Corn in the Midwest USA is approaching the time of season in which scattered plants show whole plant death. It does cause confusion as to if it involves an aggressive pathogen or is it more of a physiological nature. Diagnosis is further complicated by presence of fungi such as Fusarium species or Colletotrichum graminicola, the cause of anthracnose.
If the symptoms include all leaves wilting and stalk color becoming yellow green then brown, the problem is caused by roots not being able to supply sufficient water to leaves to remain turgid. Resulting wilt results in all physiological activity in plant to stop. Abscission layers (black layer) develop at base of each kernel. It is tempting to call the disease by the fungi found in the dead tissue but the real problem is rotting roots.
Roots become susceptible to the many microbes in the soil as sugars from the leaf tissue after pollination is insufficient for the root cells to produce the metabolites needed to ward off the microbe invasions. As more root tissue is destroyed, uptake and transport of water declines. If the transport to leaves is insufficient to meet the loss of water via transpiration, the leaves wilt. The visibility of this wilting process occurs relatively sudden. Close observation may show a slight discoloration one day and complete wilt the next day.
The individual plant that wilts was probably determined by size of root during season, supply of carbohydrates after pollination and volume of carbohydrates moved to the grain. Each of these involve multiple factors. Plant density, plant uniformity, light intensities, number of kernels, leaf diseases are among them. Genetics and environments are obviously interacting. It is better to consider those variables than simply associate it with a disease.
As the flow of carbohydrates to the developing grain intensifies, cells in leaves begin senescence. Along with these changes in cellular metabolism it is probable that production of the chemicals associated with limiting microorganisms with potentially destroying cells as they feed on carbohydrates and protein contents. The nature of the battle between the living plant material and surrounding micro-organisms.
Vigorous cells of corn plants before pollination were only overcome by microbes with specific metabolites that overcome the resistance mechanisms. Senescing cells are overcome by others that are no longer inhibited. Definition of pathogens becomes more ambiguous as these organisms attack the tissue in senescing tissue. Some fungi such as Fusarium species may have been present in the plant for much of the season, but not destroying much tissue until natural senescence begins. At what point do we call it a pathogen and not just a saprophyte?
Sampling of aspects of crop agriculture is difficult and care must to be used to draw conclusions from results of tests of samples. Biology of the plant and varying environments affect the predictability of the sample’s test result. Nearly everyone participating in agriculture realizes this problem within a short time of exposure, although it is not always expressed.
Seed producers are aware that each seed within the production field did not have exactly the same environment and that each seed can be potentially with a different parent and environmental interaction during and after the growing season. Seed producers attempt to use field and facility methods to limit potential problems that could eventually affect performance of the hybrid in grower’s fields. After using these efforts, the next challenge is to predict the success of these efforts to have good purity and germination.
Sampling of seed usually begins after ears are dried and shells. Methods are used to take general bulk sample by some randomizing technique. This bulk is the sized that includes those that represent different portions of the ear, the rounds tending to be at both ends and the flats in the center. This essentially is allowing checking seed with differing pollination dates that could affect purity. Shapes of the kernels also potentially affect germination viability.
Seed sizes are submitted to purity and germination test often before final bagging procedures have begun. Number of seed included in sub-sample to be tested varies by testing method. Effectiveness of the test in predicting the eventual seed effect on field performance is dependent upon the sampling accuracy, sample size and testing accuracy in evaluation.
Even if the initial sampling of the seed lot is done with care, there remains a randomness factor with test sample size. The percentage of seed germinating in a lot, or percent of outcross plants actually in the lot determined by the test is affected by the test size. As summarized in https://www.statisticshowto.datasciencecentral.com/probability-and-statistics/find-sample-size,
a germination or purity test of 100 seeds showing 100% has a 95% probability of actually being between 96-100% where as if the test size was 400 seeds showing 100% purity or germination, the actual has 95% probability of being 99-100%. If test result showed 96%. on a 100 seed test, the actual has a 95% probability of actually being between 90-99% where as a 400 seed test is probably between 94 and 98%.
Sampling of seed lots and testing methods including number of seed tested affect the accuracy of predicting the actual germination and purity of a seed lot.
Obtaining reliable predictions of percentage of occurrence of any biological feature within a population is extremely difficult. Hybrid seed corn in which two parent inbreds, rarely perfectly homozygous for all genetics, needs to be evaluated for potential problems with purity problems due to contamination within the parent seed or outside pollen fertilizing the ovules.
Seed producers use all reasonable approaches to limit these possibilities but environments within the seed field can affect the purity as well. Extreme dry areas can delay silk emergence but rarely delay pollen production by the male inbred. Consequently, female silks remain viable for potential fertilization by pollen from hybrid fields. Such outside pollen can be genetically segregating, resulting in genetics varying from the correct hybrid, but with each of the resulting plants different from the correct hybrid and different from each other. Corn pollen can remain viable while carried by wind for at least a mile. Lack of timely distribution of correct male inbred pollen, increases the potential contamination by foreign corn pollen.
Stressed plants in a hybrid production seed field also may cause delayed tassel production leading to the possibility of missing a few plants from having tassels removed from the female inbred parent. This can lead to self-pollination of the female parent resulting in inbreds within the hybrid seed corn.
