Separation of male and female flowers and wind-blown pollen allows open-pollinated corn plants to gain diversity of genetics. Creation of consistent characters in a hybrid, however, requires creation of pure inbreds. This process of self processing carries with it some interesting risks as well, including expression of some negative characters some of which account for the reduced size of inbreds.
Intentionally self-pollinating corn caused increasing revelation of mutations, some of which had negative effects on plant development. Corn has at least 32000 genes. As a diploid plant, having 2 copies of each chromosome, if the mutation only occurred on one DNA code in one of the pair, there would be an opportunity for the dominant form on the other member of the pair to cover up the deficiency. Self-pollination offered the chance (25%) of the new seed would be homozygous recessive and result in the expression of the mutation. What could be the effects of these mutations? Perhaps it is beneficial to our needs, like the sugary gene that gives us sweet corn, but homozygous recessives could also affect root growth, or vascular tissue size, photosynthetic rates or movement of sugars to the grain. Whereas keeping desirable ears for corn shows essentially maintained some heterosis within a variety, the open pollination nature of corn also allowed some selfing and therefore more homozygous genes for some negative characters in terms of grain production.
The move to inbred development was increasing the probability of homozygous negative recessives not being covered with dominant forms of the gene within the inbred. Not only could mutations be occurring during meiosis in the selfing generation but also those accumulated within the initial breeding population, whether from an F1 cross of two inbreds or from an established variety. Inbreds inevitably will accumulate an expression of mutations that have a negative effect on grain production performance. However, with the right combination, each hybrid parent will have a dominant form of enough negative recessive forms to overcome the major disadvantages of the other parent. Experience has shown that inbreds developed from Stiff Stalk Synthetic background have a high probability of showing hybrid vigor when crossed with inbreds developed from Lancaster Sure Crop background. However, corn breeders also are aware that the combinations for inbreds from these two backgrounds that show adequate performance to be commercially competitive are not frequent. This is apparently because of the frequency of negative recessives that become homozygous during the selfing process. Given the large number of genes in corn this does seem reasonable. Although a few inbreds may combine with a few inbreds of the other heterotic group to produce close to superior commercial hybrids, and thus be considered a general combiner, ultimately there will be one specific combination giving a truly superior hybrid, at least in the environments used for that hybrid.
One such hybrid 30-40 years ago was B73 X Mo17. Either of these inbreds would successfully combine with other inbreds but this specific combination was dominant in the Midwest with plant densities used during that period. However, this hybrid would also have stalk rot problems when stressed with higher plant densities or photosynthetic stress (cloudiness) more frequent in the Eastern corn belt. It is as if each combination of inbreds does not sufficiently cover all the pertinent negative genes, at least for the ultimate performance under all commercial environments but some specific crosses may be acceptable under some environments.
Single cross hybrids have grain production advantages because of specific combination of the genetics of the two inbred parents plus uniform plant height and pollination timing.
Essential to reproducing identical hybrid seed corn is use of homozygous parent seed. This requires inbred seed production isolated from outside pollen, which is no easy task given that corn’s basic advantage for genetic variability is its tendency for cross fertilization. Even if the parent seed is relatively clean of contaminants, the problem continues into hybrid seed production. Traditionally, outcrosses have been often defined as plants taller than the majority of plants in a field. This has been misleading. Several years ago, we intentionally made outcrosses by pollinating a female hybrid parent with pollen from hybrid plants. That seed was planted along with the correct hybrid seed for comparison. Some of the outcrosses were taller than the correct hybrid and some were much shorter. Outcross plants varied greatly in timing of pollen and silk production as well and, significantly, were all different from each other. This is consistent with what we see in seedlings (Seedling Growouts®), each outcross plant is different from the correct hybrid and different from each other.
Outcrosses that originate with pollen coming from hybrid plants are different because that pollen is the result of meiosis in the hybrid plant. Meiosis results in only one member of each of the 10 pairs of chromosomes to be represented. Minimum number of possible combinations of genetics is 2log10 or 1024 combinations of chromosomes among the pollen grain. Actually, more than that are probable because of chromosome crossovers and mutations that also occur during meiosis. Evaluation based upon taller plants is also influenced by plant height of the correct hybrid versus that of contaminating hybrid. If the correct hybrid is produced by a tall female inbred crossed with a short male inbred, it is more likely that pollen from a tall commercial hybrid will cause more ‘talls’. Even in that case, the pollen that happen to include more of the contaminating hybrid male chromosomes are likely to be smaller than the correct hybrid.
Hybrid seed with less than 1% outcrosses, tall or short, have an insignificant effect on yield. Outcrosses have become more important to those with GMO interests. If the contaminating hybrid included a gene for a GMO from one of its parents, half of the resulting pollen would include that gene. If the seed production field was intended to be non-GMO, then half of the outcrosses would include that gene. If the contaminating pollen included multiple traits on separate chromosomes then the math becomes more complicated, with each of those chromosomes having a 50% chance of being included in each pollen grain. If the trait genes were closely linked on the same chromosome then they likely would be carried together but not always because some of the linkages may break.
