ndsucpr_L_sm_PS.jpg (12513 bytes)

pscience_Logo_Lg.jpg (12372 bytes)

ISSUE 3   May 18, 2000


    Shortly after emergence, the corn plant switches to utilizing photosynthetic energy from its dependence on the stored food in the kernel. Stress after emergence can decrease starch and chlorophyll content of seedlings and limit ultimate yield. However, if the weather dries enough to encourage corn roots to penetrate deeper into the soil, the plant will be better prepared to withstand later dry weather which may offset any later weather stress effects. Wet springs can reduce yields due to poor rooting so plant preconditioning is in effect. Young corn plants are relatively resistant to cold weather, even to air temperatures just below 32F, although exposed aboveground parts may be effected. The growing point is below ground and protected with soil temperatures at the one-inch depth still slightly higher than minimum air temperatures even at air temperatures of 27F. Initiation of crown roots can be retarded as root temperatures dip from 68F to 41F. Optimum shoot growth and leaf elongation has been shown in research to be in the range of 77-95F. Leaf area in early planted corn plants is highly correlated with air temperature. Flooding besides limiting root growth reduces corn yields. Time and length of the flooding period affects yield reduction. Flooding when corn is 6 inches tall for 72, 48, and 24 hours can reduce corn yields by 32, 22 and 18%, respectively when low nitrogen availability is also a factor. Returning warm weather will help bring emerged corn plants back into growth mode and these temperatures and timely rains during the rest of the season will determine if the corn crop is on course for good yields.

Denise McWilliams
Extension Crop Production Specialist



    Low temperatures during the past few nights (May 12, 13, and 14) caused some leaf injury to corn and maybe to soybeans. Several reports today indicate corn leaf injury from completely across the southern half of Minnesota. Soybeans are just emerging and there haven’t been confirmed reports on injury, but some probably occurred. We expect regrowth to occur without a major loss in stand for both corn and soybeans.

    Corn. Substantial leaf injury can occur without any loss in stand providing good growing conditions (warm temperatures) continue for the next few days. New leaves should emerge from the corn whorl pushing the older frost damaged and dead leaves away. The growing point (source of new leaves) is located about inch below the soil surface and is not likely to have been damaged by the low temperatures (possible, but not likely unless the low temperature continued for several hours such that the soil around the growing point dropped below 32 degrees). New growth should be visible the day after the frost if the temperature is above 65 degrees. The stand can be evaluated in a couple of days to determine if some plants are not going to survive and regrow.

    Should post emergence herbicides be applied immediately? Probably not. The corn needs to recover some with new leaf tissue and be actively growing to metabolize the herbicide. That should occur within 2 to 3 days if we have good growing conditions, especially warm temperatures.

    Soybeans. Soybeans can tolerate air temperatures that are lower and for a longer period of time without showing frost injury than can corn. There are several growing points on the soybean seedling. The terminal growing point (GP) is the dominant one and is located at the top of the main stem. New leaves will grow from this GP on a normally developing soybean plant. When the terminal GP is damaged (by frost or removed when eaten by rabbits or deer), regrowth can occur from the vegetative buds
(growing points) located in the leaf axils on the main stem. The axils are the points where the cotyledons are attached to the main stem and where the petioles of the unifoliolate and trifoliolate leaves are attached. Seedling soybeans (at the growth stage where soybeans are now) will have two vegetative buds in the cotyledonary axils. Regrowth can occur from either of these vegetative buds if they have not been frozen. Often the terminal GP can be frozen without injury to the GP’s located in the leaf axils. We expect frosted soybeans should regrow from one of these GP’s. Depending upon the severity of the frost injury, some plants might not survive resulting in some stand reduction. However, this should not have a significant effect on soybean yield unless there are gaps in the row where all plants have been killed.

Dale Hicks and Seth Naeve
UM Extension Agronomists



    Look back across the years and consider the environmental conditions that increased wild oat production in your fields. More than likely, you can equate the recent few years of cool, wet springs with wild oat seed bank increases. Wild oat prefer cool springs and areas where moisture is retained. In fact, this grass weed in laboratory testing had good germination at 70F but failed fairly uniformly at 81F. Germination was enhanced by moisting seed or by rupturing the seed coat that otherwise can
cause seed dormancy. The minimum temperature for seed germination generally is 61F and the maximum temperature for any great seed germination is 81F. Light is not needed for germination. Exposing seed to the winter environment on the soil surface or deep burial may prevent weed emergence. Remember that burying seed requires many more years before weed seed viability is sapped. Wild oats are easily recognized by the panicle more open than in tame oats. The fibrous roots are limited in protecting the weed, unlike other weeds that have rhizomes. Better control of this weed in corn can be obtained with soil applied PPIs of atrazine, DoublePlay or the older Eradicane being phased out as well as with a POST of Accent, Accent Gold (on 88-day hybrids or later), atrazine, Basis Gold (on 88-day hybrids or later), Celebrity Plus, Liberty (on Liberty-Linked
corn), Lightning (on Clearfield corn), or Roundup (on Roundup-Ready corn). In soybeans, consider POST applications of Assure II, Fusilade DX, Fusion, Poast, Select/Prism, Liberty (in Liberty-Linked soybeans), Raptor, Rezult, Roundup (in Roundup-Ready soybeans) or Touchdown (in glyphosate-resistant soybeans).

