Corn stalks have a green outer rind color during most of the growing season as the outer cells are pigmented by the chlorophyll. This color continues beyond grain fill as this annual plant matures without wilting, even up to normal grain harvest. If the plant wilts because of root rot, not only do the leaves desiccate, turning from a green color to gray within a few days and then brown but the stalk color changes also. The dark green color becomes yellow-green a few days after the plant wilt. This color change progresses to yellow and a few days later to brown.
As the brown color intensifies, desiccation of the internal pith causes withdrawal of the pith from the outer rind. This changes the stalk structure from a solid rod to a tube, reducing the strength by a third, leaving it vulnerable to breakage. One can access this vulnerability by gently pushing the stalks or pinching the lower stem. Visual inspection of the color of the lower stalks to judge this deterioration also can be used to evaluate the plant’s vulnerability to lodging. Individual plants with green lower stalks a few days after grain ‘black layer’ will remain intact through harvest.
The anthracnose fungus, Colletotrichum graminicola, will cause black streaks on the outer rind even on a green stalk. This color only intensifies, however, if the plant wilts, apparently because the living cells can restrict the fungus. If there remains a green color around the black streaks, the lodging threat is not great. Another interaction with the fungus commonly occurs in the uppermost internode of the corn plant. It is often noted that this internode turns brown when remaining stalk is green. As sugars are moved from leaves to the grain, this upper internode often is depleted first resulting in senescence of this tissue. The anthracnose fungus is often found in the dying tissue. I interpret it mostly as signal that the plant is successfully moving maximum carbs to grain and not necessarily a sign of stalk rot.
Other fungi also become noticeable on the dead, brown lower stalks. Gibberella zeae produces its reproductive bodies near the nodes, Diplodia maydis produces theirs more scattered on the internode tissue and Fusarium moniliforme gives a pinkish discoloration across the internode surface. It may give us some comfort to have a name for the fungus present but it must be remembered that the cause was insufficient carbohydrate to both meet the translocation demands of the grain and the maintenance of root life. These fungi, and the many others also digesting the senescing and dead stalk tissue were not actively killing the plant.
One can see a lot by looking.
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.