​Nearly all living forms of life develop means of fighting off potential pathogens.  Corn cells produce specific enzymes to restrict and inhibit growth of most microorganisms.  Resistance to the very few than may be able to overcome most of inhibitors is usually a general compound, its effectiveness often related to amount of the inhibitor and the timing of its production.  The latter often is related to turning on its production based upon detection of the invader.
 
Most fungal species are dependent upon receiving nutrition from dead plant and animal sources partly because the anti-microbe inhibitors are not present.  However, there are many competitors for the same source of nutrition. Consequently, natural selection favored production of metabolites that ward off competitors.  This is apparent to those of us that culture bacteria or fungi in petri dishes and observe contaminants warded off by another species of bacteria or fungus.  
 
This observation in 1928 led to the initial penicillin, as the fungal species of Penicillium warded off bacteria contaminating a petri dish.  Many other antibiotics were and continue to be isolated from fungi.
 
The mushroom Strobilurus tenacellus is a fungus that spends most of its live feeding on decaying pine cones in soils of European and Asian forests.  Like many mushroom species most of the fungus is not seen until it forms the reproductive mushroom structure above ground.  Beneath the surface, however, it fights off competitors by producing a compound called strobilurin.  This compound is apparently effective against many bacterial and fungal species.  It inhibits the energy production in mitochondria and gains an advantage for this fungal species by having genes blocking the strobilurin from attaching to its own cells.  Several other wood-rotting fungi also produce similar compounds to serve the same function of fighting competitors.
 
Obviously, the activity of these compounds makes them attractive as potential fungicides for crops such as corn.  Companies have modified the compounds to make them more stable when exposed to light and allow them to attach to leaves for enough time to be effective.  Many current corn fungicides use forms of strobilurin derived from cultures of these fungi.  
 
Adequate and economic restriction of potential damage to corn grain production requires a balance of resistance systems in the corn plant and adding metabolites from fungi.
 

​Factors leading to deterioration of corn stalks are complex, as discussed previously.  In most cases the direct loss of strength comes from premature death of the plant in which it suddenly wilts before completion of grain fill.  This is preceded with destruction of roots by soil fungi due to reduction of cellular resistance.  This happens when the upper plant cannot adequately supply carbohydrates for maintenance of those root cells as sugars are also moved to the grain.
 
The wilting of plant results in withdrawal of the pith tissue from the rind, essentially changing the strength of the stalk from a rod to a tube. Stalk cell death also reduces the resistance to the fungi feeding on the cellulose and pectin of the rind, further weakening the stalk strength.
 
Fungicides could be affecting those stalk invading fungi but also could be reducing leaf pathogens during the grain fill period and therefore reducing loss of photosynthesis. This would potentially provide more carbohydrates to the roots and therefore avoiding the premature death and wilting that started the stalk strength weakening.  It would be interesting to see that hypothesis tested.
 

​Grain farmers are finding varying corn grain drying challenges in the wild, summer 2019 USA weather.  Late plantings, varying wet conditions extending into the harvest time have made for confusing grain drying to avoid mold developments while stored.  Moisture reduction in grain is due to evaporation as water must move through the pericarp basically as a physical phenomenon of water moving from higher to lower concentration.  Warmer air absorbs more water than cooler air, and consequently prime field drying after black later is best with warm, dry winds.  Open leaves surrounding the kernels on the ear, increases access to that warm dry air.  Plants reaching black layer after the warmer days of late summer, are likely to have less field drying than normal seasons, and thus slower natural drying in the field. 
 
Corn hybrid maturity classifications are often determined with some classification based upon moisture contents at time of harvest.  2019 data may not be a good year to make those classifications.
 
Seed breeding and production groups may have a more difficult time than most this year as well.  Field variability probably was greater than normal due to excessive rain interacting with soil variability.  Later planted test sites may have harvest moistures not typical for a given hybrid or predictive of performance in future seasons.
 
Drying seed is an art that includes manipulating air flow and temperatures in drying bins.  It includes consideration of outside relative humidity. As moisture is withdrawn from the seed, cellular membranes, including those of mitochondria, are potentially damaged.  Rehydration of seed for germination tests after drying can be the first indication of potential problems but often damage from does not become evident for some months later.
 
We all want to return to a normal season but what is that?

​Among the contradictions in corn culture is the need to have corn stalks maintaining upright plants through harvest but rapid deterioration in soil between seasons and /or efficient decomposition for fermentation to recover the carbon in ethanol or energy for cattle.  Primary strength during the growing season is derived from a combination of the tight connection of the pith cells to the outer rind cells, fibers and near the outer rind and thick cell walls of the outer rind cells.
 