Denise McWilliams
Extension Crop Production Specialist



    Frost was reported on May 12-14 in various regions of the state. NDAWN weather maps indicated that frost occured in the SW region on May 12, primarily in the western half of the state on May 13 and in the east central and NE regions on May 14. Crop damages being reported have been light with perhaps reseeding required in only some localized areas.

    Crop seedlings hardened by continuous low temperatures are more tolerant to light frost than seedlings hardened by alternating high and low day and night temperatures.

    Some guidelines on frost tolerance of various crops:

    Canola: Two to 6 leaf canola plants can actually withstand a considerable frost. If it get’s down to 25F then you can expect leaf wilting and yellowing, however, the plants should recover without too much leaf loss. At more severe temperatures the leaves will wilt, turn brown and then black and die. Then you would have to wait and see if any of the growing points start sending up new leaves. This takes about 4-5 days depending on the weather following the frost.

    Canola seedlings will usually recover from a light spring frost that does not damage the growing point of the plant. If a heavy frost does blacken the leaves, no action should be taken for at least 4 to 10 days. If there is green color at the growing point in the center of the frozen leaf rosette, the plant will recover and yields will be higher than if the field is worked and re-seeded.

    Flax: Flax is quite susceptible when it is first emerging. It can, in most cases tolerate temperatures of 28 degrees F if it has a couple of true leaves. Depends some on growing conditions the previous couple of days. Check to see if stem is turning black. After 2-leaf stage and hardened off it can stand temps in low 20's.

    Alfalfa: 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.

    Sunflower: Sunflower in the cotyledon stages can withstand temperatures in the 25-26 F 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. In the 2-leaf or V2 stage then 26-27 F would be the lower limit. For the 4 and 6 leaf stages, then 28-29 F is the lower limit.

    Small Grains and Corn: Our cereal grains will lose leaf tissue that freezes. New growth will follow as the growing point before jointing is protected below ground. In some cases the eventual maturity date may be delayed. For example, a 80 day corn that has a couple leaves froze off may in fact mature like an 84 day corn. Corn can recover from frost up to the V5 leaf stage, since its growing point is located about inch below ground and most likely will not be affected by the cold temperatures. New growth should be noticeable within 3-4 days after frost if the daytime temperatures are 65F or higher.

    Soybeans and Dry Beans: Dry Beans and soybeans 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. Soybeans may leaf out again after a light frost from axillary buds in the leaf axil. One of these branches will then become the main axis of the plant if the first growing point is killed. Soybeans at the cracking stage or cotyldon stage can take temperature as low as 28F for a short period of time. Once true leaves are formed, temperatures of 29-30 degrees F will cause extensive damage. Dry Beans are even less tolerant than soybeans to frost.

A good source for frost damage information is the following
web page: www.ag.ndsu.nodak.edu/aginfo/procrop/env/frost.htm

Duane R. Berglund
Extension Agronomist



    Of the few concerns expressed this week, yellowing in small grains will probably become more pronounced in the next few weeks. The most likely causes include nutrient deficiencies, disease, and environmental damage.

    Conspicuous early season yellowing, not associated with wet soil, generally indicates nitrogen deficiency. Symptoms of nitrogen deficiency are most noticeable on older plants. Typically older leaves will be yellow while emerging leaves will be greener. Tillering may also be less than typically expected. In areas of ample or excess moisture root growth may not have reached nitrogen supplies. If nitrogen deficiencies are suspected a soil test is recommended. Consult NDSU Extension Bulletin
SF-882. Also see Dave Franzen’s article in last week’s pest report.

    Sulfur deficiencies occasionally occur in North Dakota. Symptoms of sulfur deficiencies, yellowing similar to nitrogen deficiencies, often occur in patches. Deficiencies are usually associated with low organic matter soils. Only sources of immediately available sulfate sulfur, such as ammonium sulfate, will produce a response when sulfur is low or limiting. Consult NDSU Extension Bulletin SF-882.

    With recent rains development of tan spot is possible. Heavy infestations of tan spot will give fields more of a brown hue than yellow. Tan spot is most likely to occur on wheat planted on last years wheat stubble. If a wheat crop is on last years wheat stubble monitoring it for disease development should be done. Marcia McMullen included recommendations for early season control of tan spot in last week’s Pest Report.

    Its hard to remember now, but May 2nd through the 5th of this spring we had some unusually high temperatures. At this time much of our small grains were just emerging or in the one leaf stage. The soil around these very small plants was black. The combination of young plants and black soil created perfect conditions for heat canker, the plant tissue right at the soil surface is damaged. Heat canker will appear as bands on leaves as they grow. What is unique this year is that much of the wheat and barley was just emerging so the tips of the leaves were damaged. This damaged could be confused with herbicide injury. Heat canker at early growth stages in small grains does not impact productivity.

    Frost injury was certainly a possibility in this past week. In anticipation of this possibility I covered this in detail in the May 4 issue, page 3 under, "Crop Injury and Replanting Decisions." If you have tossed that issue you can get the information at


Michael D. Peel
Extension Small Grains Agronomist

cprhome.jpg (3929 bytes)topofpage.jpg (3455 bytes)tableofcontents.jpg (4563 bytes)previous.jpg (2814 bytes)next.jpg (1962 bytes)