Drought stress, freeze damage, premature leaf death
Although the wheat crop has benefited from a week of relatively mild temperatures, we are getting reports of wheat declining more quickly than expected in some areas of the state. Visits to wheat fields indicate the leaves are rapidly drying up and some fields have scattered white heads. In general, this rapid decline of the wheat can be explained by drought stress and freeze injury. In some cases, however, it appears that leaf diseases such as tan spot or Stagonospora leaf blotch also are increasing rapidly on the flag leaves, further accelerating the loss of leaf tissue. These leaf diseases appear to be a factor in the southeast and south central regions of the state where rain was more plentiful this spring.
We have seen fields in Ellsworth, McPherson, and Rice counties showing symptoms of drought stress despite recent rains. As we talk with growers in these areas it appears that parts of these counties experienced dry soil conditions and windy days that caused wheat to roll leaves and "turn blue" about 2-3 weeks ago. The wheat would have been at heading and flowering stage of growth at this time. Wheat at these growth stages is very sensitive to environmental stress, and dry conditions and/or hot temperatures will often damage developing wheat heads. This period of moisture stress would show up now as leaf-tip necrosis and scattered white heads. These white heads might be from tiller abortion or heads that were sterilized by the stress, depending on the growth stage when the stress occurred.
Some irrigated fields in western Kansas were also showing similar symptoms. Of course we would not expect moisture stress to be a problem in well-managed irrigated fields, and in this case, freeze injury to the stems appears to be an important factor. Many irrigated fields in western Kansas were jointing during multiple freeze events back in April. Evaluation of these fields indicates leaf tip-die back. This is a result of moisture deficiency, even under well-irrigated conditions. The freeze damaged the vascular tissue of the wheat plant, so that the plant has been unable to transport enough water to keep up with demand, especially now that grain fill has initiated.
Another symptom of the freeze injury is weak stem strength, which could make the plants very susceptible to lodging (Figure 3). In this situation, as temperatures increase and kernels try to fill, the plant will be under more moisture stress and it may be difficult to meet the evaporative demand of the plant even with irrigation. Kernel size and test weight may also be reduced. Another concern will be that the plant will easily lodge as it matures. The lodged plants may make grain harvest a challenge.