If the intended hybrid has the gene on the male parent only, then some or all the outcrosses from a non-trait hybrid would lack the gene. Selfing of the female parent in the seed production field would also lack the gene. These cases are most relevant to herbicide resistant traits and for that reason it is preferred that these genes are in the female parent genetics.
Grain production involves similar dynamics except that hybrid plants tend to produce sufficient pollen to reduce the probability of contamination. It does present apprehension about strict non-gmo rules, including sampling and testing procedures.
Separation of male and female flowers in corn and natural aerial distribution of pollen allowed corn to have broad genetics during the domestication of the species and adaptation of many environments. It also carries with that a small problem when we try to maintain specific trait purity such as in specific grain characteristics (ie. white or amylose corn) or GMO traits.
Genetic variability in corn allows for selection of preferred genetics of human’s use. However, efficiency of production favors each plant in a field to have same maturity and other genetic characters. This is the great advantage of single cross hybrids.
Seed producers attempt to make genetically pure hybrid seed by using homozygous parents. This is not easy. Inbreeding theoretically makes gene have the identical code on the other member of the chromosome pair and thus be homozygous dominant or homozygous recessive. Small mutations can occur along the way of developing these inbreds that can interfere with the process. A larger potential source of problems can come as the inbred in increased when unintended pollen invades the seed increase field.
Corn pollen viability can last only a few hours if the weather permits. Most corn pollen falls within a few feet to a few hundred feet, depending upon wind, although there is evidence of viable corn pollen found ½ mile from the source if wind and humidity is favorable. Fields for increasing parent seed inbreds are consequently isolated from other corn fields at long distances to minimize the contamination.
Hybrid production using pure inbred parents also must struggle with the same problem but with a few more dynamics. Both parents are inspected visibly looking for off type plants in the male and female rows, removing those that are obvious. Most off types will show some hybrid vigor over the inbreds and thus can be easily identified and removed before pollination. Hybrid seed is produced by preventing pollen from the designated female plants by either removing the tassels or use of male sterile female inbreds. The intent of the seed producer is to have adequate pollen from the male parent to cover all exposed female silk as they emerge. The small presence of foreign pollen in the air at the same time always causes the potential for contamination, making the timing of male parent pollen production essential. Environments greatly affect the success of this endeavor. Dry field conditions tend to delay silk elongation whereas it has little effect on pollen production. This can cause most male parent pollen to be released before silks are exposed. Wet field conditions allow more elongation of silks causing the risk of silks exposed before male inbred pollen is released. Exposure of silks when little intended pollen is present increases the probability of the wrong pollen landing the on the silk and thus fertilizing the egg cell in the ovule at the end of the silk. The first viable pollen grain to arrive ‘wins’!
Unintended genetics in production of hybrid seed production because of potential contamination is nearly impossible to avoid. If foreign pollen comes from a commercial hybrid field, all of the off types will not be identical. Genetics of that hybrid will include a strand of DNA from that hybrid’s male parent and a strand from the female parent. With meiosis occurring in the production of the hybrids pollen resulting in a minimum of 1024 new combinations of the 10 chromosomes in the pollen, contaminating pollen leads to many new genetics in new seed. Consequently, the outcrosses among intended hybrid seed vary greatly in appearance, some being taller than the canopy and some being shorter, along with a wide range of other morphological features. Usually contamination is associated with timing and production of intended male parent pollen thus the outcrosses often are more common within a seed size with the seed lot. First silks to emerge are from the base of the ear and thus the larger sizes. Poorly pollinated ears, perhaps from those in which silks emerged after most intended male pollen was exhausted and thus more likely to be pollinated by outside pollen results in more large, round seed. Best purity usually is in those from the middle of the ear of well pollinated ears (the medium flats).
Although a few contaminants in hybrids are common and have practically no effect on hybrid yield, they gain significance in special trait hybrids.
Zea mays plant structure with the strong distance between the male and female flowers and the ease of pollen movement in wind allows cross pollination of among corn plants. Easy transport of seed both of its historic movement from those initial development in Mexico as well as in recent times has allowed diversity among the 25-30000 genes in the species. This phenomenon is the major contributor to corn’s success to converting solar energy for human’s use.
Each gene of corn is a long chain of nucleic acid components in which one small substitution of that DNA can affect the performance of the resulting protein. That protein may be an important contributor to some physiological event in the plants cells. Diploid plants have a code for the same gene on the other string of DNA in that chromosome. This allows compensation for potential negative affect of any mutation in a gene on one of the pair of chromosomes.
Self pollinated plants have the advantage and disadvantage of eventually getting the same mutations in both members of the chromosome pairs. This can result in selection by breeders to stabilize favorable characters but the large number of genes and probability of also getting ‘negative’ genes on both members of a chromosome pair is especially high. Inbred plants have little genetic diversity among each other. This allows for retaining genetics when in a controlled field environment like a seedstock increase isolation where all plants have identical, homozygous genes. Unfortunately, some of the affect is negative affect of some genes affecting plant size and perhaps some specific traits such as resistance to a pathogen. It is the corn breeders objective to match inbreds that will have dominant genes to reduce the negative affect when crossed with a specific inbred.
