​History of corn’s origin in tropical environments to its spread around the earth is fascinating to me.  Not only did the people in Mexico select mutants that held onto the seed instead of instantly shattering and scattering seed before harvest, but they also found mutants in the Teosinte plants that stored more starch in endosperm.  As people migrated through that part of Mexico carrying the seed to their environments. They identified and selected mutants that allowed reproduction in the new locations.
 
Among the most significant is adaptation the reduced length of growing season in the temperate zones of earth. Tropical corn, such as those originally in Mexico, flowered when day light time was reduced, the daylength sensitivity that affects many flowering species.  Those short days occur too late in tropical corns life, resulting in tropical corn in temperate zones to not complete filling of grain before frost.  Adaptation to temperate zones require emphasis on accumulation of heat to trigger the change of the apical meristem from producing more leaves to production of flowers. 
 
One study that I did many years ago compared the heat units to time of apical meristem showing a tassel to the maturity rating for many commercial hybrids. Timing of that differentiation, occurring in June correlated very closely with our final maturity ratings for those hybrids.  This supported the hypothesis that it is the heat units beginning immediately after planting that is most significant in determining the maturity of a corn crop. Heat after switching the growing points from producing stem and leaf tissue to tassel and ear tissue has an influence, but the earlier season affect is greater.  Maturity in most corn belt corn is controlled by several genes affecting response to accumulating heat soon after planting.  Tropical corns are also influenced by heat but other genes affecting response to number of hours of continuous darkness have a greater effect on time to flowering.


As our climate changes the genetic diversity available to us will allow further adaptations in corn.

​The changing environments and dynamics of genetics allows and demands a continual need for diversity in corn and, actually, all other aspects of life, including humans. There is no reason to think these dynamics will end. Genetic mutations allowed the origin of Homo sapiens and its adaptation to many environments including those caused by pathogens.  Genetic mutations allowed the origin of corn from a weedy grass species in central Mexico and further adaptations to grain production around the earth.   Natural mutations in this annual plant species and slight changes in the DNA code frequently has no recognizable affect but some provides the needed resistance to a future pathogen, perhaps also undergoing mutations.
 
Efforts of corn breeding programs to continually select the best genotypes for today’s environments, as well as those by corn growers to provide good environments will be critical each year.  This requires continual effort to select those best genetics for the time and place.  We celebrate diversity in corn and humans.

Today is Thanksgiving day in the USA.  People everywhere should be thankful for the multiple of efforts with the amazing Zea mays. Those people in central Mexico several thousand years ago who selected and planted seed from the mutants in the wild plant Teosinte that held its seed instead of scattering it.  And then those of that time that chose, and propagated the seed that with larger endosperms, and thus, more starch.  We are thankful for the multiples of people that carried the seed throughout the Americas, allowing and selecting for mutants adapted to their environments and personal food desires.  
 
Native Americans had already selected multiple corn varieties adapted to their local preferences by the time Europeans ‘discovered’ the American continents.  Some carried the seed corn plant vigor expressed with the crossing of certain varieties.  Multiple people experimented with ways of developing genetic purity through inbreeding and identifying specific crosses between corn inbreds that expanded the potential grain yield for multiple uses of the grain of this species.  We are thankful for the multiples of people’s efforts during the past and that which continues today.
 
Happy Thanksgiving to everyone.

​Variety parent seed is identical to the final seed product.  Parents of hybrid seed are not identical to the hybrid.  Single cross corn hybrids are inbreds selected primarily for favorable performance when combined as a hybrid but each with homozygosity for repeatable genetics.  This inbreeding process does result in some genetic expression of negative characters in one parent that are covered up by the other when combined as hybrids.  After identifying such a combination of inbreds, selection of which parent will become the source of the seed and which will become the pollinator becomes significant to the commercial production of hybrid seed.
 