Stalk components after harvest range among hybrids.  About 50% of the solid weight is composed of carbon but most of it is involved in complex molecules such as cellulose, hemicellulose and lignin.  Although lignin composition is only about 7% of the stalk, it is the most difficult to digest and often is wrapped around the more easily decomposed cellulose molecules.
 
Multiple fungal species in the soil produce enzymes capable of breaking and modifying the lignin molecules. Tree wood, mostly composed of lignin, is slowly destroyed by fungi specializing in production of lignocellulolytic enzymes.  These initial wood rotting species are succeeded by other fungal species that enzymatically degrade the cellulose into its components.  Genetic variation among fungi and competitive pressure for obtaining the energy locked up in corn stalks provides multiple sources to break down the complex carbon compounds that provided strength for the corn stalk previous to harvest.
 
Among the challenges for all interested in corn is to identify hybrids that produce stalks that remain upright through harvest but can be efficiently digested by cows, fermentation and soil organisms.

Published in Corn Journal 11/1/2018

​The fungal genus Fusarium is a ubiquitous inhabitant of corn and other grasses.  Several species of Fusarium are also associated with their sexual stage of the genus Gibberella. Fusarium is recognized microscopically by the shape of their asexually produced spores (conidia) that are produced in abundance and dispersed by wind currents.  Distinction between Fusarium species requires specific lab methods but quick microscopic exam for the curved, multicellular, hyaline conidia leads to a quick analysis as Fusarium.
 
Fusarium species do not tend to be aggressive pathogens of vigorous, living corn tissue but almost more of an inhabitant, not actively killing cell tissue but more of a scavenger of dying or dead tissue.  Some Fusarium species such as F. verticillioides (formerly named F. moniliforme not only produce multicell conidia but are known to produce single cell microconidia that apparently can move in the vascular system of a corn plant.  Ease of movement within and outside of the corn plant and the ability to infect weakened and dead corn tissue allows for this fungus to be found in nearly all dead corn tissue.  This can include leaf tissue injured by insects, hail or other physically injury.  Active leaf pathogens such as Exserohilum turcicum (cause of northern leaf blight, apparently ward off Fusarium invasion via antibiotics.  
 
Nearly every dead stalk will have a Fusarium species among its inhabitants.  If the dead tissue includes the more easily identified symptoms of Gibberella zeae (the sexual stage of Fusarium graminearum) the diagnosis will be Gibberella stalk rot.  If black streaks typical of the anthracnose that will be the announced cause.  Diplodia maydis, now called Stenocarpella maydis, is distinguished by its symptoms as well.  If none of these symptoms are evident, the ever-present Fusarium, is diagnosed as the cause of Fusarium stalk rot. 
 
The actual cause of death of the stalk tissue is the complex interactions of photosynthesis and distribution of carbohydrates during grain fill of the corn plant. The fungi present are able to digest the dying and dead tissue.  If none of the easily identified fungi associated with stalk rot are found, there is always Fusarium stalk rot.  The ease of identifying a fungus in the tissue, implying the case of the early death of the plant, can lead to avoiding the diagnosis of why the plant died before completion of grain fil. 

​Gibberella zeae is a fungus belonging to a group of fungi called Ascomycetes.  This group are defined by production of spores after meiosis in microscopic sacs (ascus is Greek for sac).  It is typical of this group of fungi to also have asexually produced spores that are distinct and, in some cases, identified separately, and given distinct names.  The asexual stage of this Gibberella zeae is Fusarium graminearum.  Although formal nomenclature protocol requires naming a fungus by its sexual stage either name is commonly used to identify this same fungal species.
 
This fungus receives nutrition from weakened or dead plant tissues, primarily grasses.  Often initial invasion of the tissue is initiated by the spores (conidia) of the Fusarium portion of the life cycle.  The fungus quickly produces more spores on corn debris, spreading readily in a corn field.  Spores landing on exposed silks germinate and grow down the silk channel into the tissue within and around the kernels.  As the kernels develop within the moist environment of the tight husks, the fungus spreads to appear as a pink mold.  Haploid nuclei within the Fusarium hyphae fuse to form a brief diploid nucleus.  This causes the fungus to produce a new structure, called a perithecium, a distinctive black body on the surface of the infected host. Within the perithecium, meiosis occurs, resulting in haploid ascospores within the ascus.  Ascospores are released, perhaps in the following season.  These infections result in the fungus further reproduction and spread of the fungus.
 