After identifying the pair of inbreds that make desired hybrids with maximum expression of desired traits a commercial hybrid can be produced. Controlled pollination of one parent onto the other parent is essential for resulting seed to uniformly express the hybrid genetics. Few pollen from the female plant landing on its own ear silk, will result in seed with the female inbred and not that of the hybrid genetics. Such ‘self’ will be small, unproductive plants in the hybrid field. If female silks are pollinated by the wrong plants, such as from other hybrid fields, resulting seed will not be identical to the desired hybrid. Pollen produced by hybrid plants are not genetically identical to each other and therefore such contaminating pollen on a female plant in a hybrid seed production field results in seed with wide variation. Each outcross plant from this type of contamination will be different from the desired hybrid but also from each other, varying in plant height, ear placement and size and other noticed traits.
Corn diversity benefits availability of traits and needs to be carefully controlled to maximize its advantages.
Zea mays is a species that has sufficient morphological distance between the male and female flowers that most pollen does not fertilize its mother plant. This character is unique from most other crop plants in which the stamens and pistils are parts of the same flower. Although other species may have other strategies to reduce self pollination, like timing, the distance between the ear shoot and the tassel plus abundance of pollen that easily floats in air encourages pollination of other corn plants rather than self pollination.
This characteristic of corn promoted significant genetic diversity ultimately resulting in ease of adaptation to a wide range of environments as humans selected the genetics suited to their needs and environments. This is diversity to us as we selected genes for less starch in sweet corn, endosperm explosiveness in popcorn, soft starch in floury corn and hard endosperm of flint corn. Corn varieties are adapted to short seasons of temperate zones and to longer seasons near the equator. Even as pathogens evolve adaptation of some corn genetically controlled resistance, genetic variability within the species is eventually identified to produce the metabolic products needed to resist the pathogen.
Open pollinated varieties maintain genetic heterozygosity as meiosis after fusion of male gamete with that of the egg cell, creates new mixes of the genes. Consequently, each seed in an open pollenated corn pollinated plant when at least one of the parents is heterozygous will have some genetic variation from adjacent seeds. Corn breeders have used this principle to get new, desired genetic characters.
Genetic variability is advantageous when identifying useful characters, but not when attempting to get optimum grain production. The advantage of crossing two homozygous plants of two distinct genetic backgrounds is producing predictable and consistent hybrid plants. Controlled selfed pollination for several generation ultimately results in homozygosity, with essentially each seed having identical genetics. This selfing process also creates homozygosity of some genes that have negative affects on the plants, including characters affecting seed size and volume. Identifying homozygous inbreds that essentially overcome these negative genes of another hybrid parent is the plant breeders job for selecting potential high performing hybrids.
Cross pollination characteristic of corn has allowed genetic variability and human effort to select and self pollinate to create inbreds and then crosses to make specific hybrids, allowing this species to be a major international crop.
As many humans on earth face current interactions with a deadly virus, it is a reminder of significance of genetics and environmental interactions that affects our lives. Our genetic complexity affects each individual’s ability to defend against the Covid-19 virus but even that is qualified by the multiple individuals medical conditions because of past ‘environments’ and probably genetic changes in the relatively simple virus genes.
Human genetic diversity exists because of our evolution under multiple selection pressures of our past environments. All humans, despite evident, minor visible trait differences such as skin, hair and eye color, have other less evident genetic differences witnessed only by slight differences among an individual family.
Corn has a genetic history somewhat similar to humans. Its ancestor was a related species and perhaps some crossing with another species but with human’s intervention it was moved to multiple environments and selection pressures allowing diversity of characters we could easily see. Yet, within each environment there was selection for other favored, less easily seen, characteristics. Just as with human the vast majority of genetics remain close to being identical to others of the species.
Corn genetics that were best for desired traits in the season of an individual field in 2020 probably will not be the same as the best hybrid in another field nor even the best for the same farm in 2021. Small differences in 2021 environments will favor slightly different genetics affecting expression of traits not noted in 2020. Some traits observed in 2020 environments may not be expressed in all environments. We are dependent on maintaining genetic diversity within the corn species because of environmental variability.
Humans also are dependent upon diversity not for those relatively simply inherited characters such as skin, hair and eye color but also for those less obvious genetics that our species successful adaptation to a forever changing environment. Success of our species, and of corn, is dependent upon genetic diversity. The biology of both species favors continual genetic interactions and diversity.
Those occupants of dying stalks, roots and ears will be there next spring as they continue to look for nutrition on organic material to digest. Living plants and animal tissue evolved with resistance to most of these organisms but we are all dependent on these organisms to release the components to be used again in the new generation.
Seeds are surrounded by many microorganisms mostly feeding on organic matter that is mostly dead. The photo, copied from our publication of Stalk Rot of Corn power point, shows the multiple colonies grown from a particle of soil in the spring of a corn field. These organisms are competing for the carbs locked up in the dead debris in the soil. Fusarium species are among these that can also penetrate some living corn roots.
