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.
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.