The cool temperatures this past weekend (it was in the 30s in much of central and northern Illinois Sunday morning) have caused most cornfields to show some signs of cold injury. There is not much outright frost injury, but when leaves are exposed to the sky and the night temperatures fall to 36 or 37, it is not unusual for the surface of the horizontal parts of leaves to experience freezing or near-freezing temperatures. Darker soils radiate heat to leaves, which warms the leaves and may decrease damage, while plants in lighter soils and those with less tillage and more surface residue may show more damage. Wetter soils do not cool as quickly, so damage may be less where rainfall occurred before the low temperatures. |
Most of the damage from such an event, however, takes place when the sun comes out strong after the low temperature, as it did almost everywhere on Sunday. Young corn leaves that have experienced temperatures of 40 degrees or less suffer physiological damage to membranes that function in photosynthesis. When light hits such leaves, its energy canít be captured normally, but rather causes further damage to the leaf, usually bleaching out much of the green color. The result is a silvery or "sun-scorched" appearance of the leaves, typically with more damage to the horizontal parts of leaves higher on the plant. Even without such specific leaf damage, there has been a general loss of green color in fields, such that a lot of fields look sickly now compared to last Thursday or Friday. The high winds on Thursday and Friday also contributed to this problem, shredding leaves as they were blown against the ground and in some cases causing some injury and plant cutting from blowing soil particles.
Even though the color change is not something we would have wanted, we think that this injury is unlikely to have much effect on the crop. The crop is generally off to a fast start, and even though this color change means a temporarily lowered ability to absorb and use sunlight, the loss of a few days of photosynthesis should not decrease yield potential. Except for those areas that are drying up and look (and are) dead, much of the leaf area on the plants will green back up once warm temperatures return. Of course, we canít expect the crop to grow very fast when itís not photosynthesizing very well.
I traveled in southern Illinois in the past week and saw that, while many fields are planted and emerged, the corn on the lighter-colored, poorly drained soils did not seem to be particularly thriving, even though stands were good. That was before the high winds and cool temperatures, so those fields likely look worse now than when I saw them. Dry surface soils may be contributing to this, especially if the nodal roots are having trouble growing out into the soil. Dry surface conditions and lack of root growth often mean difficulties in the plant taking up enough nutrients; that, coupled with low temperature injury, is probably keeping affected plants growing very slowly, even if temperatures are favorable.
The cool temperatures have slowed up growth considerably, but corn planted the first half of April has reached the V5 stage (5 leaf collars visible) here at Urbana, and is close to a foot tall. Corn planted the last week of April is in V3 or pushing V4. As I indicated last week, stands and stand uniformity are generally very good. One way to assess this is to look at fields from the side Ė across the row direction Ė and note if there are spots where soil is more apparent, indicating lower stands or less growth. There are such spots where rain has been short since planting, mostly due to slow growth from inadequate water supplies. But most fields show much better uniformity than normal.
Wheat Condition Mixed
Dr. Fred Kolb and I made a rather long "wheat loop" tour on May 11, from Champaign to Carmi to Mt. Vernon, Belleville area, and past the research center at Brownstown, in Fayette County. Most of the wheat fields look very good from the road, with good color and very good head numbers. Most had flowered about a week before, such that the "berry" (developing kernel) had grown to one-fourth to one-half its expected length. Most of the essential leaf area--the upper two or three leaves--was quite healthy. We did see some small cereal leaf beetle larvae, and this should be watched closely.
We did see some evidence of wheat streak mosaic in some fields, but almost as much damage from barley yellow dwarf virus, which is spread by aphids. The BYDV was particularly evident on the variety trial at the SIU Belleville research center. We were probably looking too early to detect Fusarium head scab if it has infected kernels, but rainfall was not constant during flowering, and so we donít expect a lot of this problem in most fields. The cool, dry weather this week has been positive for the crop, but of course has slowed the rate of development. The crop is still a week or so ahead of normal development, and so should still be ready to harvest at least a few days earlier than normal, unless it stays cool.--Emerson Nafziger