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Issue No. 6, Article 7/May 14, 2010

Hail, Frost, and Flood: What's Next for Corn?

The good news is that 94% of the Illinois corn crop was planted by May 9, keeping 2010 on track as one of the earliest plantings we've ever had. Most of the crop has emerged, and most stands are good. For what it's worth, 82% of the crop is rated good to excellent. That rating this early in the season generally means that the crop has emerged reasonably well, but at this time in the season such ratings can change quickly as we wait to see what the weather will bring.

In some parts of Illinois the weather has not been very kind to the new corn crop. There was some hail damage in early May in parts of Macoupin and nearby counties, in which newly emerged corn plants were cut off at ground level, with some mud coverage as well. According to a report from Robert Bellm, most of these fields recovered quickly and almost completely, with a large majority of plants showing new growth within 3 to 4 days. Warm temperatures the first week of May helped, and it's unlikely that such fields will show lingering effects from the loss of the small amount of leaf area so early. It also helped that most hailstones were apparently small enough that few plants took direct hits, which can push the seedling down into the mud and prevent new growth.

The next hazard that the corn crop faced was low temperatures on Sunday morning, May 9. Temperatures dropped to 30 to 32 degrees over a large chunk of the northern half of Illinois, with below-freezing temperatures between I-74 and I-80, including in Champaign and Peoria. Corn we had planted in early April was at stage V3, and that planted April 21 was at V1. Neither showed any direct effect of low temperature when I saw them at 8:45 a.m. on May 9. We normally expect temperatures of 31 or 32 degrees to cause injury to young corn leaves, and they often result in death of the leaf tissue that is most directly exposed to the "cold" sky, due to radiative heat loss that can lower leaf temperature to below the air temperature.

One reason we saw little direct leaf damage on May 9 was that it was cloudy during the first part of the night before the low morning temperatures. This reduced radiative heat loss and also reduced the duration of the lowest temperatures. A more important reason was the breeze that came up early that morning, probably at about the time of lowest air temperature. Moving air stirs the air around the leaf to prevent leaf temperature from dropping below air temperature. It also helps pick up sensible heat from the soil, which can help raise the temperature of the air near the ground.

Because there was not much leaf loss from the low temperature does not mean that the temperature did no damage. Most fields on Monday morning had a pale color that resulted from some physiological effect of low temperature on leaf tissue. Such physiological damage greatly reduces the ability of the leaf to absorb and transform sunlight energy, in which case light energy causes even more damage, often bleaching leaves to a very pale color. Such damage can be repaired over time, but plants grow very little in the meantime. Fortunately, there was not much bright sunshine on Monday, so lasting damage to leaves was minimized. Leaves looked a little better by Tuesday, but rain and clouds were moving in. We will have to wait for warm weather and sunshine before we see a return to the darker green leaf color that signifies fully functional leaves.

A larger concern after heavy rainfall in parts of Illinois this week will be standing water and, for the crop recently planted, the possibility of emergence problems. There were a few reports of death of germinating seeds in fields planted a few days before heavy rain in parts of western and southwestern Illinois, but this is not widespread. We think that such seeds simply ran out of oxygen and that shoots died before emergence. Seedlings that have emerged and have roots are more resilient, but there is a very good chance that plants that stand in water for more than two or three days will not survive, especially if temperatures go up. Higher temperatures mean less oxygen in the water and also faster seedling metabolism rates, so plants run out of oxygen sooner.

Even if plants survive, their regrowth can be slow due to poor conditions around the roots. Diseases can also invade plants that stand in water; one example is the downy mildew fungus that can carry crazy-top, whose symptoms won't appear for weeks after the water is gone. In any case, we often see plant size and health diminish as we move from the edge to the middle of low areas where water stood. If the size of the drowned-out area is large enough to justify a repair-planting after it dries up, it might be a good idea to plant into the area around the edge with living but slow-growing plants as well, in order to replace sickly plants with healthy ones.--Emerson Nafziger

Emerson Nafziger

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