Soybean: Last Monday, May 16, air temperatures dropped to high 20s/low 30s causing some freeze injury to soybeans. Soybeans in low areas of the field are most likely to be affected. Plants should be assessed for damage at least five days after suspected injury to inspect for regrowth. If damage occurred above the cotyledons, the plant will likely recover. If damaged occurred below the cotyledons, the plant will not recover. Look for a discolored hypocotyl (the “crook” of the soybean that first emerges from the ground) which indicates that damage occurred below the cotyledons. The soybean plant pictured will not recover. If soybeans were not yet emerged at the time of the freeze, they should be fine.
If widespread freeze damage occurred, it is not too late to re-plant (see the Late-Planted Soybean article in this issue of the CORN newsletter).
Although early planted corn has been severely damaged by recent frosts in some areas, the effects of the low temperatures on corn survival will probably be negligible for the most part. In past years, we have observed that corn that was in the process of germinating or as far along as the V1 stage (one leaf collar visible) survived freezing soil temperatures in April with little impact on crop performance or plant stand. Agronomists generally downplay the impact of low temperature injury in corn because the growing point is at or below the soil surface until V6 (six leaf collars visible), and thereby relatively safe from freezing air temperatures. Moreover, the cell contents of corn plants can sometimes act as an "antifreeze" to allow temperatures to drop below 32 degrees F before tissue freezes, but injury to corn is often fatal when temperatures drop to 28 degrees F or lower for even a few minutes.
Effects of low temperatures on germination are far more serious when combined with snow and freezing rain. When dry corn seed absorbs cold water as a result of a cold rain or melting snow, “imbibitional chilling injury” may result. However these conditions were largely absent following the recent frosts.
Several studies have indicated that cool conditions following frost could lead to continued plant mortality as a result of bacterial stalk rot diseases. In Ohio, we’ve observed extensive stand loss in early corn subjected to heavy rains shortly after hail damage and defoliation. The increased plant mortality was associated with what appeared to be bacterial soft rot.
To assess the impact of freezing temperatures on emerged corn, check plants about 5 days after the freezing injury occurred (and preferably when growing conditions conducive for regrowth have occurred). New leaf tissue should be emerging from the whorl. You can also observe the condition of the growing point (usually located ½ in to 3/4 in below the soil surface) by splitting seedlings lengthwise. If the growing point appears white to light yellow and firm several days after the frost, prognosis for recovery is good.
For more information on frost effects on early planted corn, check out the following:
McMechan, J. and R. Elmore. 2016. Risk of Freeze Damage in Early Planted Corn. Cropwatch. University of Nebraska – Lincoln. http://cropwatch.unl.edu/2016/risk-freeze-damage-early-planted-corn (verified May 20, 2016)
Nielsen, RL (Bob). 2001. Symptoms of Low Temperature Injury to Corn and Soybean. Corny News Network, Purdue Univ. online at http://www.kingcorn.org/news/articles.01/Frost_Corn_Soy-0418_Gallery.html (verified May 20, 2016)