Hybrid seed corn producers are well aware of these potential problems and use multiple methods to avoid purity problems. Despite their field management and care, there are circumstances that are difficult to overcome. Consequently, checking the purity and germination of the resulting seed needs to be done after the seed is harvested.
Each kernel of seed corn can be distinct in origin. Those at the base of the female parent ear were probably fertilized a few days earlier than those at the tip. It is possible that the source of pollen could be different simply because of timing conditions at that location of the field. Seed producers are aware of these possibilities and significant problems to hybrid corn performance are rarely released to sales. Testing for purity of the hybrid seed sizes allows the eventual discard of any highly contaminated seed sizes from those being sold.
Seed companies give considerable effort to produce and sell pure hybrid seed. These are tasks easily overlooked as one views uniform hybrid corn fields from the roadway.
A few corn plants on the edges of fields are showing the deformed tassels as predicted with the very wet spring. Corn Journal summarized the main factors of standing water relationship with this disease in the issue dated 6/18/19 and 5/30/17 blogs. It is always somewhat surprising because the disease symptoms are most evident after tassels develop but the infection occurred a few months previous to symptom development. Here is a copy of the 6/18/19 blog concerning this disease.
Excessive rain in much of the USA Corn Belt in 2019 with ponds of water in areas of fields can encourage infection by a fungus that has swimming spores. Corn Journal blog of 05/30/17 may be appropriate for this year as well.
One of the effects can be infection by an organism called Scleropthora macrospora. This is a fungus-like organism belonging to a group of organisms called Oomycetes. Also, in this group are pathogens causing Downy Mildew and Pythium diseases of corn and other plants. Common among these are the ability to form thick walled spores to withstand stress environments that can release swimming spores when in water-saturated soil. S. macrospora infects more than 140 grass species in addition to corn.
The source of infection of corn is often grasses near a low spot or edge of a field. Oospores in the flooded living and dead leaves release swimming spores (zoospores) when close to the corn submerged leaf tissue these zoospores release a germ tube that infects the plant. The filaments (hyphae) grow towards the meristems throughout the life of the plant. This can initially be seen as fine stripes in the leaves, but the most obvious symptom is proliferation of leafy aberrations of the tassel- the crazy top symptom. Scleropthora macrospora also can grow to the ear bud meristem, causing similar multiple ears from a single node- but no grain.
Related oomycetes occurring in warmer, subtropical and tropical environments can cause similar symptoms. These downy mildew diseases can also cause the proliferation of the tassels and ears. Susceptible genotypes can have severe grain loss from these diseases. Scleropthora macrospora infection is usually limited to a very small area near grass in a low part of the field.
Infection occurs when the plants have less than 6 leaves. Symptoms that show late in the season, but the problem began with excessive rain that occurred only a few weeks after planting. That early moisture that may contribute to large yields can allow this pathogen to form these unusual corn structures in a few spots of the field. In addition, it is just part of the interesting biology of corn.
Individual plants in corn fields are starting to show red leaves as the season approaches the final days of grain fill. Close observation of these plants reveals that they have very few kernels, generally because of poor fertilization. These plants probably were silking only after most pollen in the field was gone, perhaps because of late emerging seedlings or other causes of late silk emergence such as moisture stress.
Red pigments are caused by production of a flavonoid known as anthocyanin. Many plant species produce anthocyanins, especially in reaction to stresses such as low temperatures, diseases or insect damage. Anthocyanin compounds accumulate in cells as water soluble compounds in cell vacuoles. It is derived from glucose in a synthesis and is linked to accumulation of glucose within the cells. It has the effect of absorbing light with effect of reducing photosynthesis.
It is not completely clear the advantage to the corn plant to reduce glucose production by absorbing less light for photosynthesis. Perhaps it reduces the callose development in phloem tissue that could reduce the flow of glucose to the few developing kernels. It is clear that it is related to accumulation of glucose in the leaf tissue because of reduction of transport to grain.
Corn has benefited from human’s selection process annually for the past 10000 years. This has occurred over multiple environments with preference towards stability of desired characteristics of the grain. This usually led towards increase in grain storage of starch. With the realization of value of hybrids between parents from unique heritage, the combination of those genetics added to the greater grain yield. Combinations of 30000 genes from both parents creates a stability in multiple environments.
Genetics affecting root size and direction, essential to water and mineral uptake for the plant is also influenced by genetics affecting efficiency of transport of carbohydrates from the leaves to supply energy for the growth. Volume of carbohydrates produced in leaves is influenced by multiple genes affecting leaf size and intracellular dynamics. Even resistance to most leaf diseases involves 3-4 genes directly limiting the pathogen.
Genetics affect the timing of the production of pollen and emergence of female stigmata (silk). Genes contribute to movement of water to the ovules for extension of the silk from the leaves surrounding the ear shoot. Number of ovules, potential size of endosperms, quantity and strength of hormones causing the flow of carbohydrates to the pollinated ovules is affected by genes.
All of these genes are selected for stability under multiple environments, some with annual extremes of mineral, water and sunlight supplies. Multiple genes contribute to stable performance of successful corn hybrids.
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 possibility.
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About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.