The corn kernel is a fruit.  The outer layer, the pericarp, is a structure of the female plant.  It does not include any genetics of the inbred chosen to be the pollen source.  The bulk of the corn seed within the kernel is the endosperm where storage of starch is made available as energy utilized for germination of the seed.  Cells within the endosperm include 2 copies of female plant chromosomes and one of the pollen parent.  Only the embryo has equal genetics from both parents of hybrid seed.
 
Commercially acceptable female parents of hybrids need to have reliable and consistent elongation of silks even when under some moisture stress. Silks need to be receptive to fertilization after pollenated.  High number of ovules is favored.  Pericarp structure must be inclined to withstand stress with minimal cracking. The most important character of the female parent is consistently high percentage of germination.  A major factor linking this to the female seed parent is the genetics of the mitochondria within the embryo cells.  Mitochondrial genetics originate only from the ovule.  These sources of transforming energy stored as carbohydrates into that needed for cell metabolism are full of membranes that can be damaged by rapid swelling when water infuses into dry seed.  Maintenance of the integrity of these membranes become essential to the germination process.  Tolerance of natural stresses on emergence of silks, of pathogens and stresses on pericarps and of function of mitochondria are all associated with the female parent of a corn hybrid.
 
Pollen sources for hybrid seed production do have some responsibility as well.  Most critical is reliable and timely production of live pollen grains.  Release of pollen grains from the anthers is affected by genetics, as the anther chambers must dehisce as the relative humidity drops.  Timing with the presence of receptive silks on the female parent is essential.  It is probable that part of the pressure for selecting parents that increase grain yield involves shifting the genetics for energy needed to produce pollen to that of more grain results in less pollen.
 
Commercial hybrid corn breeding programs identify which hybrid parent is best as the female or male based upon quantity and germination of the seed.  These are determined by the genetics affecting the biology within the corn seed.

​As it became apparent to some corn breeders in the early 1900’s that consistency and repeatability of genetics in corn required creating homozygous inbreds from populations frequently expressing heterosis.  The enigma was that inbreeding greatly reduced the volume of hybrid seed to be planted but the production of grain from those hybrid seed was greater than produced by indigenous varieties.  A few academic corn breeders pushed the idea of using the hybrids as parent seed to make double cross hybrids to overcome the seed volume problem.  They encouraged several farmer seed producers to adapt this concept in the 1930s.  The significance of heterosis resulting from crossing specific inbreds became obvious to many during the 30’s, stimulating investigation into the genetics and botany of corn in academia and entrepreneurship among farmer breeders.  
 
As more farmers switched to using hybrid seed, public and private corn breeders increased inbred breeding programs.  New synthetic populations were created by breeders by crossing seed from existing varieties, selecting for heterosis by crossing with opposing inbreds, recycling the best, testing again and repeating the cycle to create new improved populations from which new inbreds could be created.  Stiff stalk synthetic population created at Iowa State University became and continues to be a powerful source of new inbreds that commonly used as female parents of hybrids.  Populations derived from varieties with origin in Eastern USA and grossly identified as Lancaster often became sources of inbreds expressing heterosis with stiff stalk derived inbreds.  
 
Breeding efforts to select more productive seed parents, improved seed production methods and economics of corn grain led to the introduction of single cross hybrids in the USA in the late 1960s.  A similar pattern developed in the multiple environments on other continents as well.  Continual selection by humans from the diverse genetics selected by previous human generations has led to continual improvement of grain productivity of this species. Its biological features of separation of male and female flowers, C4 photosynthesis, easily transported seed and 30-40000 genes has served us well.

​As it became clear that corn hybrids could create large boosts in yield when two unrelated inbreds were crossed, it became clear that the biggest gain came when the inbreds were derived from distinct heterotic groups.  Lancaster Sure Crop was created in Lancaster county Pennsylvania.  It featured long, flinty ears with disease resistance consistent with the humid eastern USA environment.  Meanwhile, the Reid dent corn, developed from the accidental crosses of New England Flint with Southern dent in central Illinois spread throughout the corn belt with many sub-varieties selected for differing moisture and soil stresses.  These two heterotic groups became major sources of inbreeding in the 1920 and 30’s as hybrid development advantages were clear.
 
