It should come as no surprise that, after a warm, dry period of weather in late March and early April, rain has fallen and temperatures have turned cooler, representing a return to more "normal" (well, perhaps more average--the range of what is normal is fairly broad) weather conditions for this time of year. The latest official report indicates that about 2 percent of the state's corn crop is planted, in one of the earliest starts to planting that we have ever had. Planted acres are mostly in central Illinois, with rather substantial acreages reported in the Springfield and Decatur areas. Some of the earlier-planted fields have emerged.|
With temperatures to be in the day/night range of 30s/50s over the next few days, how can we expect the planted corn crop to react? Probably not very well, or at least, the crop won't be very "pretty" until the weather warms back up. Temperatures in the 30s at night, especially if the weather is clear, may very well frost off the emerged leaf area, especially that area that is oriented horizontally, and thus more exposed to radiational cooling, which drops surface temperatures below air temperature. Even if such leaves don't die, they can often be badly injured, turning pale yellow to almost white in color, often with some dead areas around the edges or at the place where the leaf surface was most horizontal.
Though injured leaves of young plants will often make the crop look bad, there is usually little or no lasting effect of this on the growth and yield of the crop. The growing point--the tip of the stem--is below ground level until the plant has five or six fully emerged leaves (i.e., with visible collars), which requires an accumulation of about 350 growing degree-days after emergence. Until that time, the soil around the growing point provides some protection against low air temperatures. Even if leaf area is killed, temperatures around the growing point are usually high enough to encourage the growing point to continue to initiate leaves and for the leaf growth that takes place above the growing point. This arrangement also means that new leaf tissue will continue to push up out of the soil after the emerged leaf tissue is injured or killed, thus restoring the ability of the plant to resume photosynthesis and growth.
All of this means that, as long as the growing point stays alive and active, leaf loss or injury in young plants is only a temporary setback. Such a setback does interrupt the development of the plant, and will delay subsequent developmental events, even pushing tasseling and silking back, probably by about the number of growing degree-days lost to the interruption of growth. Fortunately, that number of growing degree-days is generally quite small--probably less than 50 in most cases--due to the ability of the plant to resume growth quickly. Each new leaf requires about 65 growing degree days to emerge, so crops with damaged or killed young leaves usually show little effect beyond their being a leaf stage or less behind in their development.--Emerson D. Nafziger