We can be thankful for naturally occurring mutations. It is basic to providing the eventual variability that has driven and continually drives evolution. It allowed the deviants in Teosinte that was selected by people in Mexico 10000 years ago and the multiple selections in corn as it was moved worldwide since then. Most research has verified that most of these genetic mutations result in recessive genes and thus the presence of the mutation is not often expressed in a diploid present in which the dominant member of the paired gene is expressed. The su genes resulting in sweet corn is only expressed when the recessive gene is expressed in both members of the diploid plant. Same is true of the mutants wx for waxy. This is true for multiple other homozygous recessive traits.
Occurrence of mutations can be an advantage or a disadvantage. In most cases, being recessive, the mutation may not be detected by performance of the hybrid. Selfing to achieve homozygosity during the inbred development reflects the negative affect of making some recessive genes more homozygous. This is reflected in reduction of plant size from the heterozygous parent used for inbred development. The selection process with each generation does allow elimination of some negative homozygous recessives. Double haploid systems do not allow generational selection because the homozygous condition is fixed.
Expression of hybrid vigor when an inbred is crossed with another specific inbred is mostly due to dominant versions of the negative recessive genes of the inbred parents. That is probably why prospective commercial hybrids are from crosses of inbreds with distinct ‘families’, each not likely to share the same negative recessive versions of important genes. Corn has 40000 genes, including some negative recessives, perhaps due to mutations. The seed industry uses hybrid testing, and inbred development to select for hybrid performance. Further selection among those near-inbreds can allow for selection against the few negative traits found among some plants to improve inbred performance in hybrid production. That has been consistent with our experience in our proprietary Rapid Inbreeding® program. Diversity is good.
Differences among all living things is driven by mutations. A simple change in the nucleic acid position within the DNA or RNA code can affect the structure of the protein being produced by a gene. That protein’s function among the several affecting production of some cellular process ultimately can be significant to the organism’s appearance or function. Every cell division is vulnerable to such slight changes in these types of slight changes in DNA. Those occurring during cell division of cells not involved in reproduction result in a group of cells differing from adjacent cells is called a chimera. It could result in a distinct streak in a leaf of one corn plant. Most chimeras do not continue to the next generation if the mutation was not present in the nuclei of the reproduction cells.
Mutations during meiosis are the main source of genetic diversity in organisms with two sets of chromosomes (diploids). Multiple mutations accumulate over generations resulting in distinctive characteristics among genotypes. Organisms with shorter life cycles are likely to gain diversity quicker. Selection of those best fitting an environment can include variations utilizing different physiological processes to accomplish this success. The randomness involved in mutations also includes differences that we humans do not know are of necessary function but simply are present.
These genetic differences among corn varieties can be detected by comparing plant structures, proteins or DNA. Professional Seed Research, Inc. compares leaf structures of three leaf plants when growing in uniform environments, allowing the distinction between expected phenotype versus those produced by the wrong parent, such as selfing or pollen from wrong male parent. Careful observation of mature plants also makes this distinction. In all of these methods, we detect diversity but not necessarily significant differences in desired function of the plant.
Although diversity is frequently random or at least without recognized useful function, we use it as a means to detect distinct varieties. Taxonomists specialize in observations of these differences to define species. Humans use phenotype expression of facial characters to distinguish among individuals. It is best, at this point, not to comment about those human characteristics most desirable to me, but just as with corn, some diversity due to genetic diversity would seem less functional than others.
Among the 30000-40000 genes in corn, some mutations have occurred over time to be useful to humans and some are distinctive but not obviously useful in today’s environments. In general, we should celebrate diversity in all organisms. Mutations are great!
Our exposure to diversity, in environments, plants, pathogens and in our species shows that diversity is basically good even if it causes some temporary stress. And we have the ability to adjust to changes and move on. This blog from Corn Journal on 7/5/2016 illustrates how it works in corn.
One study of a single corn inbred (B73) indicated that it had 30,000 genes. We benefit from a species with a huge genetic potential and a pollination system that encourages new mixes of genetics. This has allowed the species to be used for food in a wide range of environments. Some of those genes are turned on in response to the many microbes searching for the products of photosynthesis for their nutrition. Microbes have genetics too! Some, like rust and smut fungi, survive by attacking living corn cells, drawing carbohydrates to the cell, and then moving on via spores before the host cells respond to the fungus. Many fungal pathogens of corn simply kill a limited area of the leaf tissue, feed on the dead tissue, produce spores and infect new areas. Corn varieties differ in how quickly and strongly they respond to the invasions.