In 1935, George Sprague began intercrossing 16 inbreds and 4 inbred parents that were mostly from these many sub-varieties of Reid dent corn.  This became known as Iowa Stiff Stalk Synthetic population (SS).  New inbreds from this populations were test crossed with Lancaster inbreds, the successful SS inbreds were intercrossed, new selfs made, crossed again to Lancaster inbreds.  These cycles were continually resulting in more populations at Iowa State University.  Other public and private corn breeding efforts built on Stiff Stalk Synthetic, selecting from portions of it as well as constructing their own genetic populations.  Inbreds from these efforts continuously improved hybrid performance as national corn yields increased. A third heterotic group, Iodent, was developed by mostly private breeders in the USA.  Internationally, similar processes of identifying inbreds that combined with unrelated inbreds to give hybrid vigor.
 
The cause of heterosis lack some clarity but most of the evidence implies that negative, recessive genes in one inbred are overcome by the dominant version of the gene in the other inbred.  Given that a corn plant has at least 30,000 genes, and that any selfing, whether intentional to make inbreds or coincidental in open pollinated varieties, probability of homozygous recessive genes is high. Despite some randomness in matching dominant genes for the recessives of the other parent, inbreds derived from heterotic populations increase the probability of covering up the negative recessive genes.
 

​Separation of the male and female flowers of corn and ease of distribution of corn seed from its origin in central Mexico to the various environments of the earth increased the genetic variability in the species.  Selection by humans for those plants best fit for their individual use also reduced genetic variability of some other traits.  Some favorable corn traits are expressed best when a specific gene pair includes at least one dominant form of a gene.  But if the plant is heterozygous (one dominant and one recessive) for that gene, one quarter of the progeny created by pollination of the female flowers by pollen from the same plant will be homozygous recessive for that gene. That gene will not not be expressed in the plant growing from that seed.  
 
Corn has a large number of genes and rarely does homozygosity of a detrimental recessive gene have an overwhelming affect on the plant, but accumulation of these events does detract from maximum growth and expression of favorable traits by corn.  Regardless, the saving of seed in the various environments resulted in accumulation some homozygous recessive genes unique to that variety.  
 
This is the enigma of selection of corn for uniformity that allows for uniformity of characters needed to get efficient and maximum harvest.  Self pollinating preferred plants also increases the probability of eliminating the expression of favorable traits in the next generation.  Selfing increases uniformity but detracts from many physiological processes involved in the ultimate goal of maximum grain yield. The boost in yield resulting from crossing plants from two of the distinct corn populations that were developed in different environments over many years apparently occurred because each had different negative recessive genes covered up by the dominant gene of the other population. The selfing process to obtain uniformity increased probability of negative traits but crossing with certain inbreds from another genetic family could overcome the negative traits of each. 
 
Realization of this advantage of hybridization led to major advances in corn culture. Selfing of corn for favorable traits in today’s corn culture and hybridization with selected inbreds continues to lead to improved corn hybrids.

​Separation of male and female flowers in corn allows for spreading of pollen among corn plants and, consequently, some mixing genes. However, much considerable amount of pollen falls on the silk of the same pollen producing plant, resulting in increasing homozygosity among the genes on seed saved from from each generation. Although those saving seed from desirable plants, the next season would have some of the desirable characters recognized by the seed savers but would also show the affect of homozygosity of some genes that would detract from maximum performance. Thus, this practice of annually saving seed in the various areas tended to select for characteristics favored but also resulted in eventual selection of less less obvious genetics inhibiting maximum expression of the capacity of grain yield in this species.

