​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 must not forget that naming the fungus most obvious on the dead stalk is not the same as identifying the cause of the plant wilting early and the resulting weakening of the stalk tissue.  Best to look further into the dynamics of all factors that kept the plant from finishing the season with green and solid lower stalk.

​As the sugars in the corn plant move to developing kernels, each individual plant differs in number of kernels and supply of stored sugars as well as daily differences in photosynthesis.  Shading of leaves by adjacent plants differs especially if distance between plants differs or if leaf pathogens differ.  Water and mineral differences also could differ with soil differences.  Although genetics may be the same, small environmental differences can affect total photosynthesis even between adjacent plants in a corn field. 
 
Transportation of sugars to the ear is largely affected by number of ovules pollinated and genetics of the hybrid.  After the first 10 days after pollination, there is a constant and consistent pull of sugars to the kernels.  This is daily, regardless of daily variation of photosynthesis caused by cloudy and dark days.  Sugars are transported from storage in stalk tissue if not available from leaves.
 
Sugars in the stalk are also used to supply energy for root cell metabolism.  Root tissue started deteriorating shortly after pollination, but the speed of that deterioration is at least partly determined by sugar supplied from above ground sources.  Metabolism of root cells is essential for resisting invasion by fungi in the soil.  Plugging of the vascular system by these fungi interferes with the water intake from the soil and transport in the xylem to the stem and leaves.  
 
Meanwhile, transpiration of water through leaf stomates continues at a rate determined by usual dynamics, hot dry days requiring more water transported from roots to keep the leaves turgid.  Furthermore, water is the solvent for the sugars to be moved from the leaf cytoplasm to and in the phloem to the developing kernels.
 
Water in the stalk also contributes to the stalk strength, especially by keeping the pith cells swollen and adjacent to the outer rind cells, essentially keeping the stalk with the physical strength of a rod.
 
A lot of things are happening in each corn plant that we can’t see but each of these plants is essentially on its own, as dictated by its genetics and environment. 

​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.

​Below is from blogs that I wrote in 2015.  The principles of factors involving stalkrot continues in fields today.
 
With a B.S. degree in Botany from Iowa State I was lucky to teach biology in Sarawak as a Peace Corps volunteer in 1963 and 1964.  Probably like all teachers, I soon discovered how little I knew about my subjects.  Among the many benefits of that job was the short term.  It was assumed that one would leave, and for me that meant grad school and a chance to learn more about plants and fungi. After more studies in Botany and Mycology I took a job with a seed corn company in which studies of plants and fungi was forced to face the realities of corn grown in many environments and considered as a crop instead of individual plants.  I was hired because of the corn industry concerns that their crop’s vulnerability to disease had been exposed with race T of southern corn leaf blight in 1970.  I, nor the seed company, had real clear ideas of what a plant pathologist should do in a company breeding corn seed varieties.  I had a lot to learn.
 
After southern corn leaf blight danger subsided, corn breeders advised me that their toughest problem was obtaining resistance to stalk rot.  So I was a guy with an interest in biology of plants faced with a complex problem.  Why does stalk rot occur in one plant but not the next?  Single cross hybrid plants are, at least theoretically, genetically identical. The fungi accused of causing stalk rot are ubiquitous and surely readily available to attach each plant. So, why this plant and not the next?

A dead plant with yellow lower stalk is adjacent to a genetically identical plant with a green live lower stalk. Why? I first hypothesized these could be the plants that emerged late, as if it developed from seed that germinated slowly because of deteriorating quality. In the field nursery with many hybrids tags were put by plants that showed only 3 leaves and other tags placed by seedlings just emerging when most adjacent plants were showing 5 leaves. These plants were followed through the season.  Late emerging plants definitely did not develop stalk rot. Some of those that were only emerging when tagged actually disappeared but if not they were completely barren. Those tagged with 3 leaves had narrow stalks and tassels that were much smaller and with fewer branches than surrounding plants, silks emerging later than other plants and averaged about 20% of the yield of majority plants- but no stalkrot.  I showed the plants to the fellow that evaluated winter growouts for purity of new seed lots. He said that confirmed his opinion that some plants he saw that looked like they could be inbred selfs were actually late emerging plants.  To make sure that these were not inbreds, the next year I hand planted 5 hybrids leaving interplant space for planting the same hybrids later between the initial planting.  These intentional late emergence plants looked the same as in the first year’s observation.  Late emergers apparently suffer from competition resulting in having very small stalks and yield but the stalks remain green.  Late emerging plants do not yield but also do not explain why adjacent plants do not behave the same in terms of stalk rot.
 
The concept that late emerging plants do not perform well and that seed quality is a significant component to good grain yields was observed by many others before that experiment in 1973, but it was new to me.  It also set me on path to find a better method of evaluating purity of hybrid seed lots as well as finding other explanations as to why a dead plant would occur adjacent to a green one only a few inches apart. It is probably significant that commercial hybrids of 2015 have a lot less ‘flex’ than those of 1973, but the basic importance of uniform emergence remains.