One fungus that I find interesting is Bipolaris zeicola. It was formally known as Helminthosporium carbonum. There are genetic variants of this species that apparently feeds only on dead leaf tissue, often caused by insect damage or simply physical injury. These variants apparently lack the genetics for either penetrating the live corn leaf tissue or overcoming the resistance system of most corn varieties. At least one variant of this fungus produces a toxin that kills corn plant cells but most corn varieties have dominant gene that effectively blocks this toxin. However, very occasionally, a mutation of that dominant corn gene does occur while developing new inbreds. If this mutation, now a recessive gene, becomes homozygous during the inbreeding process, the inbred is vulnerable to the toxin. The result is practically no defense to this variant (race 1) of B. zeicola. The pathogen kills small leaf area of leaf, produces spores and spreads to new leaf tissue and eventually causes the whole corn plant to die early. Because susceptibility is recessive, and the dominant toxin-resistant gene is present in most corn inbreds, this creates a problem for seed producers but not for hybrid growers. Good that corn has genetic diversity.
2019 diverse US corn belt environments was a strong reminder of all the significance of diversity among corn hybrids. This principle applies to all areas of earth in which corn is planted. Following is from Corn Journal on 11/13/2018 attempting to discuss corn diversity.
Corn’s history and biology has resulted in diversity beyond what most of us see in any single season. Advantages of hybrid plant uniformity for yield, harvestability, disease and pest resistance and genetic repeatability requires development of homozygous inbred parents. Each of many seed companies produce multiple hybrids each year and there are about 40,000 genes in each corn plant that are available to influence something, whether needed or not.
Corn researchers in 1920’s became aware of a need to collect and share many of the genetic sources in corn, forming a Maize Genetics Cooperation Stock Center- it’s history is summarized at http://maizecoop.cropsci.uiuc.edu/mgc-info.php. This collection started and continues to emphasize mutants affecting some identified trait, such as those involved in sweet corn, waxy corn or amylose corn and many that may not have a specific economic advantage but are useful in understanding some biochemical pathway in the corn metabolism. Study of these mutants contributed to location of genes on each chromosome and add to growing knowledge of corn DNA codes for many traits.
Despite these efforts, corn’s genetic diversity is large due to selection by humans over diverse environments. Our experiences with ‘new’ diseases as a pathogen such as the bacterium causing Goss’ wilt suddenly appears, with a previously unknown susceptibility gene in corn became part of popular corn hybrids, or susceptibility of race T of southern corn leaf blight associated with mitochondrial gene in t-cytoplasm male sterile corn. Resistance to Maize Chlorotic Mottle Virus was found in corn genetics in USA after it occurred in Kansas in the 1970’s and in Africa in 2016.
Often the strong resistance to these diseases are associated with single genes already present in corn apparently without intended human selection and without known selection pressure in absence of the disease. Perhaps there was exposure somewhere in its history where the gene was favored but also it is likely that randomness of mutations, segregation of genes during miosis, cross pollination and historic diversity of corn’s environments have provided many genes for characters that we have yet to identify. These genes must be influencing multiple internal aspects of absorption of light wavelengths, translocation of carbohydrates, absorption and movement of minerals, water uptake and conservation, and structures of leaves. Among this diversity is the future adaptation needed for changing environments.
Breeders witness diversity within their nursery as they see differences in plant structures and growers see differences among hybrids in performance each year. At Professional Seed Research Inc., we see differences among hybrids in structures of seedlings (Seedling Growout®). Genetic diversity will continue to be an important contributor to this crop as it interacts with changing environments.
Breakage of stalks in the 2nd or 3rd internode above the soil is a big concern during harvest. Multiple studies have been done attempting to sort out the dynamics of physical strength of the stalk and fungi associated with lodged stalks. Multiple fungi capable of digesting the cellulose and lignin of corn cells surround the corn plants in the field. Most of these are warded off by the anti-microbe metabolites of living corn cells. Fungi that successfully attack dying or dead cells, producing recognizable fungal structures such as Diplodia (Stenocarpella) maydis, Gibberella zeae, Colletotrichum graminicola and Fusarium sp. as well as several others that are found in the deteriorated stalk.
