Gibberella zeae is a fungus belonging to a group of fungi called Ascomycetes. This group are defined by production of spores after meiosis in microscopic sacs (ascus is Greek for sac). It is typical of this group of fungi to also have asexually produced spores that are distinct and, in some cases, identified separately, and given distinct names. The asexual stage of this Gibberella zeae is Fusarium graminearum. Although formal nomenclature protocol requires naming a fungus by its sexual stage either name is commonly used to identify this same fungal species.
This fungus receives nutrition from weakened or dead plant tissues, primarily grasses. Often initial invasion of the tissue is initiated by the spores (conidia) of the Fusarium portion of the life cycle. The fungus quickly produces more spores on corn debris, spreading readily in a corn field. Spores landing on exposed silks germinate and grow down the silk channel into the tissue within and around the kernels. As the kernels develop within the moist environment of the tight husks, the fungus spreads to appear as a pink mold. Haploid nuclei within the Fusarium hyphae fuse to form a brief diploid nucleus. This causes the fungus to produce a new structure, called a perithecium, a distinctive black body on the surface of the infected host. Within the perithecium, meiosis occurs, resulting in haploid ascospores within the ascus. Ascospores are released, perhaps in the following season. These infections result in the fungus further reproduction and spread of the fungus.
Gibberella stalk rot and ear rot are usually identified presence of the perithecia, rough black structures produced on the outside of the stalk or kernel tissues. Kernel infections often are also a concern because this fungus tends to produce chemicals such as zearalenone and deoxynivalenol (DON). This probably the most significant concern of problems from this fungus. The stalk phase is more of a secondary invasion of dying stalk tissue because of senescence of plant tissue because of the balance of carbohydrates within the maturing corn plant. These can be some satisfaction in having a name for the fungus, but analysis of the cause is more significant to reducing the future problems.
Fungi invading grain while on the ear vary by season, location and hybrid. In many cases the actual invasion of the grain occurred through the silk during pollination time of the season. The channel in the silk in which the germinating pollen tube grows towards the ovule is often the same channel for invasion by a fungus. This channel closes after the pollen tube passes, essentially closing the path for the fungal mycelium. Avoidance of fungal invasion into the ovule and therefore the developing grain is highly dependent on pollination occurring soon after silk emergence from the husks around the young ear.
Circumstances that tend to delay pollination include drought pressure at flowering time, causing delay in silk emergence but not in release of pollen. Less pollen available results in scattered kernels on ears or no grain at ear tips because pollen was expired before the silks leading to these ovules were gone. Aspergillus and Ustilago (smut) species produce spores during dryer environments.
Extensive rain during pollination time can also result in more infected grain. Pollen is not released from anthers when wet from rain. Silk emergence is enhanced with high water tension. These dynamics tend to delay pollination of exposed silks, as well as encouraging sporulation of Fusarium, Gibberella and Diplodia species.
After entering the ovule, these fungi can successfully invade the developing embryo and endosperm. In many cases this becomes evident as a molded kernels and in some cases the infections are not evident but only show later if the grain is not dried relatively quickly to 15.5% moisture.
Other environmental factors such as previous infected corn debris to provide fungal spores are important. Genetics of hybrids influence the vulnerability to silking and pollen timing problems when plants are under stress. Hybrids differ in kernel composition and development and spread of infection if fungi invade the ovules. Rapid drop in kernel moisture after black layer can be related to husk characters like thickness and looseness, essential to rapid loss of grain moisture in the field.
It is important to use care in analyzing cause if severe ear rot occurs. Was it the genetics of the environment that was primary?
As corn matures, and grain fill is completed, the usual challenge is to allow the grain to reach maximum loss of moisture before harvest without losing ability to be harvested due to lodging. This usually depends upon stalk strength after completion of grain fill. Hybrids do vary on the cellular structures of rind tissue, affecting the ability of the rind to be punctured with a penetrometer. Cellulose and lignin synthesis in the stalk tissue during plant growth is involved and affected by combination of genetics and minerals.
Although the rind resistance to breakage is significant, a solid stalk, with the pith tissue intact and attached to the rind, creates the strengthening dynamic of a rod versus that of a tube after the pith is pulled away from the rind when the plant wilts. This can be detected in a simple push test of plants after grain fill. Those with hollow stalks will easily pushover. These individual plants will also show symptoms of invasion of fungi such as species of Diplodia, Colletotrichum, Fusarium or Gibberella. These are the fungi most often identified on dead stalks, but several other species also attack the cell wall components of the rind. Presence of these fungi often lends credence of blaming the stalk vulnerability to the lodging on these fungi.