Gibberella stalk rot and ear rot are usually identified presence of the perithecia, rough black structures produced on the outside of the stalk or kernel tissues. Kernel infections often are also a concern because this fungus tends to produce chemicals such as zearalenone and deoxynivalenol (DON).  This probably the most significant concern of problems from this fungus.  The stalk phase is more of a secondary invasion of dying stalk tissue because of senescence of plant tissue because of the balance of carbohydrates within the maturing corn plant.  These can be some satisfaction in having a name for the fungus, but analysis of the cause is more significant to reducing the future problems.

​Fungi invading grain while on the ear vary by season, location and hybrid.  In many cases the actual invasion of the grain occurred through the silk during pollination time of the season. The channel in the silk in which the germinating pollen tube grows towards the ovule is often the same channel for invasion by a fungus. This channel closes after the pollen tube passes, essentially closing the path for the fungal mycelium. Avoidance of fungal invasion into the ovule and therefore the developing grain is highly dependent on pollination occurring soon after silk emergence from the husks around the young ear.
 
Circumstances that tend to delay pollination include drought pressure at flowering time, causing delay in silk emergence but not in release of pollen. Less pollen available results in scattered kernels on ears or no grain at ear tips because pollen was expired before the silks leading to these ovules were gone.  Aspergillus and Ustilago (smut) species produce spores during dryer environments.
 
Extensive rain during pollination time can also result in more infected grain.  Pollen is not released from anthers when wet from rain.  Silk emergence is enhanced with high water tension. These dynamics tend to delay pollination of exposed silks, as well as encouraging sporulation of Fusarium, Gibberella and Diplodia species.
 
After entering the ovule, these fungi can successfully invade the developing embryo and endosperm.  In many cases this becomes evident as a molded kernels and in some cases the infections are not evident but only show later if the grain is not dried relatively quickly to 15.5% moisture.
 
Other environmental factors such as previous infected corn debris to provide fungal spores are important. Genetics of hybrids influence the vulnerability to silking and pollen timing problems when plants are under stress.  Hybrids differ in kernel composition and development and spread of infection if fungi invade the ovules.  Rapid drop in kernel moisture after black layer can be related to husk characters like thickness and looseness, essential to rapid loss of grain moisture in the field.  
 
It is important to use care in analyzing cause if severe ear rot occurs.  Was it the genetics of the environment that was primary?

​As corn matures, and grain fill is completed, the usual challenge is to allow the grain to reach maximum loss of moisture before harvest without losing ability to be harvested due to lodging.  This usually depends upon stalk strength after completion of grain fill.  Hybrids do vary on the cellular structures of rind tissue, affecting the ability of the rind to be punctured with a penetrometer.  Cellulose and lignin synthesis in the stalk tissue during plant growth is involved and affected by combination of genetics and minerals. 
 
Although the rind resistance to breakage is significant, a solid stalk, with the pith tissue intact and attached to the rind, creates the strengthening dynamic of a rod versus that of a tube after the pith is pulled away from the rind when the plant wilts.  This can be detected in a simple push test of plants after grain fill.  Those with hollow stalks will easily pushover.  These individual plants will also show symptoms of invasion of fungi such as species of Diplodia, Colletotrichum, Fusarium or Gibberella.  These are the fungi most often identified on dead stalks, but several other species also attack the cell wall components of the rind.  Presence of these fungi often lends credence of blaming the stalk vulnerability to the lodging on these fungi.
 
Intactness of the pith tissue, however, is probably the most significant factor in occurrence of late season lodging of corn stalks.  This occurs when the plant wilts previous to completion of grain fill because insufficient carbohydrate is available to fill the grain and maintain life in the root.  Wilting causes the withdrawal of the pith from the rind, thus significantly weakening the stalk strength.  Fungi readily invade the dead tissue but the main strength weakening preceded their invasion.
 
Stalk lodging is affected by many factors.  Genetics and pre-pollination environments affect ear height, root growth, photosynthesis, leaf disease, stalk rind and storage of carbohydrates in stalk pith tissue. Genetics and post-pollination environments affect the ability of individual plants to obtain completion of grain fill and stalk strength until harvest.

​Each kernel in a corn ear is a fruit.  As with most other fruits, sugars are transported to the kernel through the vascular system from the leaves and the stem.  Plant hormones like auxins and gibberellins produced in the seed embryo meristems actively guide the sugars to the seed within the fruit.  Corn kernels have only one seed. Although much of the sugar is moved outside of the embryo to the endosperm, sugar also provides energy for growth and development of the embryo.  As the embryo matures, auxin production is reduced.  Consequently, physiological demand for sucrose supply to the kernel is reduced. 
 