Some of the seed savers witnessed the extra vigor when some of these isolated populations were mixed, showing the advantages of intentionally crossing between them. James L. Reid’s father, Robert Reid used a seed variety called Gordon Hopkins adapted to their Ohio area in the mid 1800s. He and his son James L. Reid migrated to Illinois in the late 1800s and brought along some of the corn seed. Robert Reid started intentionally crossing and selecting seed of desirable plants in Ohio, and later with his son in Illinois, developing a popular variety known as Reid yellow dent corn. It included some New England Flint and Southern Dent corn genetics as well. This variety was sold and distributed to many areas of the USA but was especially adapted to the environments of the Eastern half of the USA.

The detrimental affect of inbreeding and the advantage of crossing between between varieties became clear to students of corn breeding in the early 1900’s. Inbreeding allowed the reliable reproduction of some traits but with the depression of others.

In 1935, George Sprague began intercrossing 16 inbreds and 4 inbred parents that were mostly from these many sub-varieties of Reid dent corn. This became known as Iowa Stiff Stalk Synthetic population (SS). New inbreds from this populations were test crossed with Lancaster inbreds, the successful SS inbreds were intercrossed, new selfs made, crossed again to Lancaster inbreds. These cycles were continually resulting in more populations at Iowa State University. Other public and private corn breeding efforts built on Stiff Stalk Synthetic, selecting from portions of it as well as constructing their own genetic populations. Inbreds from these efforts continuously improved hybrid performance as national corn yields increased. A third heterotic group, Iodent, was developed by mostly private breeders in the USA. Internationally, similar processes of identifying inbreds that combined with unrelated inbreds to give hybrid vigor.

The advantage of crossing of inbreds derived from different populations was becoming understood but would take some time and effort to economically produce hybrid seed.

​Before intentional production of hybrid corn seed, corn growers selected varieties of seed best suited for growth in their environment and grain use. Leaf disease resistance was a more important character in the southeast United State, whereas ability to silk when under dry conditions became more significant in the western areas of the central USA. Other regional environment differences resulted in selection of different genetics in corn varieties.
 
This tendency for corn growers to select seed that best met their desired characteristics led to distinct regional varieties but also inadvertently led to some homozygosity of some negative genes.  This was happening worldwide as corn was distributed and then locally selected annually for desired characteristics.  As seed was selected locally and annually, the desired characters were expressed, and genetics affecting those characteristics became ‘fixed’, inbreeding caused homozygosity of not only the desired genes but also of some other genes.  That affect ultimately limited the total grain production of the saved seed variety.
 
Occasional experiences of mixing of races showed the extra vigor the yield repression when ‘races’ of corn were crossed. It is written that an Ohio corn grower accustomed to planting a southern dent variety supplemented the poor emergences in his field with planting a northern flint corn in the gaps of the field. He saved seed from that field after harvest, as usual, for the next season.  That seed resulted in a largest grain yield, the plants having the advantage of the dominant genes of one variety overcoming the recessive negative genes of the other variety.
 
This phenomenon was commonly noted by professional corn breeders as understanding of corn genetics became better understood.  Vigor repression in corn was clearly linked to inbreeding and restoration of vigor was understood to occur when some varieties were crossed. Inbreeding was needed to get expression of some characteristics but also led to homozygosity of some negative recessive genes. If crossed with the certain other varieties that had dominant forms of those genes, the resulting progeny did not express the negative characters.  This attracted the attention of academic and commercial people in the early 1900’s as they began to experiment with the practicality of corn hybrids. 
 
 

​Corn’s unique biology encouraged diversity beginning with peoples’ selections of its desirable characteristics over its history.  It continues to carry the essential C4 photosynthesis mechanisms attributed to its origin in a tropical environment and the separation of male and female flowers allowing the easy cross pollination with other corn plants. 
 
The C4 photosynthesis system allowed large potential for converting light energy into carbohydrates. Separation of male and female flowers allowed the diversity and ultimately the broad genetic base and adaptation by humans to their use of corn.  Being an annual plant also allowed quick adaptation to diverse environments as humans distributed the seed with their desired attributes. This happened long before humans were describing and understanding of genetics and biology.
 
