Root rot comes first
Stalk rot nearly always begins as a root rot. Root rot by organisms is nearly always occurs because the root suffers from lack of sugars supplied from above ground to parts of the plant. The symptom we see is plant wilting and lower stalk turning brown but the below the soil surface, the root is deteriorating.
Sugars produced in pre-flowering corn plants supply the basic energy and carbohydrates for root growth and metabolism just as photosynthesis provides similar tasks for leaf and stalk growth. Hormones such as cytokinins produced in growing points are linked to the movement of sugars to the above and below ground parts of the corn plant. Roots are hard to study but research has shown corn root size begins to detract about 10 days after flowering due to root rot. This rotting can be gradual and may have no above-ground visual effect.
Movement of sugars to newly formed kernels is slow for the first 10 days after pollination, with 80% of the deposit occurring during the next 40 days, at the rate of about 2% of the total per day. That movement is linked to the hormone production associated with each new embryo in the ear. This pull to the ear is constant during that period regardless of reductions in photosynthetic rates due to cloudy weather or leaf disease. Sugars come from other sources such as those stored in the corn stalk pith cells. It also becomes a major competitor with the root cells in need of the sugars for the metabolism to prevent invasion by the multiple microorganisms in the soil with enzymes to destroy root tissue.
If the reduction in photosynthesis during this grain fill period is drastic and is combined with a large pull of sugars to the developing kernels, root destruction by pathogens can cause sufficient interference with uptake and transportation of water to the leaves. Failure to replace water lost by transpiration, causes the plant to wilt. A plant with bright green, turgid leaves suddenly turns gray in color and limp in structure.
An extreme example of the stress of too strong of movement sugars to the ear is observable in the outer row of a corn field where those few plants with 2 fully-pollinated ears show early wilt symptoms. In the canopy of the field, those wilted plants will either have more kernels than adjacent plants and/or show some signs of reduced photosynthesis such as borers causing upper leaves to be removed, leaf damage from foliar disease or uneven spacing allowing shading from adjacent plants.
There are genetic factors influencing root structure, number of kernels, amount of sugars translocated to each kernel, photosynthesis rate per plant and reactions to environmental stresses. Early wilting of plants not only allows the progression of fungi associated with stalk rot but also directly weakens the strength of the stalk.
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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.