​If a corn plant draws more carbohydrates to the ear during those critical 60 days after pollination than it can supply with current photosynthesis and storage in the stalk, it depletes the supply needed to keep root cell’s metabolism. The deterioration of root cell metabolism allows invasion of organisms in the soil and inability of the roots to transport water to the plant parts above the soil.  Transpiration from the leaves continues until all available water is gone.  Then the plant wilts. Symptoms of the wilt become slightly visible for a few days before all leaves turn gray and droop.  We call this premature death. These wilted plants occur as individuals, often surrounded by green plants that did not overdraw on its carbohydrate supply to fill its kernels. Often these individual wilted plants have more kernels than those adjacent green plants or have excess photosynthetic stresses.
 
The wilted plants initially have green outer rind to the lower stalk, but the pith tissue inside the stalk has puled away from the outer rind, as part of the wilting process. This weakens the stalk strength by one third.  The outer rind slowly turns from a green color to yellow and shows symptoms of invasion of fungi as they digest the remaining cellulose and proteins in the stalk cells, further weakening the stalk strength.
 
The critical stresses leading to stalk rot occur during the kernel fill period, those 60 days after pollination. If a plant makes it through that time without wilting, it probably will not get premature root ands stalk rot by normal harvest time.  Inspection of corn plants about 60 days after pollination allows the grower to access the to predict probability of stalk rot and lodging in the field that season.  Individual gray plants with yellow and brown color to outer rind of the lowest 2-3 above-ground nodes are most likely to lodge with slight wind pressure.  These plants easily fall with slight pressure.  One can also easily pull the plants up from the soil as the dead roots offer little resistance.
 
Why did that plant commit more to kernel fill that its photosynthesis could supply?  Was it shaded by adjacent plants because of inadequate spacing of seed, did it have excess damage to leaves from pathogens or insects.  Did the environment of the roots cause less root mass or destruction from pathogens or insects?  Perhaps the genetics of the hybrid encouraged a higher kernel number in that environment than could be supported under the photosynthetic stress of the season.
 
Having a few early wilted plants in a field can be a sign that the hybrid maximized its ability to produce grain yield for that season’s environment.  One can learn a lot with inspection of the field shortly after the 60-day kernel fill period.

​Sugars are transported to each kernel for days 10-50 after pollination if most corn plants if field conditions are favorable. Then the transport slows for another 10 days.
 
​Each kernel in a corn ear is a fruit.  As with most other fruits, sugars are transported to the kernel through the vascular system from the leaves and the stem.  Plant hormones like auxins and gibberellins produced in the seed embryo meristems actively guide the sugars to the seed within the fruit.  Corn kernels have only one seed. Although much of the sugar is moved outside of the embryo to the endosperm, sugar also provides energy for growth and development of the embryo.  As the embryo matures, auxin production is reduced.  Consequently, physiological demand for sucrose supply to the kernel is reduced. 
 
Reduction of sucrose in the cells at the base of the kernel causes the balance of ethylene and auxin to change.  Ethylene increase causes the layer of parenchyma cells closest to the kernel base to lose cell wall contents, while those adjacent cells away from the kernel gain wall thickness.  Eventually an abscission layer forms cutting off all movement of sugar into the kernel and water movement away from the kernel thru the stem tissue (the cob). 
 
Research by J.J. Afuakwa, Crookston and R.J Jones (Crop. Sci. 24. 285-288) showed that reduction of sucrose available to the kernel was a major factor in induction of the abscission layer (black layer).  Although it is mostly related to maturation of the embryo, it could be induced by other factors reducing sucrose supply to kernels.  Reduction of photosynthesis by leaf disease or frost damaged leaves could result in shortened time to black layer.  Early plant death, perhaps from root rot, causing leaves to wilt and thus removing sugar supplies induce black layer within a few days.  
 
Plants with green leaves 60 days after pollination now will undergo slow maturation with abscission layers forming at the base of each leaf. Those abscission layers cut off the transport of sugars from the leaves and the transport of water to the leaves and removal of water from the plant via transpiration. Reduced competition with kernels for sugars stored in the stalk pith tissue allows roots to slowly age.
 
Plants that did not manage to make it the full 60 days with green leaves likely had photosynthetic stress reducing the ability to meet the demand by kernels for sugars. As a result, root tissue on that plant lost its ability to adequately battle the multiple potential pathogens.  Loss of living root tissue during the kernel fill period results in less water uptake and consequently early wilting of the entire corn plant. Kernel filling stops at that time.
 
 

​Competition for carbohydrates between kernels in developing ear and plant metabolism sites especially in the roots of a corn plant is established 10 days after pollination. This is a battle being fought individually by each plant in a corn field, as each plant can have slightly different environmental factors influencing its ability to produce carbohydrates and different numbers of kernels.
 
