Name that stalk rot
Assigning a name to the dominant fungus in a rotting corn stalk implies that an aggressive pathogen attacked the plant and therefore caused the plant to be vulnerable to lodging. Symptoms of black streaks on outer rind indicates that the problem was anthracnose, caused by Colletotrichum graminicola. Small dark pycnidia emerging from rind tissue near the node indicates that Diplodia maydis was dominant. Perithecia of Gibberella zeaeon surface of rind and pink color inside the rind of the lower stalk suggests that this was the main cause of the rot. Fusarium verticilloidesis always present in rotting stalks and therefore can be assigned as the cause if none of the above symptoms are present.
It may be comforting to assign a name for the stalk rot, but it can lead to avoiding the more difficult analysis of the actual cause of the plant dying before normal completion of grain fill. The predisposition of the stalk to invasion by any of these fungi involves the complex biology of that corn plant as it moves available sugar to the grain. If the plant did not supply enough carbohydrate to meet that demand, root tissues became deficient of the metabolic energy needed to fend off the multiple soil organisms capable of invading and digesting weakened root tissue. Similar deficiency in lower stalk tissue likewise decreases resistance to the many fungi, including those named above. If roots are sufficiently damaged that water uptake cannot keep up with water loss from transpiration from the leaves, the plant wilts. This further weakens the stalk as the pith tissue withdraws from the rind, changing it from the strength of a rod to that of a tube.
Although the fungal related cause of the stalk rot may be easy to analyze, the more important analysis should involve the basic reasons that the plant did not reach the season with sufficient sugar to maintain root and stalk living tissue until completion of grain fill. Did a leaf disease result in reduced photosynthesis? Did early season environment factors, such as water supply cause better silk emergence than normal for that hybrid but late season stress such as lack of water or cloudy weather reduced photosynthesis? Was the plant density too great for that hybrid for that season? Nitrogen-potassium ratio was wrong for that season? Best hybrid for the previous season may not have been the best for this season and, likewise, the one that had stalk problems in a season with more stalk problems may be the best the next season.
It is good to analyze for potential causes of stalk rot in the field when it occurs, harvest before severe lodging and then put together the potential causes of insufficient photosynthesis to meet the translocation demand to the ear without causing excessive root tissue death. Having a name for the obvious stalk rotting fungus may seem comforting but getting at the main biological cause can lead to reduced problems in the future.
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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.