D. R. Hicks and S. L. Naeve
Agronomy and Plant Genetics
May 15, 2000
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 ½ 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 now? 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.
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