As people started more intense farming methods, they increased their efforts to select corn seed with desirable characteristics adapted to their environments. The broad genetic base encouraged by its separation of sexes, allowed selection of plants with flowering and grain maturity appropriate for the frost-free season.  They also selected seed from their crop with desired grain hardness and volume.  Eventually multiple farmers selected their own seed from many areas of the earth.  In North America, the soft starch form in the grain was desired whereas in the northeast states those with hard endosperm starch was selected.  These two extremes in starch were also selected elsewhere corn was distributed.
 
Diversity of genetics within Zea mays also allowed selection of plants with root growth appropriate for the local soils and water supplies. Also selected were genetics that allowed close timing for emergence of female flowers and pollen resulting in successful grain development. Plants with desirable resistance to local pathogens and insects were selected by individual farmers.
 
This selection of genetics within regions resulted in some restriction of genetic diversity within that region and eventual limits in grain production to only to a small fraction of what is expected by today’s corn growers. That began to change in the late 1930’s and continues today.  That phenomenon will be discussed in the next Corn Journal issue.

​ 
Corn’s unique biology and history contributes to the crop’s worldwide distribution and productivity today.  About 8-10000 years ago, probably in the Balsas River valley of Mexico, someone or perhaps several people, found a mutant teosinte plant in which the hard fruit casing did not extend fully around the enclosed single seeded grain, the kernel.  Someone realized that this made the kernel very edible and consequently propagated some of the seed.  It was recently discovered that a single mutation in the TGA1 gene in Teosinte, changed a single nucleotide in the DNA code of this gene.  This resulted in an amino acid change (asparagine instead of lysine) component of a protein critical to the development of the seed case in Teosinte and corn. Not only were the starch components of the seed more easily extracted by humans, but removal of the hard case allowed for greater growth of the kernels.  This was only one of several mutations that ultimately resulted in we know as maize but we are appreciative that people a very long time ago, recognized the advantage of this mutation. A description of the mutation is in http://phys.org/news/2015-07-tiny-genetic-tweak-corn-kernels.html.
 
Other mutations assisted humans as they moved from a plant with about 20 seeds per ‘ear’ encased in a hard covering, into a plant that were easily used for food.  It is notable that the initial mutation was to an annual plant version of Teosinte, thus allowing for selection of new genetics each year. A thousand years could equal 40-50 generations of humans but is 1000 generations of corn, allowing a lot of opportunity for genetic changes as humans made selections for adaptation to the environments and their food.
 
There is archeological data supporting that the early corn, although originating in a valley in south central Mexico, was moved to the highlands of Mexico.
 
The first European exposure to corn was about 500 years ago.  Europeans called it corn because that was the common name for other grains. Seed was moved to Europe where it spread from even further to Africa and Asia, again with locals selecting for adaptation to their conditions. This included a wide range of required time from planting to harvest, disease pressure and local food uses.
 
Within the USA, corn that moved through the Southwest and then north and east tended to be flint types whereas the Southeast corn was floury types perhaps with genetics influenced by Caribbean corn migration.  Flint corns in the Northeast USA tended to have fewer kernel rows than the semi-dent types of the southeast.  As people became more stationary, with each farmer having the opportunity to saved seed that favored their environment, food and livestock needs, multiple uniquely genetic varieties developed across the continent. Although local selections were based upon gross performance, ultimately selections were having effects on root structures, photosynthesis parameters, moisture stress tolerance, germination in cool soils, and disease resistance.  Separation of the male and female flowers of corn became a major asset to avoid complete inbreeding as farmers kept desirable ears to save for the next season.  Most pollen from an individual corn plant does not fall on its own silk, almost guaranteeing cross pollination in open fields. Until 80 years ago, open pollinated varieties were the sources of corn, with yields in the USA usually less than 30 bushels per acre. 