Intactness of the pith tissue, however, is probably the most significant factor in occurrence of late season lodging of corn stalks. This occurs when the plant wilts previous to completion of grain fill because insufficient carbohydrate is available to fill the grain and maintain life in the root. Wilting causes the withdrawal of the pith from the rind, thus significantly weakening the stalk strength. Fungi readily invade the dead tissue but the main strength weakening preceded their invasion.
Stalk lodging is affected by many factors. Genetics and pre-pollination environments affect ear height, root growth, photosynthesis, leaf disease, stalk rind and storage of carbohydrates in stalk pith tissue. Genetics and post-pollination environments affect the ability of individual plants to obtain completion of grain fill and stalk strength until harvest.
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.
Early formation of black layer before normal time for full transport of sugars to the kernels results in reduced grain weight. Shortened time to black layer also reduces some of the water replacement in the grain normally occurring as water moves out through the vascular system. Consequently, black layered kernels may have higher moisture percentage than usual. Some of these biological factors contributed to the grain production differences in USA Midwest in 2019.
Stalk rot of corn occurs when sufficient root tissues dies of starvation. This happens when its energy source, carbohydrates stored in the stalk, are depleted because of excessive draw to the grain versus the supply from leaves and stalk. Size of daily movement to grain is determined by genetics and environments. Soil moisture during time of silk extension is significant to more ovules being pollinated. Supply of carbohydrates during the season is determined by multiple factors: Hours and days of intense light, utilized by it C4 photosynthesis method to supply energy for more plant growth, storage of carbs in the stalk tissue as well as sugars to move to the grain after pollination. Individual plant environments such as competing with adjacent plants for light and mineral uptake can influence success in normal completion of transport of sugars to all pollinated ovules on each plant.
Seasons with extreme late planting dates in temperate zones have extra factors. Some fields will be fewer stresses in early season, resulting in less ovules pollinated, resulting in less grain but less stalk rot. Late planting dates still requires about 55 days after pollination to complete movement of sugars to the grain. It can be stopped with freezing the phloem within the stem tissue, with the result of light grain weight. Such a freeze may not result in stalk rot as remaining stalk tissue may remain intact.
If the below freezing temperatures were not sufficient to kill the stem phloem but did result in leaf death, depletion of the sugars in the stalk are intensified. This can increase death of stalk pith tissue, allowing stalk rotting fungi to digest the cells and weakening the stalk strength.
We should expect variable field results in Midwest USA in 2019.
Severity of below freezing temperatures while a corn plant is moving sugars to the grain is dependent on what tissue freezes. Leaves are most vulnerable because of exposure to the cold air. Ice crystals form in the leaf cells, killing the individual cells including those with chloroplasts. Among these chloroplasts are those in guard cells of the stomata. Without photosynthesis the day after freezing, the guard cells do not open to allow transpiration.
Water movement from the root tissue to the upper plant is a physical phenomenon in which each molecule evaporating through open leaf guard cells, is replaced by a molecule of water because of water’s molecular structure causes bonding with each other. Water is essentially pulled from the roots through the xylem structures of the vascular system because of this bonding. If water is not utilized or transpired, leaves do not receive new supplies of water, further causing wilting of the leaves after frost or a more severe freeze.
Sugars are transported through living phloem cells in vascular tissue. During grain fill, sugars are drawn from the leaves and the stored reserves in the stem. Death of leaves eliminates movement from the leaves. If the low temperatures were not severe enough to kill the phloem and other stem cells, movement from stem to grain continues.
Below freezing temperatures during grain fill will cause some loss of expected grain weight, but the severity will depend upon whether stem phloem is killed and the supply of sugar reserves in the stalk.
As if the stresses that reduce photosynthesis isn’t enough to offset the balance of movement of sugars during grain fill, the 2019 USA Midwest had extreme water in the early season, even after corn was planted. Sugar movement, stimulated by hormones produced by meristem tips, goes to apical meristem of shoot prior to flowering and to root tips. After pollination the concentration of apical meristems in the ear redirects the flow towards the grain, competing with the flow towards the root tips.
If root growth was inhibited by extreme moisture in the soil, perhaps by low oxygen supply, does it result in fewer root tip meristems? If so, does this reduce its capacity to attract sugars during the season and does this increase the vulnerability to the root pathogens. This would result in increasing probability of the plant wilting during grain fill as well. Stalk rot follows after plants wilt.
Deterioration of stalk quality follows plant wilting during the grain fill period. Wilting is caused by root tissue unable to absorb enough water to be transported to leaves. Loss of water from leaves occurs through leaf stomata via evaporation. Dry, windy environment around leaves causes more rapid transpiration. Early season environment affects root growth and mid-season environment affects the size of grain sink. Genetics determine how the plant reacts to these environments. These multiple factors determine whether the grain successfully completes normal grain fill on plants with green stalks or not.