Fusarium is a fungus genus composed of many species. This group of fungi asexually produce spores called conidia that move in the wind. Most of these species also have a sexual stage in which the haploid hyphae fuse to form a diploid structure (ascus) in which the nuclei undergo meiosis, producing haploid ascospores, to produce more hyphae with new genetic combinations. The fact that the asexual and sexual stages are separate in time and, sometimes, location adds to the difficulty for specialists to associate the two. The ‘rules’ of naming these fungi declares that the sexual stage is primary. Consequently, on fungus commonly infecting corn seedlings, stalks and ears is known as Fusarium graminearum but also by its sexual stage name of Gibberella zeae, another common Fusarium infecting seedlings, stalks and ears is Fusarium verticillioides also known for the sexual stage name of Gibberella fujikuroi. Adding to the naming confusion, the previous name for F. verticillioides was Fusarium moniliforme.
The names are confusing, but biology of these fungi is not much better. They tend to mostly be weak pathogens. They can enter weakened root and mesocotyl tissue of the germinating seed. Host resistance seems to limit the fungus ability to destroy much tissue, but it appears to simply live in the corn tissue without causing visible damage. It is not unusual to find at least a few seed germinating on a paper towel that look normal but have some hyphae of Fusarium verticillioides growth. This species frequently can be isolated from corn leaf tissue with no apparent damage. Likewise, Fusarium species are nearly always isolated from stalks of corn plants. Often, if no symptom associated with another pathogen, such as the black streaks in outer rind caused by the anthracnose fungus (Colletotrichum graminicola), we tend to call it Fusarium stalk rot. The death of the stalk mostly was a biological problem of the corn plant and fungi like Fusarium was there, among others, to assist in the digestion of dead stalk tissue.
Fusarium is the name of genus of a fungus group of species that are common saprophytes and plant pathogens. Ability to kill living cells varies among Fusarium species but certainly they can feed on dead cell tissue. Among the many fungal species feeding on dead corn stalks almost always a Fusarium species will be present. If no diagnostic indicators of Gibberella, anthracnose or Diplodia are present, it becomes easy to call the disease Fusarium stalk rot.
Fusarium species that are often found in dead and live corn plant stalks initially grow between cells, producing enzymes to digest the walls of the pith cells and fibers. These walls are composed of complex cellulose compounds combined with proteins and basic carbohydrate molecules, formed by biological activity within the living cells and secreted through the cellular membranes. These compounds protect the cellular contents by structure and some active resistance products as well. Cell wall components include cellulose, arabinoxylans, hydroxycinnamates, pectins, glycoproteins and lignins.
Fungi initially grow between the cells, secreting enzymes to digest the cell wall structures. Meanwhile living corn cells produce compounds such as Jasmonic acid, abscisic acid and salicylic acid to limit the growth of the Fusarium species. This battle begins with the first incursion of the fungus, perhaps with injury to the seedlings or perhaps with corn borer. Artificially infecting the stalk with any of the fungi associated with corn stalk rot shortly after pollination usually shows infection limited to area near to point of inoculation. As the tissue matures the fungi seem to more rapidly spread, digesting more cell walls. Breaking down the complex compounds allows the fungus to utilize more simple carbohydrates for their own metabolism.
Digestion of the cell wall components weakens the strength of the stalk beyond the sudden decrease of strength that occurred with the plant wilted after root rot. The wilting caused the pith cells to withdraw from the rind, effectively changing the structure from a rod to a tube. This alone is believed to reduce strength by one-third. Corn hybrids that tend to construct more and stronger cells in the rind can have less lodging but the physical change from a rod to tube is difficult to match by cell wall strength alone.
Maintaining sturdy stalks until harvest is a balance among producing sufficient carbohydrates for filling all kernels, avoiding wilt by delaying death of root cells and maintaining stalk cells alive to fend off potential pathogens such as Fusarium
Less common in the USA corn belt but occasionally regionally destructive is Head Smut. It is not easily distinguished from common smut, but it has distinctive characters and resistance is more common. This smut-causing fungus is Sphacelotheca reiliana. This smut shows mostly in the complete destruction of the ear and the tassel of corn. This species also attacks sorghum, Johnson grass and Teosinte. The diploid teliospores of this fungus, shed from the smutted ears and tassel remain dormant in the soils for many years but when activated by the root exudates from susceptible corn seedling, the spore undergoes meiosis, producing short hyphae and then haploid basidiospores. These spores germinate to form hyphae that fuse with compatible haploid hyphae, resulting the cells with two haploid nuclei. From this point the fungus infects root cells, apparently growing between the cell wall and the cell membrane of the affected cell.
Absorbing the nutrients from the infected, live cell, the fungus continues growing cell to cell towards the meristems of the seedling. From there it continues to advance without killing corn cells as they undergo cell division and eventually producing the ear and tassel. Disease symptoms from the infection are absent as the fungus absorbs nutrients but does not kill tissue nor penetrate the cell membranes.