Those corn breeders contributed to the wide genetic diversity for current corn breeders to tap into for the latest traits needed for successful crops.

​As the corn crop is being harvested in North America one can easily marvel how a tropical grass called teosinte that reseeded itself by easily detaching mature seed and allowing easy scattering for the next generation.  One also has to marvel at the humans that recolonized the genetic potential, before we knew about genes, of this weedy plant with a few seed growing in central Mexico.  The path to modern corn progressed by people some 8-10000 years ago by selecting the mutants that did not scatter the seed, that deposited larger amounts of carbohydrates in the kernel, and had other characteristics allowing for better harvests and human use as they gathered the seed.
 
People who traveled through those areas found they could carry this seed and later successfully found mutants that would adapt to new environments.  This plant’s ease of cross fertilization provided more genetic diversity, and normal mutation rates, provided increasing diversity for many plant and kernel characteristics desired by each group of people as well as the carbohydrate nutrition needed for human health.  
 
The genetic features that evolved previous to human effort probably included a photosynthesis mechanism that allows more efficient use of atmospheric CO2 and sunlight than most plant species. This C4 photosynthesis has an advantage over most plants’ C3 photosynthesis by continuing to increase production and storage of carbohydrates even with the brightest of sunlight.  
 
Humans from the food-gathering folks to all of those today working with this crop learned to cultivate it in multiple environments including soils, nutrients and to manipulate and select genetics best meeting the needs of people.  Corn has come a long way and it will continue as we learn more of the potential of Zea mays.

​​Resistance to stalk rot fungi involves so much of the corn plant’s biology and the environment that it does not become an easy trait to express in hybrid descriptions.  On the other hand, there are differences in the tendencies to develop stalk rot when the plants are under certain environments.  Hybrids differ in reaction to favorable pre-pollination conditions, some committing to greater movement of carbohydrates to the grain at the detriment of carbohydrate availability to the roots.  Reactions to late season photosynthetic stress also varies among hybrids.
 
The gradations of these variables and hybrid reactions do not allow absolute stalk rot resistance ratings possible.  Expression of stalk rot rating is much like expression of corn yield- absolute values are not appropriate but are only meaningful in relation to other hybrids or acceptable performance.  It is in this regard that evaluation of stalk rot vulnerability of experimental hybrids by plant breeders needs to be done in hybrid yield tests.  Stalk rot vulnerability is a hybrid phenomenon that may be influenced by the inbred parents, but it is mostly the product of heterosis, with the combination of the parent genetics affecting the probability of stalk rot problems.  Consequently, it is evaluation of the hybrid that is critical.  Also, just as with yield testing, commercial seed breeders are interested in predicting the stalk rot vulnerability in the field where the hybrid will be used.
 
Plot yields can be taken with accuracy, but evaluation of stalk rot requires human observation. Counting lodged plants is relatively easily done from the harvest combine but this method does not consider the rotted plants still standing but ready to lodge with the next strong wind.  An alternative is to rate the stalk condition by walking each plot, giving it an acceptability rating.  If all plants are strong and with green lower stalk color, the plot is scored as excellent.  If too many plants are weak and ready to lodge, making it unacceptable, then it can be scored as extremely unacceptable.  Of course, some plots would be scored as intermediate.  This method should be applied at all plot locations.  The final summary can be expressed as the percentage of plots or location in which a hybrid had acceptable levels of stalk rot.  This should allow an estimation of the frequency of stalk rot problems expected for each hybrid.  Test plot locations will not represent all the environments that a commercial hybrid will need to balance yield, stresses and stalk rot but growers will evaluate annually in their field conditions.
 