Individual corn plants that have brown lower stalks are invaded with several fungi that can be identified in the dead stalk tissue. Some, such as Diplodia (Stenocarpella) maydis, Gibberella zeae, Colletotrichum graminicola and Fusarium verticilloides, become obvious in the deteriorating stalk tissue and therefore the naming of the stalk rots as Diplodia, Gibberella, Fusarium and Anthracnose. Although these are the most frequent and most easily identified fungi found in those plants with rotted stalks, the underlying cause of the plant death involved more complicated biology of carbohydrates to the root tissue as the plant moves sugars to the grain. If the movement to grain is too great for the supply from the leaves and stored carbs in the stalk tissue, roots suffer without sufficient energy to meet the root metabolism needs. Eventually deteriorating root tissue succumbs to destruction by soil microbes, resulting with the plant wilting. This allow many fungal species to advance into the dying and dead stalk tissue, destroying the structural strength of the stalk.
Beyond naming the dominant fungus present in the dead stalk, it is important to identify the cause of insufficient supply of carbohydrates. Potential causes are shading from other plants, leaf disease destroying leaf tissue, insufficient sunlight, leaf removal from corn borer, or hail damage.
Stalk rot of corn is a problem in some fields somewhere each year. Complexity of environments and genetics makes conclusions of causes very difficult. Predicting whether this year’s performance is likely to reoccur is not easy.
More perspectives on corn stalk rot can be found by using the search on this page in Corn Journal. It is a subject that I (and others) have studied for a long time.
That wilted corn plant, often surrounded by green plants with carbohydrates still being transported to the grain, did not have sufficient supply of carbohydrates available to meet the draw to the ear. This may have been due to reduced photosynthesis because of leaf tissue destruction from leaf disease or hail, shading from adjacent plants, insufficient potassium available or dark cloudy days.
Movement of carbs to the grain is directed by hormones produced in growing points, each embryo having an apical meristem producing auxins to direct the flow. Genetics affects the amount per day and number of meristems affects the total flow per day. If daily photosynthesis in leaves is insufficient to meet this demand, reserves stored in the corn stalk are drawn upon. Sugars that are being stored in grain are also required to maintain life in the root cells. Depletion of stalk sugar reserves available to roots, eventually weakening those cells ability to resist invasion by soil microbes.
Eventually, root rot reduces water uptake and transport that water availability to transpiring leaves causes desiccation of all plant tissue. All leaves on this plant show the wilting symptom by a gray appearance and pointing downwards. Wilting also causes the pith tissue, previously attached to the inner layer of the rind tissue to shrink away from that attachment. Stalk cells die. Chloroplasts in the outer rind cells die, resulting in the lower stalk tissue turn from a green color to yellow-green and eventually brown, while adjacent plants continue to have a green color.
Abscission layers form at base of all leaf tissues immediately after tissue wilting. That includes the formation of black layer at base of each kernel. Consequently, these kernels will not have as complete grain fill as the adjacent green plants that continue to receive the flow for the 55 days after pollination. If the major cause of the early wilt was from producing more kernels than the adjacent green plants, the total grain weight on the wilted plant may not be much different although the weight per kernel will be less.
Cause and effect of wilting corn plants is dynamic with multiple interactions between the corn biology and environments. Assessment of these potential causes when they occur could be useful in preventing significant yield and harvest problems in the future.
As the corn plants approach completion of grain fill about 40-50 days after pollination, some individual pants will change in color. These changes can indicate the effect of the season on the plants.
Plants in which all leaves are gray, then brown, pointing towards the ground, have wilted because the roots could not provide sufficient water to meet transpiration needs. These have root rot caused by lack of sufficient sugars moved from the leaves, allowing soil microbes to invade and destroy the root tissue. Movement of limited supply of sugar moved to the grain was preferred over movement to roots. That individual plant did not produce sufficient photosynthesis to maintain both the root life and grain fill. Such plants will develop hollow stalks and be invaded with fungi. These plants are likely to lodge by harvest time.
Some individual plants may turn red gradually during this time period. These plants usually have only a few kernels. Sugars accumulate in the leaves because not enough hormone driven transport is causing the sugars to be moved, or perhaps because of interference of movement because of insect damage to the vascular system. Chemical processes in the cells with accumulating sugars transpose the sugar molecules to anthocyanin, providing the red color in the leaves. This is usually an indication of poor pollination, perhaps because this individual plant emerged late as a seedling and therefore missed much of the pollen from other plants.
Nitrogen deficiency is indicated when several plants have yellow lower leaves while upper leaves remain green. This is common in areas of fields that were water-logged.
Desired color of maturing plants indicating a successful yield has white ear husks while other leaves remain green and turgid all the way to completion of grain fill, about 55 days after pollination. The occasional wilted plant can be a sign that the field got maximum grain allowed by that season’s environment.
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.