After the host plant’s meristematic cells at the upper tip of the stalk and ear change to producing pollen mother cells and ovules, the fungal cells also change. The two compatible nuclei in each fungal cell fuse to form diploid single cells. These fungal diploid cells fill and destroy the host cells in the tassel and ears before pollination. The individual, diploid fungal cells develop thick walls resulting in the first sign of the disease as a loose powder forms instead of ears on the corn plants. These teliospores are then spread by the wind, the thick wall giving them survival until another host plant is found for repetition of the cycle.
Fortunately, high levels of resistance to head smut is more common than susceptibility. Some of the resistance is thought to be from differences in the root exudates in the initial induction of teliospore germination. Some comes from the successful recognition of the initial invasion, triggering the response by the host of producing enzymes to keep the pathogen from reaching the meristems. Perhaps it involves the speed of the plants cell divisions, escaping the fungus while infected cells produce anti-fungal chemistry. Regardless of the method, most corn hybrids are reasonably resistant.
The problem historically has been with popular, susceptible hybrids being planted for consecutive years, allowing an increasing concentration of teliospores, until the right environment such as warm, light soils at time of corn seed germination results in noticeable high infection rate. Switching to more resistant hybrids in future seasons reduces the occurrence of head smut.
Biology of corn includes the biology of pathogens.
Multiple stresses during the 2020 corn growing season likely produced environments encouraging ear smut. The biology of the fungus and the corn plant are closely related to the occurrence of corn ear smut.
Ustilago maydis is the fungal pathogen of corn causing common smut disease. Its biology includes a fungal version of sex, a portion of which involves corn. The fungus produces thick walled spores called teliospores that overwinter in the soil for many years. Teliospores have diploid nuclei (2 sets of chromosomes). When moistened, the teliospore nuclei undergo meiosis, resulting in 4 individual cells, each with haploid (monoploid) with one set of chromosomes. These cells are called sporidia. Sporidia that land on corn tissue may germinate but cannot enter the corn plant until they are united with sporidia of a different mating type. After the two mating types fuse, the two nuclei stay separate but the fungus forms special structures to enter the corn plant cells. Often it is through wounds but also directly through corn cells as they are elongating, as the combination of host cells and fungus form galls. Within this mass the fungal cells, having two monoploid nuclei per cell, now fuse the nuclei, forming the diploid single nucleus stage of the fungus life cycle. These diploid cells form the thick cell walls as they become teliospores to be released as the corn plants are harvested.
Smut galls may form on leaves, tassels, aborted ear shoots at the leaf nodes and the main ear shoots. The ear shoots are the most damaging to the grain yield, of course.
Corn silks that are not pollinated soon after emergence are vulnerable to infection by Ustilago maydis. The fungus grows down the silk channel infecting the cells surrounding the ovule. If pollen successfully reaches the ovule before the fungus, the smut fungus is inhibited and no gall will form. Subsequently, the timing of pollen release and silk emergence becomes a significant factor in formation of galls on corn ears. Rainy days may encourage silk emergence, but delay pollen release or drought may delay silk emergence until there is limited pollen available. Complete absence of kernels but ears full of smut are a sign that the ear failed to receive pollen. These factors contribute also to the occurrence of smut at the tips of ears as these are from the last silks to emerge, perhaps when no pollen was available.
Another Fusarium species, Fusarium graminearum, forms a sexual stage of the fungus when its mating types combine. That sexual stage is identified by the name of Gibberella zeae. This fungus is associated with Gibberella stalk rot and wheat scab. The fungus is common in corn debris, producing huge numbers of Fusarium spores during much of the corn-growing season. Spores germinate on the silks, with the fungal filaments growing down the silk channel towards the ovule. Generally, if pollen tube growth reaches the ovule first, the following collapsing, dry silk tissue effectively stops the fungus. Silks are most vulnerable during cool, wet weather as pollen spread is poor but fungal spore production is high. This results with a prolonged period of silk exposure. Later infection of the kernels appears to be related to kernels physically injured by hail or insects. Husk leaves tightly wrapped around the ear also appears to be related to spread of this mold within the ear.
Early infected kernels will fail to develop completely, will be light in weight and often will not germinate if planted as seed. Later infection can spread to cover much of the ear with a mold as the fungus spreads from the initial infection area. This mold produces mycotoxins including deoxynivalenol (DON). This toxin is associated with severe health problems in swine. The fungal spores produced among the grain also can be detected by swine and cows, causing them to reject the grain. I recall a personal situation in which I was a sent a corn sample from seed dealer who claimed that newly harvested grain was rejected by pigs and calves. Observation of the grain in my lab showed lots of Fusarium spores- so much that I asked to see the storage bin where the grain was stored. A single hand full of the grain from the bin revealed a cloud of spores. The grain was dried in the bin by air being blown from the bottom up through the bin. This air, however was moved over an accumulation of previous year’s debris below the grain. A sampling of that debris revealed that it was heavily infected with Fusarium. The new crop was being inoculated with the fungus as it was being dried.