Stalk rot resistance ratings should not be considered as absolute resistance by hybrids against a specific late season fungus but more of the balance of photosynthetic stress and translocation of carbohydrates under most field conditions. 
(Article from Corn Journal 10/19/2017)    

There is a tendency to give the identity of the most obvious fungus in a rotting stalk as the cause when significant lodging occurs in a corn field. It is Diplodia, or Anthracnose or Fusarium or Gibberella because these are the most easily identified fungi on the rotting material. In fact, multiple other fungi are probably in the same stalk rotting the dead stalk tissue.  Most importantly, these fungi did not cause the stalk tissue to die but entered after the stalk tissue died.  Living corn tissue fends off these and the many other potential saprophytes in and on soil debris. Any of these fungi successfully invade the stalk tissue when the plant tissue no longer produces the metabolites to fend off these potential saprophytes.
 
When analyzing the cause of significant stalk rot in a field, one needs to look for the reason the plant lost its ability to fend off the fungus and lost its strength to stay upright.  A third of that strength is due to the lower stalk tissue that has pith tissue connected to the outer rind cells, essentially forming a rod.  These pith cells are large parenchyma cells filled with liquid and stored, soluble carbohydrates.  These carbohydrates are available to the roots for their metabolism and, when called upon by the hormones directing flow, also to the newly forming kernels.  If previous and continuing photosynthesis in the plant produces enough carbohydrate to meet the demand of the kernels and the roots the plant maintains succulent, living pith tissue throughout the stalk and thereby maintaining the stalk strength.  Potential invaders are warded off by the metabolism of those living cells.
 
Discerning the cause of why one plant died and lodged and not all plants require a more diligent search than simply naming the most obvious fungus.  Pith tissue died prematurely and shrunk away from the stalk rind, changing the structure from a solid rod to a hollow tube.  Furthermore, those multiple saprophytes easily invaded the dead tissue, further weakening the stalk.

​The analysis of why has to include reasons why was there not enough carbohydrate in the pith tissue of the rotted stalk to maintain living pith cells.  It is not as simple as naming the dominant fungus in the dead stalk

​Several aspects of the tar spot fungus, Phyllachora maydis. Appears to be unique corn leaf pathogens.  It belongs to a group of fungi called Ascomycetes, because its sexual reproduction stage involves formation of an ascus after the fusion of two nuclei.  Miosis occurs to produce 4 haploid single cells each of which divides to produce 8 ascospores within a sac called an ascus.  A thick-walled group of cells form black ‘stromata’ on the surface of the leaf enclosing several of these spore-containing asci.  With the right environment, such as a humid warm night, the spores are released from the ascus and easily moved in the wind.  After germinating on the surface of the corn leaf, the hyphae establish specialized structure that grows into the leaf.  Apparently, within susceptible hosts, the fungus absorbs nutrients and reproduces soon, again releasing spores into the air.
 
Many corn leaf pathogens are ascomycetes but most of the fungal spread is done be asexual reproduction, with conidia the pathogen spreading mechanism, with the sexual stage being restricted to end of season reproduction.  Although genetic mutation can occur within the fungus before conidial reproduction, this opportunity for development of new genetic combinations is not as profound as with fusion followed by meiosis such as in meiosis following the sexual union of nuclei.  This may allow variants to continually develop within this pathogen.
 
Being an obligate pathogen, only known to grow on living corn leaves, certainly makes study more difficult but eventually some of the variation will become understood. Currently no other host has been identified but genetic variability would make this a possibility.  Widespread outbreaks like in summer of 2021 seemed to favor this disease.  Perhaps variants will also adapt to less humid weather as well.
 
This disease has potential to cause significant damage to corn and must be studied with urgency.

​Tar Spot of corn became an important problem in Northern USA corn belt states in 2021.  It has drawn the attention of several corn pathologists that summarized their observations of the past few years in Oct. 2020 publication of Plant Disease Vol.104, No. 10.  The main pathogen causing the disease is an obligate pathogen, meaning that it can not be cultured on artificial media.  This characteristic hinders rapid investigations of the disease.  Its spread appears to be related to temperatures and humidity, again making intense annual studies difficult because of inconsistent occurrence.
 