Methods of Evaluating Stalk Quality in Corn, published in 1970, (https://www.apsnet.org/publications/phytopathology/backissues/Documents/1970Articles/Phyto60n02_295.PDF ) is a summary of the dynamics crushing strength of lower stalk pith and rind versus intensity of Diplodia maydis. Both pith integrity and rind thickness are significant contributors to the crushing strength. Their study and others point out that the Diplodia fungus grows only in dead pith tissue, and, therefore, correlation of this fungus with weakened stalks is mostly related to death of pith tissue.
When individual plants wilt, usually because of root rot, the pith cells dehydrate, pull away from the rind and lose production of the metabolites needed to restrict growth of the fungi of the stalk. This results in weakening the strength of the stalk by changing the dynamics of pith attaching to the rind plus allowing the growth of fungi that can break down the rind cells.
It is interesting to observe (from the roadside) multiple fields in our area with very little stalk lodging. Plants are obviously dead from maturity and low temperatures. In general, lower stalks that make it to black layer without wilting, maintain strength for a long time. Dynamics involving environment, genetics of response to environment and vulnerability to root rot are significant in corn stalk lodging.
Nearly all living forms of life develop means of fighting off potential pathogens. Corn cells produce specific enzymes to restrict and inhibit growth of most microorganisms. Resistance to the very few that may be able to overcome most of inhibitors is usually a general compound, its effectiveness often related to amount of the inhibitor and the timing of its production. The latter often is related to turning on its production based upon detection of the invader.
Most fungal species are dependent upon receiving nutrition from dead plant and animal sources partly because the anti-microbe inhibitors are not present. However, there are many competitors for the same source of nutrition. Consequently, natural selection favored production of metabolites that ward off competitors. This is apparent to those of us that culture bacteria or fungi in petri dishes and observe contaminants warded off by another species of bacteria or fungus.
This observation in 1928 led to the initial penicillin, as the fungal species of Penicillium warded off bacteria contaminating a petri dish. Many other antibiotics were and continue to be isolated from fungi.
The mushroom Strobilurus tenacellus is a fungus that spends most of its life feeding on decaying pine cones in soils of European and Asian forests. Like many mushroom species most of the fungus is not seen until it forms the reproductive mushroom structure above ground. Beneath the surface, however, it fights off competitors by producing a compound called strobilurin. This compound is apparently effective against many bacterial and fungal species. It inhibits the energy production in mitochondria and gains an advantage for this fungal species by having genes blocking the strobilurin from attaching to its own cells. Several other wood-rotting fungi also produce similar compounds to serve the same function of fighting competitors.
Obviously, the activity of these compounds makes them attractive as potential fungicides for crops such as corn. Companies have modified the compounds to make them more stable when exposed to light and allow them to attach to leaves for enough time to be effective. Many current corn fungicides use forms of strobilurin derived from cultures of these fungi.
Adequate and economic restriction of potential damage to corn grain production requires a balance of resistance systems in the corn plant and adding metabolites from fungi.
Factors leading to deterioration of corn stalks are complex, as discussed previously. In most cases the direct loss of strength comes from premature death of the plant in which it suddenly wilts before completion of grain fill. This is preceded with destruction of roots by soil fungi due to reduction of cellular resistance. This happens when the upper plant cannot adequately supply carbohydrates for maintenance of those root cells as sugars are also moved to the grain.
The wilting of plant results in withdrawal of the pith tissue from the rind, essentially changing the strength of the stalk from a rod to a tube. Stalk cell death also reduces the resistance to the fungi feeding on the cellulose and pectin of the rind, further weakening the stalk strength.
Fungicides could be affecting those stalk invading fungi but also could be reducing leaf pathogens during the grain fill period and therefore reducing loss of photosynthesis. This would potentially provide more carbohydrates to the roots and therefore avoiding the premature death and wilting that started the stalk strength weakening. It would be interesting to see that hypothesis tested.
Grain farmers are finding varying corn grain drying challenges in the wild, summer 2019 USA weather. Late plantings, varying wet conditions extending into the harvest time have made for confusing grain drying to avoid mold developments while stored. Moisture reduction in grain is due to evaporation as water must move through the pericarp basically as a physical phenomenon of water moving from higher to lower concentration. Warmer air absorbs more water than cooler air, and consequently prime field drying after black later is best with warm, dry winds. Open leaves surrounding the kernels on the ear, increases access to that warm dry air. Plants reaching black layer after the warmer days of late summer, are likely to have less field drying than normal seasons, and thus slower natural drying in the field.