DON has been shown to accumulate in grain stored at moisture higher than 20.5%. One should assume that the fungus is ubiquitous and that monitoring the grain drying and storage is important to avoiding this toxin problem. (Corn Journal 03/01/2018)
Nutrition and moisture in corn silks allow the fast movement of the pollen tube towards the ovule and contribution of the male genetics to the next generation. Those same favorable silk characteristics also can be used by invading fungi. Rapid deterioration of the silk tissue after pollen tube growth offers protection within a few days after pollination, but environments and genetics can have a drastic effect on the time of silk vulnerability and the biology of potential invaders. Aspergillus flavus gets much attention because of its dangerous toxin produced on infected corn. Fusarium verticilloidesis, another common invader of corn kernels through silk infection that can produce a mycotoxin (i.e. fumonisin). Others such as Diplodia maydis and Gibberella zeae also can utilize the silks and initial entry into the ear.
These fungi are mostly saprophytic feeders on plant debris and intensity of their spore production is greatly dependent of corn debris from the previous season near the new crop plants. Their biology also is influenced by the environment affecting competition with other saprophytes feeding on debris and production of spores when the silks are exposed.
Duration of silk vulnerability is also associated with environment. Cool, moist weather a few weeks before normal pollination may cause silks to be exposed before pollen is produced- and may favor Diplodia(Stenocarpella) maydis. Extended dry, warm periods during the pre-pollination time, may cause pollen production before silk elongation and exposure but favor Aspergillus flavussporulation and distribution by the time the un-pollinated silks do emerge. Fusarium species (including Gibberella zeae) produce massive numbers of spores under most environments.
Plant pathologist have shown that one can induce ear infection by directly spraying the silks with the spores of each of these pathogens. These studies have shown evidence of resistance variance among genotypes but usually only on a scale and not of absolute absence of disease. Evaluation for resistance from natural infection is not easy. One can record occurrence of infection within plots, but each genotype may not be exposed to the same environments, including time of silk exposure. One does need to use care before drawing conclusions about ear rot susceptibility based upon single location observations.
Ear rots are prime examples of the complex biology of host and pathogens interacting with environments. Ear rot may not be noticed until harvest, but the problem involved the dynamics occurring at pollination time of the season.
This Corn Journal blog written in 2016 pretty much applies to USA corn in 2020.
There have been numerous studies comparing inbreds and hybrids for the fiber strength of stalks. Perhaps these results are somewhat helpful to predicting standing corn at the end of a season, but I suspect that it ignores the biggest factors affecting standability at harvest time. When the stalk collapses in the lower internodes preharvest, the pith tissue has pulled away from the rind of the stalk. This reduced the structural strength by 1/3 as what was once a rod now becomes a tube. This happened because the root died earlier, reducing water uptake followed by wilting of the plant. This dessication of pith cells caused withdrawal from the rind. The dead cells, now having limited resistance to fungi readily invading the tissue and digesting the cell walls, further weakening the strength of the stalk.
The time between black layer and harvest level grain moisture is the best time to evaluate stalk quality. A simple push test of several plants in many areas of a field can give one a good idea the crop’s vulnerability to lodging. Basically, those that are strong, soon after black layer, will not decrease in strength during the rest of the season. One should never forget, however, the stalk rot of 2016 in one field is not necessarily an indication of the behavior of the same genetics in the same field next year. Evaluations across several environments is critical to predictions of yield and stalk quality.
The individual plant that wilted and prematurely died because of root rot but is surrounded by living corn plants does not necessarily have drier grain at harvest time. Grain moisture replacement by starch formation in grain stops when the abscision layer (black layer) forms at the base of the kernel. In most corn plants this happens between 55-60 days after pollination at about 30% moisture in grain. Those prematurely dead plants that wilted earlier form abscision layers about a day after the wilting, at a higher moisture percentage.
Grain drying in the field after this time is an evaporation process. Moisture must travel through the pericarp of the kernel at a rate determined by the relative humidity surrounding the grain. Pericarp thickness must be a factor but also plant structures such as cob volume and husk leaves length, thickness and adherence to the grain are major factors.
‘Water runs down hill’ as Professor Loomis in my plant physiology class of 1960 would say to emphasize that it goes from a high concentration to a low one. Grain evaporation rate is very much dependent on relative humidity immediately surrounding the kernels. Transpiration from the senescing, but green, leaves in plants for a time after kernel black layer contribute to higher humidity in the field, including the area surrounding that single dead plant. Eventually, senescence of these plants halts transpiration, leaving the ear structures as the only barrier to response to atmospheric factors such as relative humidity and wind affecting the dry-down of the grain. The individual plant that wilted and prematurely died because of root rot but is surrounded by living corn plants does not necessarily have drier grain at harvest time. (Corn Journal, 9/22,2016)
As harvest season approaches, standability in the corn field becomes a primary interest to the grower. Appearance of the lower stalk is an indication of the plant’s vulnerability to lodging from stalk rot. Those stalks that remain green 60 days after pollination are likely to have solid interiors of the stalk and will not break before harvest. These plants did not have premature wilting and thus the cells within the stalk did not withdraw from the rind. One can judge this by observation alone in plots. Plants with deteriorating stalks show discoloration beginning with a slight pale yellow, and then a darkening yellow that eventually turns to brown.
