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Issue No. 6, Article 8/April 29, 2005

Return to "Normal"

One principle we should always remember about Illinois weather is that "normal" is not the same as "average." We often use these words interchangeably, but it helps to think of normal as weather that "doesn't surprise" us, while average is just the middle of what is often a rather wide range of what we should consider as normal events on any given day or week. Thus the month of April 2005 has been normal but not very close to average, at least during the first half of the month, when rainfall was low and temperatures were high.

Weather during the past week has been more average, including the drop in temperature and the rain that fell on most of Illinois. Corn planting was even ahead of last year's record pace, with 64% of the crop planted by April 24 this year. Progress has slowed in most areas this week, though progress in Illinois is well ahead of the progress in Iowa, Minnesota, and most other states, and more than double the national figure of 30% planted. Illinois remains in great shape with respect to corn planting progress, though the return to lower (more average) temperatures means that emergence and early growth will slow.

Of more immediate concern is the effect of freezing temperatures on April 23 and in some places on the 24th. Fortunately, most of the corn in the northern parts of the state, where Sunday morning temperatures were below freezing, has not yet emerged and so should be safe. An unusual feature of this cold spell, however, is that the lowest temperatures occurred under breezy conditions. Radiational cooling, by which the leaf temperature can drop below the air temperature, is much less a factor when air is moving around the leaf, so leaf temperatures do not drop below air temperature. This helped, but the temperature was still near freezing in some places in central Illinois, and there are reports of damage to leaf tissue on early-planted corn that had two or three exposed leaves.

Corn in a planting date study at DeKalb was about 3 inches tall when air temperature fell to about 28û briefly and stayed below freezing for about 5 hours on Sunday morning, according to Lyle Paul. I saw this corn on Monday of this week, and most of the aboveground leaf tissue was dead and lying flat on the ground. There were occasional plants that seemed almost unaffected, with no obvious cause for the differential damage. The crown, where the growing point is located, was well established on these plants, and the belowground tissue seemed to be unaffected by the death of the tissue above ground.

Mike Roegge, Extension educator at Quincy, reported similar symptoms in some fields in western Illinois, especially in bottomland fields. Some of this might have been caused by reduced wind speed in that area and also by cold air drainage into low-lying fields. The photos that Mike sent show plants with more living tissue near the ground, and in most cases the percentage of plants affected was considerably lower than what we saw in DeKalb. But the same phenomenonof a plant "frozen and flat" next to one with very little damage--was evident in Mike's photos.

While we have every reason to believe that these frozen-off plants will grow back from the growing point (tip of the very small stem, now located about 3/4 inch below the soil surface), this regrowth will not be very fast under the cooler temperatures we have now. Also, we have learned to be a little bit cautious about saying that every plant with a "protected" growing point will grow back with no effect on yield potential. We have removed all of the leaf area from small plants in some studies, and, in some rare instances, plants do not grow back at all, or they grow back but fail to become productive.

Mike Hellmer, with Pioneer, sent some photos from 2001 in which plants frost-killed to the ground did not regrow. April 2001 started out much like April 2005, with dry, warm weather and early planting. The low temperatures on April 16, 17, and 18 that year were 30, 29, and 29, respectively, at Urbana, and highs on those days were 39, 40, and 44, respectively. That's much more severe than the temperature drops we had this year, and it's probable that failure of such plants to regrow in 2001 was because of penetration of freezing or near-freezing temperatures down to the growing point itself.

The photos that Mike Hellmer sent show what looks like healthy growing point tissue after the 2001 event, but there was little unaffected tissue above the growing point. The amount of healthy leaf tissue above the growing point indicates the amount of immediate reserves for regrowth, and this tissue is also the first to appear as plants grow back; hence, it's helpful to see how much healthy looking leaf tissue there is above the growing point. We think that the seed reserves should also be available to help regrow new leaf area when the tops freeze off, but we don't know if that happens in every case, especially when plants are older before the injury and the process of moving materials from the seed to the crown through the mesocotyl has slowed or stopped. Nor can we be certain that the growing point always is as healthy and functional as it might look; any browning near the growing point might indicate injury. Thus, while we think damaged plants will regrow, it will be useful to watch plants closely over several days to see when the first new leaf tissue appears above ground and make sure the regrowth process continues strong once it warms up.

One concern with freeze injury is whether the new leaf tissue can grow back through the dead tissue without getting "caught." This happens well most of the time, especially when the dead tissue dries out so that it shreds easily. Work done some time ago in Wisconsin showed that it doesn't help to try to mow off the old tissue to help ease the way for the new. Even on plants that don't show death of leaf tissue, temperatures in the 30s at night followed by bright sunlight usually mean that the green color of leaves bleaches out quickly, and it takes several days of warmer temperatures for the healthy green color to return. The "sand blasting" that some emerged plants have had from recent winds will contribute to the sickly appearance of leaf surfaces. Soil particles moved across the young leaf surface can damage the waxy coating, making the leaves look dusty and causing leaves to lose water too quickly, perhaps even wilting some. Damaged leaves may not recover very well from this, but they should be replaced quickly by new leaf tissue so that the loss of the first two or three leaves, while not a good thing, should not affect yield potential.

One issue that Mike Roegge raised was the uneven competition that will result from uneven plant sizes as damaged plants regrow next to undamaged plants. We have done some simulated hail studies over the past 3 years in which we cut off small plants next to undamaged plants at different leaf stages, starting at V3, or with plants about 6 to 8 inches tall. At Urbana, cutting off 25% of the plants at V3 caused an average yield loss of about 10 bu/acre, and the cutoff plants yielded less than a third as much as intact plants. There was some compensation by adjoining plants, and this treatment was more extreme than happens when young plants are unevenly frosted, but it is likely that uneven plant size resulting from uneven frost damage will reduce yield potential by a few bushels. Such a potential loss is not large enough to suggest replanting or trying to damage intact plants to "even things up," but it might occur in those fields that show such symptoms not only from freeze damage but from any kind of uneven damage of young plants.--Emerson Nafziger

Emerson Nafziger

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