, and disposal.

4.  You MUST wear the specified personal protective equipment even though you may be risking only your own safety by not wearing it.

    In summary, the use directions and instructions are not advice, they are requirements.

Andrew A. Thostenson
Pesticide Program Specialist


    Several homeowners have noticed their turfgrass "heading out" or forming seedheads at this time of year. Their concern seems to focus on a couple of points:

1.  Should they allow the seeds to mature and drop to thicken up their lawns?

2.  Why is their lawn doing it, and someone else’s not?

    The first question is easy to answer in one word: "No!" The answer to the second question will explain why the first answer is no.

    Cool season turfgrasses (Kentucky Blue, Tall Fescue, Creeping Red Fescue, and Perennial Ryegrass) are affected by day length. Kentucky bluegrass grown under short days (8 hours) has a different habit of growth than one grown under our present day length (12+ hours of daylight). The long day plants will tend to be taller, producing more upright growing leaves, while the same plants under short days will produce plants more prostrate in habit.

    Along with this extended growth, some species will tend to form seedheads - induced and developed under increasing day length and cool spring time temperatures. The most common example is annual bluegrass which plagues golf course superintendents during both the spring and fall seasons. Generally speaking, when the first day of summer arrives, and the days begin to get shorter, most grass species will stop producing seed heads - with the exception again being the annual bluegrass (Poa annua). This very adaptable species has the ability to form seedheads under cool, moist conditions - generally found in spring and fall seasons - whether the days are long or short.

    From a turfgrass management point of view, it is better to have the grass remain vegetative, rather than begin its reproductive cycle. As managers, we encourage this via regular fertilization and mowing practices. Maintaining an adequate level of nitrogen through the growing season and mowing as needed - every 3 days early in the season, to every 7-10 days later on - will contribute to minimizing seedhead formation.

    By this time in July, seedhead formation in turfgrass will be a faded memory!

Ron Smith, Ph.D.
NSDU Extension Horticulturist and Turfgrass Specialist

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