Various fungi invade the dead interior tissue of such a corn stalk. Multiple species can be present in such stalks although a few get the most attention because of their distinct features on the stalk. Colletotrichum graminicola, the fungus associated with anthracnose is distinguished by narrow black streaks on outside of rind but is also present inside the infected stalk.
Gibberella zeae, sexual stage of the fungus Fusarium graminearum, shows distinct black ‘perithecia’ on the outside of the rind near the stalk nodes. Ease of rubbing these off the stalk is diagnostic of Gibberella stalk rot. Inside such a stalk usually has a pink discoloration caused by the asexual stage of this fungus that is known as Fusarium graminearum.
Another common fungus found in rotted stalks is Diplodia maydis, also known as
Stenocarpella maydis. This fungus frequently is found on the outside of the stalk nodes as a black sticky structures known as pycnidia. The fungus in only known to be present on corn and overwinters in the soil.
Nearly all corn plants are exposed to another Fusarium species, Fusarium verticillioides (also known as Fusarium moniliforme. Some think it exists in corn plants mostly as a nearly saprophytic occupant rather than an aggressive pathogen, but it is easily found in nearly all rotted stalks.
There are several other fungi that have been isolated from deteriorated stalks, but we like to name a stalk rot as caused by the most easily identified. Ultimately, however, the actual cause is the physiology of that plant. That plant had insufficient carbohydrate to complete the fill of grain and maintain root life. The plant wilted, cells in stalk die, withdraw from rind (reducing strength by 1/3) and lose resistance to invading fungi. It is most important to analyze the cause of this lack of the photosynthetic stress and translocation balance in these rotted plants
Summer of 2020 in Midwest USA corn-growing areas has been unusually stressful. Early season rain caused delayed planting in many fields, extreme winds in Iowa resulted lodging in healthy plants, absence of rain in many areas in northern Illinois resulted in parts of fields to have plants with yellow leaves long before normal maturity. This is not the usual precursor of stalk rot.
Nitrogen is a major component of chlorophyl and thus contributes to the green color of leaves. This mineral, along with others, is absorbed through the roots along with water and transported to the leaves through the vascular system. Lack of water in soil makes the nitrogen less available in the soil and thus less available for this transportation. The result is the plant leaves slowly turns from dark green to yellow. This also affects the physiology of the developing kernels resulting in abnormally incomplete transport of carbohydrates. Symptoms of late season drought in multiple plants in areas of a field have all plants with completely yellow leaves.
Premature plant death leading to stalk rot, on the other hand, usually occurs with individual plants turning gray, not yellow. These plants wilted from inability to withdraw sufficient water from soil to match the water loss from translocation. The lack of water for leaves was caused by roots dying because of insufficient carbohydrate supply. Contributing to the lack of carbohydrates was the competition with the grain. Individual plants with excessive movement to grain, depleted the stored carbohydrates of the stalk and thus the source of energy for life in roots. Insufficient root life in root hairs resulted in reduction of water absorption and thus wilting of that individual plant. These plants frequently show a slight gray appearance a few days before complete wilting and then all leaves become gray. Loss of turgor results in all leaves to turn downwards. The ear likewise turns down. Adjacent plants may remain green with upright ears until completion of grain fill, normally about 60 days after pollination.
Stalks of the plant death caused by drought and those from wilting also differ. Yellow plants may have sufficient carbohydrate and water in stalk tissue to maintain strength whereas the wilting plant has collapse of pith cells weakening its strength. Both conditions are not good and may ultimately cause lodging but at least it is probably that the additional kernels on the wilted plant can make up for the light kernel weight whereas the yellow plants probably have fewer kernels.
As a corn plant moves carbohydrates to the developing grain from the leaves and storage pith cells of the stalk, the rate of flow may vary from adjacent corn plants. This rate is determined by genetics and nutrients affecting the flow per kernel and the number of kernels forming on each individual plant. Environment of each individual plant in field can have sufficient differences that affect number of kernels and amount of photosynthesis in the plant.
Those individual corn plants that seem to suddenly turn gray during grain fill have a permanent wilt. It is not unusual for it to seem sudden, because the plant looked as green as others in the field just a few days previous. However, closer observation of these individual plants, reveal a few early signs of wilting. The upright ear starts to point downward, the leaves get a sort of faded green color that can be noticed a couple days before all of that plant’s leaves turn gray. This symptom is not just the top leaves – it is all leaves on the plant. The water transportation from the root has stopped. Probably the continual chain of water molecules in the xylem tissue has been broken, ruining the capillary action needed for supplying the rest of the plant for water needs.
Wilting causes cell functions to stop; no more photosynthesis, no more movement of sugars and minerals, and no more movement of sugars into the grain. In fact, the kernel forms an abscission layer at the base of each kernel soon after permanent wilt, cutting off all movement of sugars into the grain- or water from the grain. Loss of potential carbohydrate storage in the grain of a wilted plant is determined by the number of days the filling period was cut short. Grain fill between about 10 days after pollination and day 50 is about 3% per day, between day 50 and day 60 it is about 1% per day.
The contradiction can be that having more kernels on the plant, ultimately caused the roots to die early, resulting in a wilt that cuts off the flow of carbohydrates to kernels. So, there are more kernels than on adjacent plants but less carbohydrate per kernel because of the wilt.
