Summer of 2020 in Midwest USA corn-growing areas has been unusually stressful. Early season rain caused delayed planting in many fields, extreme winds in Iowa resulted lodging in healthy plants, absence of rain in many areas in northern Illinois resulted in parts of fields to have plants with yellow leaves long before normal maturity. This is not the usual precursor of stalk rot.
Nitrogen is a major component of chlorophyl and thus contributes to the green color of leaves. This mineral, along with others, is absorbed through the roots along with water and transported to the leaves through the vascular system. Lack of water in soil makes the nitrogen less available in the soil and thus less available for this transportation. The result is the plant leaves slowly turns from dark green to yellow. This also affects the physiology of the developing kernels resulting in abnormally incomplete transport of carbohydrates. Symptoms of late season drought in multiple plants in areas of a field have all plants with completely yellow leaves.
Premature plant death leading to stalk rot, on the other hand, usually occurs with individual plants turning gray, not yellow. These plants wilted from inability to withdraw sufficient water from soil to match the water loss from translocation. The lack of water for leaves was caused by roots dying because of insufficient carbohydrate supply. Contributing to the lack of carbohydrates was the competition with the grain. Individual plants with excessive movement to grain, depleted the stored carbohydrates of the stalk and thus the source of energy for life in roots. Insufficient root life in root hairs resulted in reduction of water absorption and thus wilting of that individual plant. These plants frequently show a slight gray appearance a few days before complete wilting and then all leaves become gray. Loss of turgor results in all leaves to turn downwards. The ear likewise turns down. Adjacent plants may remain green with upright ears until completion of grain fill, normally about 60 days after pollination.
Stalks of the plant death caused by drought and those from wilting also differ. Yellow plants may have sufficient carbohydrate and water in stalk tissue to maintain strength whereas the wilting plant has collapse of pith cells weakening its strength. Both conditions are not good and may ultimately cause lodging but at least it is probably that the additional kernels on the wilted plant can make up for the light kernel weight whereas the yellow plants probably have fewer kernels.
About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.