Corn hybrid maturity classifications are often determined with some classification based upon moisture contents at time of harvest. 2019 data may not be a good year to make those classifications.
Seed breeding and production groups may have a more difficult time than most this year as well. Field variability probably was greater than normal due to excessive rain interacting with soil variability. Later planted test sites may have harvest moistures not typical for a given hybrid or predictive of performance in future seasons.
Drying seed is an art that includes manipulating air flow and temperatures in drying bins. It includes consideration of outside relative humidity. As moisture is withdrawn from the seed, cellular membranes, including those of mitochondria, are potentially damaged. Rehydration of seed for germination tests after drying can be the first indication of potential problems but often damage from does not become evident for some months later.
We all want to return to a normal season but what is that?
Among the contradictions in corn culture is the need to have corn stalks maintaining upright plants through harvest but rapid deterioration in soil between seasons and /or efficient decomposition for fermentation to recover the carbon in ethanol or energy for cattle. Primary strength during the growing season is derived from a combination of the tight connection of the pith cells to the outer rind cells, fibers and near the outer rind and thick cell walls of the outer rind cells.
Stalk components after harvest range among hybrids. About 50% of the solid weight is composed of carbon but most of it is involved in complex molecules such as cellulose, hemicellulose and lignin. Although lignin composition is only about 7% of the stalk, it is the most difficult to digest and often is wrapped around the more easily decomposed cellulose molecules.
Multiple fungal species in the soil produce enzymes capable of breaking and modifying the lignin molecules. Tree wood, mostly composed of lignin, is slowly destroyed by fungi specializing in production of lignocellulolytic enzymes. These initial wood rotting species are succeeded by other fungal species that enzymatically degrade the cellulose into its components. Genetic variation among fungi and competitive pressure for obtaining the energy locked up in corn stalks provides multiple sources to break down the complex carbon compounds that provided strength for the corn stalk previous to harvest.
Among the challenges for all interested in corn is to identify hybrids that produce stalks that remain upright through harvest but can be efficiently digested by cows, fermentation and soil organisms.
Published in Corn Journal 11/1/2018
The fungal genus Fusarium is a ubiquitous inhabitant of corn and other grasses. Several species of Fusarium are also associated with their sexual stage of the genus Gibberella. Fusarium is recognized microscopically by the shape of their asexually produced spores (conidia) that are produced in abundance and dispersed by wind currents. Distinction between Fusarium species requires specific lab methods but quick microscopic exam for the curved, multicellular, hyaline conidia leads to a quick analysis as Fusarium.
Fusarium species do not tend to be aggressive pathogens of vigorous, living corn tissue but almost more of an inhabitant, not actively killing cell tissue but more of a scavenger of dying or dead tissue. Some Fusarium species such as F. verticillioides (formerly named F. moniliforme not only produce multicell conidia but are known to produce single cell microconidia that apparently can move in the vascular system of a corn plant. Ease of movement within and outside of the corn plant and the ability to infect weakened and dead corn tissue allows for this fungus to be found in nearly all dead corn tissue. This can include leaf tissue injured by insects, hail or other physically injury. Active leaf pathogens such as Exserohilum turcicum (cause of northern leaf blight, apparently ward off Fusarium invasion via antibiotics.
Nearly every dead stalk will have a Fusarium species among its inhabitants. If the dead tissue includes the more easily identified symptoms of Gibberella zeae (the sexual stage of Fusarium graminearum) the diagnosis will be Gibberella stalk rot. If black streaks typical of the anthracnose that will be the announced cause. Diplodia maydis, now called Stenocarpella maydis, is distinguished by its symptoms as well. If none of these symptoms are evident, the ever-present Fusarium, is diagnosed as the cause of Fusarium stalk rot.
The actual cause of death of the stalk tissue is the complex interactions of photosynthesis and distribution of carbohydrates during grain fill of the corn plant. The fungi present are able to digest the dying and dead tissue. If none of the easily identified fungi associated with stalk rot are found, there is always Fusarium stalk rot. The ease of identifying a fungus in the tissue, implying the case of the early death of the plant, can lead to avoiding the diagnosis of why the plant died before completion of grain fil.
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.
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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.