Net supply of carbs is influenced by environments, like light intensity, in which the rate of photosynthesis drops directly with less light. Cloudy days result in lass carbs produced.  Leaf disease, reducing photosynthesis due to less leaf area, as does shade from adjacent plants. We have selected genetics programed to move carbs to non-photosynthesizing tissue like the roots with excess stored in stalk.  During the grain fill time of the corn plant, newly produced carbs are transported to the kernels as well as those stored in the stalk pith tissue. The draw to kernels is constant, influenced by genetics and minerals, regardless of daily photosynthesis factors. Competition between kernel development and root cell metabolism for carbohydrates reserves stored in the stalk tissue becomes more intense if daily photosynthesis is reduced during the 40-day period of maximum draw by developing kernels.
 
Energy supplied by the carbohydrates pulled to the root tissue is used in its cells’ metabolism for normal function including producing the compounds needed to ward off the multiple organisms in the soil attempting to devour the root tissue. Defense of the living, functioning root tissue is essential to the rest of the corn plant, as the minerals and water absorbed by roots and transferred upwards are essential to function.
 
It is a battle essentially between roots and kernels for carbs that is fought by each individual plant in a corn plant especially between day 10 and day 50 after pollination.  If carb supply is not sufficient to meet both needs, kernels win the race. The roots degenerate prematurely and are unable to supply the water to leaves to match the loss of leaf water due to transpiration.  As a result, the plant wilts.  Kernels’ win is temporary, as the wilted plant no longer can transport carbs to kernels. 

​Photosynthesis is the engine driving corn from the seedling to maturity.  Previous generation's photosynthesis provided the energy for the seedling to emerge from the soil. Current generation photosynthesis provides more than sufficient energy for the metabolism to build tissue to construct a plant with expansive roots, large leaves, and 6-9 feet of stalk within a few months of seedling emergence.  The corn plant not only provides the energy for the building materials for its new structures and daily metabolism but also has excess carbohydrates that are stored in the stalk pith cells.
 
Then, at midseason, it produces flowers.  After pollination for the female flowers, hormones in the plant shifts the direction of flow of the carbohydrates from leaves and stalk pith cells to the the new embryos and its storage compartment, the endosperm.  Environment and genetics determine the rate of flow of carbohydrates to the new fruit of the corn plant.  The number of fruits (kernels) also becomes a big influence on the total draw from the carbohydrate supply.
 
Flow for first 10 days after pollination is relatively slow but then it speeds to a faster, relentless pace for the next 40 days.  That daily pace of movement of carbs to the ear on the plant continues regardless of daily variation in photosynthetic rates due to environmental variables.  If cloudy weather slows photosynthesis, the reserves in the stalk provide the difference.  If leave disease reduces leaf area available to light energy conversion to carbohydrates, storage from early days is called upon for movement to the new kernels.
 
This high rate of flow of carbs to the new kernels slows after day 50 for about 10 days until it is cut off with the formation of an abscission layer at the base of each kernel.  If the other plant parts such as the root tissue survived the competition for stored carbohydrates in the stalk, slow senescence occurs in the plant cells. This is the outcome desired by all corn growers but environmental stresses can interfere with the completion of maximum deposit of carbs in the kernel on living plants.

​About 5-6 weeks after pollination is a good time to evaluate corn hybrids for resistance to leaf disease. This Corn Journal blog from 2017 discusses some of the dynamics in resistance to leaf diseases.
 
Leaf epidermal cell’s walls and the waxy leaf surface provide the first line of defense against microbes.  Pathogens adapted to overcoming this defense set off the next defense system after penetrating the leaf.  This is initiated by the plant detecting the presence of the intruder. Plant cells nearby detect the presence of a protein exuded by the pathogen. Such proteins are called effectors, as they are detected chemically by host cells near the invader.  Upon detection, these adjacent host cells produce potential microbe-inhibiting compounds such as reactive oxygen, nitric oxide, specific enzymes, salicylic acid and other hormones to effectively thwart the pathogen growth.  Much initial reaction is limited to host cells adjacent to the infection site.
 
Resistance to corn leaf pathogens such as Exserohilum turcicum, cause of northern leaf blight, Cercospora zeae-maydis (gray leaf spot) and Bipolaris maydis (southern corn leaf blight)
Involve detection of that specific pathogen and production of more general antimicrobial products in the immediate area of the pathogen.  These two steps are inherited independently. Perhaps the pathogen detection system is more specific to the pathogen, accounting for a corn variety being more resistant to one pathogen than another.  On the other hand, I am suspicious that if two pathogens arrive in the same area of the plant, only one will survive, as if the plant reacts to the first one by producing general resistance compound that inhibit the infection by the second one to arrive in the same area.
 
The system described above is referred to as general or horizontal resistance.  It is controlled by 3-5 genes for products to detect and reduce spread of the pathogen.  Horizontal resistance is expressed in corn plants by fewer leaf disease lesions.  Evaluation of varieties for this type of lesion has some ambiguity however, because the number of lesions or amount of leaf damage is also affected by the intensity of disease pressure.  Heavily diseased leaves from the previous season in fields of low tillage, with frequent early season rain can result in more leaf lesions in a variety of good general resistance to a pathogen than will occur in one of poor resistance with little disease pressure.