And that is just the beginning of the problem for the grower who needs to harvest the corn.
Pursuit to understand why and how corn stalk rot develops was certainly undertaken by many researchers before my attempts. There were research publications done before corn hybrids were commercialized, when farmers mostly saved seed from the variety in the field. But these plants were not genetically uniform. Superior yields of hybrids and eventually the realization that highest yields came from single cross hybrids in the mid to late 1960’s, it became clear that even when each plant in the field was genetically identical, stalk rot could not be explained by genetics alone. Pappelis at Southern Illinois University showed that the pith cells senesced after pollination and at rates faster in genotypes that tended to get more stalk rot. Mortimore’s studies in Canada indicated that stalk rot is always preceded by root rot. Ullstrup, at Purdue, did studies showing that root tissues start senescing soon after pollination. Foley, at Iowa State, published that when the pith tissue pulls away from the stalk rind as when a plant wilts, the structural strength is reduced by 1/3 by simply changing from a rod to a tube. Other studies showed that one could increase occurrence of stalk rot by artificially shading plants after pollination. Corn borers were associated with stalk rot occurrence. Leaf diseases like Northern Leaf Blight were seen to increase stalk rot. Corn breeders were (and still are) frustrated that some of the highest yielding hybrids tend to be the most vulnerable to stalk rot. Although pathologists usually identified a few fungi, such as Fusarium moniliforme, Gibberella zeae, Diplodia maydis, and Colletotrichum graminicola as frequently found in the dead stalks, actually many other fungi were also present. But if root destruction occurred first, were these only quick invaders of tissue that was on its way to death? And then there was the observation made by many that one often could see, in a corn row along the edge of a field, that the plant that wilted first had two ears, instead of the one ear on most of the other live ones in the same field. Why that plant?
Annual plants such as corn undergo physiological changes after flowering, especially in corn that is genetically selected to maximize capture of products of photosynthates in the grain. Flow of carbohydrates within the plant are directed by hormones produced in meristems. Before flowering that flow went to growing leaves and roots near meristems. Excess carbs were stored in parenchyma cells in stalk tissues. After flowering, hormones direct the flow towards the developing kernels.
Genetics and environments influence the intensity of the flow. Hybrids that tend to have more total starch in the ear either because of more kernels or larger kernels are favored by humans but risk early death of roots and leaf tissue that still require the energy provided by carbohydrates for cellular metabolism. Environments that reduce optimum photosynthesis during the grain fill period accelerate the depletion of carbohydrate reserves stored in the stalk tissue. In some hybrids, perhaps all, the depletion becomes most evident in the stalk tissue near the flag leaf, eventually resulting in an abscission layer to form at the base of the flag leaf, cutting off water to that leaf and eventual wilting of the leaf. Fungi such as Colletotrichum graminicolaare able to invade the outer rind of that small stalk tissue with typical anthracnose symptoms. This loss of productive photosynthetic tissue in the small leaf is insignificant and could be indicating good grain fill. Loss of significant root tissue is more important.
The challenge of the corn breeder is to select hybrids that have the balance of maximum grain production capturing all carbohydrates available without causing too much damage to needed life functions in the plant. The challenge of the grower is to provide environments that maximize this
Sugar is the product of photosynthesis, a process at which corn is especially good. The sucrose form of sugar is moved (translocated) from the photosynthetically-active leaf (source) to sinks such as growing leaves, roots and, eventually, seeds. Hormones, mostly cytokinins, direct direction of the flow. Translocation occurs through the phloem portion of vascular bundles through cell membranes at the cost of some energy. Cytokinins are mostly produced by the newly developing cells at growing points such as tips of root branches, leaf buds, growing leaf tips and embryos in newly formed kernels. We humans selected from the Teosinte ancestor, plants that not only met the minimal needs of producing seeds to assure a future generation but also those with extra storage of carbohydrate in the fruit (grain) for our own consumption. To do this we selected for excessive photosynthesis, temporary storage of excess carbohydrates in the pith of the stalk and eventual movement of it to the grain. This was not done cheaply. We had to get more leaf area and more root tissue to not only support the plants but also to uptake the water and nutrients to grow the bigger plant and to initiate the larger grains. All of this required more energy. After pollination, the newly formed embryo in each kernel begins to produce the cytokinins directing the flow of sugar towards it. This is occurring at the same time that root tips are not as prolific and consequently producing less cytokinin.
It takes about 10 days after pollination for the flow to each kernel to gain full speed. Varieties, and environments, differ in the flow rate per kernel but from day 11 to about day 40 the flow per kernel appears to be constant. Production of sugars per day may be affected by cloudy days, or leaf damage but the power of the individual kernel sinks remains strong during that time. Any shortage of new sugar is replaced by sugars stored in the stalk pith tissue. After the 50th day, the draw per day is reduced until finally an abscission layer is formed at the base of the kernel in which the phloem tissue no longer can move the sugars. However during that 60-day period the root is competing with the kernels for sugars and our attempt to capture the maximum carbohydrate in the grain.
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.