The spores of this fungus are easily spread in wind, apparently for several miles.  Spores can overwinter in corn debris, infect corn pants at all stages and set up spread within and outside the field if humidity is relatively high for prolonged times.  There appears to be no complete resistance among today’s corn hybrids but a range of susceptibility from those that seem to have few lesions to those that are completely killed before normal black layer.  Uneven and somewhat unpredictable spread of the disease has complicated the evaluation of disease resistance.  We at PSR attempted to rate hybrids for resistance among hybrids submitted by seed companies for evaluation of resistance to other diseases.  In 2021, our nursery seemingly suddenly showed most plants with Tar Spot.  The black spots on most leaves in many hybrids often interfering with other disease symptoms.  Leaves appeared to die prematurely.  Those with mostly green leaves were tentatively appearing as more resistant to tar spot with the potential that relative maturity was complexing the ratings.  Checking those that had green leaves 7 days later, remained green, consistent with actual resistance and not maturity as the main factor in reaction to this disease.
 
This 2021 small nursery was isolated from other corn for at least a mile.  We saw tar spot in the field 2 years ago but very little last year, supporting the hypothesis that this year’s infection was mostly from spores spread from some distance.  Although our field has been in corn for several years, it had very little of last year’s debris before planting.  
 
This disease was identified in corn in Mexico in 1905, but, I think, only identified in corn in the Midwest area in 2015. Hopefully more knowledge will be gained from the 2021 experience, and we will find more on how to protect corn.  Corn genetics, weather, pathogens and agriculture are in constant change resulting in new disease occurrences. 

​Corn stalks have a green outer rind color during most of the growing season as the outer cells are pigmented by the chlorophyll. This color continues beyond grain fill as this annual plant matures without wilting, even up to normal grain harvest.  If the plant wilts because of root rot, not only do the leaves desiccate, turning from a green color to gray within a few days and then brown but the stalk color changes also.  The dark green color becomes yellow-green a few days after the plant wilt.  This color change progresses to yellow and a few days later to brown. 
 
As the brown color intensifies, desiccation of the internal pith causes withdrawal of the pith from the outer rind. This changes the stalk structure from a solid rod to a tube, reducing the strength by a third, leaving it vulnerable to breakage.  One can access this vulnerability by gently pushing the stalks or pinching the lower stem.  Visual inspection of the color of the lower stalks to judge this deterioration also can be used to evaluate the plant’s vulnerability to lodging.  Individual plants with green lower stalks a few days after grain ‘black layer’ will remain intact through harvest.
 
The anthracnose fungus, Colletotrichum graminicola, will cause black streaks on the outer rind even on a green stalk.  This color only intensifies, however, if the plant wilts, apparently because the living cells can restrict the fungus.  If there remains a green color around the black streaks, the lodging threat is not great.  Another interaction with the fungus commonly occurs in the uppermost internode of the corn plant.  It is often noted that this internode turns brown when remaining stalk is green.  As sugars are moved from leaves to the grain, this upper internode often is depleted first resulting in senescence of this tissue. The anthracnose fungus is often found in the dying tissue.  I interpret it mostly as signal that the plant is successfully moving maximum carbs to grain and not necessarily a sign of stalk rot.
 
Other fungi also become noticeable on the dead, brown lower stalks.  Gibberella zeae produces its reproductive bodies near the nodes, Diplodia maydis produces theirs more scattered on the internode tissue and Fusarium moniliforme gives a pinkish discoloration across the internode surface.  It may give us some comfort to have a name for the fungus present but it must be remembered that the cause was insufficient carbohydrate to both meet the translocation demands of the grain and the maintenance of root life.  These fungi, and the many others also digesting the senescing and dead stalk tissue were not actively killing the plant.
 
​One must not forget that naming the fungus most obvious on the dead stalk is not the same as identifying the cause of the plant wilting early and the resulting weakening of the stalk tissue.  Best to look further into the dynamics of all factors that kept the plant from finishing the season with green and solid lower stalk.

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

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