Reduction of sucrose in the cells at the base of the kernel causes the balance of ethylene and auxin to change.  Ethylene increase causes the layer of parenchyma cells closest to the kernel base to lose cell wall contents, while those adjacent cells away from the kernel gain wall thickness.  Eventually an abscission layer forms cutting off all movement of sugar into the kernel and water movement away from the kernel thru the stem tissue (the cob). 
 
Research by J.J. Afuakwa, Crookston and R.J Jones (Crop. Sci. 24. 285-288) showed that reduction of sucrose available to the kernel was a major factor in induction of the abscission layer (black layer).  Although it is mostly related to maturation of the embryo, it could be induced by other factors reducing sucrose supply to kernels.  Reduction of photosynthesis by leaf disease or frost damaged leaves could result in shortened time to black layer.  Early plant death, perhaps from root rot, causing leaves to wilt and thus removing sugar supplies induce black layer within a few days.  
 
Early formation of black layer before normal time for full transport of sugars to the kernels results in reduced grain weight.  Shortened time to black layer also reduces some of the water replacement in the grain normally occurring as water moves out through the vascular system.  Consequently, black layered kernels may have higher moisture percentage than usual. Some of these biological factors contributed to the grain production differences in USA Midwest in 2019.

​Stalk rot of corn occurs when sufficient root tissues dies of starvation. This happens when its energy source, carbohydrates stored in the stalk, are depleted because of excessive draw to the grain versus the supply from leaves and stalk. Size of daily movement to grain is determined by genetics and environments.  Soil moisture during time of silk extension is significant to more ovules being pollinated.  Supply of carbohydrates during the season is determined by multiple factors:  Hours and days of intense light, utilized by it C4 photosynthesis method to supply energy for more plant growth, storage of carbs in the stalk tissue as well as sugars to move to the grain after pollination.  Individual plant environments such as competing with adjacent plants for light and mineral uptake can influence success in normal completion of transport of sugars to all pollinated ovules on each plant.
 
Seasons with extreme late planting dates in temperate zones have extra factors.  Some fields will be fewer stresses in early season, resulting in less ovules pollinated, resulting in less grain but less stalk rot.  Late planting dates still requires about 55 days after pollination to complete movement of sugars to the grain.  It can be stopped with freezing the phloem within the stem tissue, with the result of light grain weight.  Such a freeze may not result in stalk rot as remaining stalk tissue may remain intact.
 
If the below freezing temperatures were not sufficient to kill the stem phloem but did result in leaf death, depletion of the sugars in the stalk are intensified.  This can increase death of stalk pith tissue, allowing stalk rotting fungi to digest the cells and weakening the stalk strength.  
 
We should expect variable field results in Midwest USA in 2019.

​Severity of below freezing temperatures while a corn plant is moving sugars to the grain is dependent on what tissue freezes.  Leaves are most vulnerable because of exposure to the cold air.  Ice crystals form in the leaf cells, killing the individual cells including those with chloroplasts.  Among these chloroplasts are those in guard cells of the stomata. Without photosynthesis the day after freezing, the guard cells do not open to allow transpiration.
 
Water movement from the root tissue to the upper plant is a physical phenomenon in which each molecule evaporating through open leaf guard cells, is replaced by a molecule of water because of water’s molecular structure causes bonding with each other.  Water is essentially pulled from the roots through the xylem structures of the vascular system because of this bonding.  If water is not utilized or transpired, leaves do not receive new supplies of water, further causing wilting of the leaves after frost or a more severe freeze.  
 
Sugars are transported through living phloem cells in vascular tissue.  During grain fill, sugars are drawn from the leaves and the stored reserves in the stem.  Death of leaves eliminates movement from the leaves.  If the low temperatures were not severe enough to kill the phloem and other stem cells, movement from stem to grain continues. 
 
Below freezing temperatures during grain fill will cause some loss of expected grain weight, but the severity will depend upon whether stem phloem is killed and the supply of sugar reserves in the stalk.

​As if the stresses that reduce photosynthesis isn’t enough to offset the balance of movement of sugars during grain fill, the 2019 USA Midwest had extreme water in the early season, even after corn was planted.  Sugar movement, stimulated by hormones produced by meristem tips, goes to apical meristem of shoot prior to flowering and to root tips.  After pollination the concentration of apical meristems in the ear redirects the flow towards the grain, competing with the flow towards the root tips.
 
If root growth was inhibited by extreme moisture in the soil, perhaps by low oxygen supply, does it result in fewer root tip meristems?  If so, does this reduce its capacity to attract sugars during the season and does this increase the vulnerability to the root pathogens.  This would result in increasing probability of the plant wilting during grain fill as well.  Stalk rot follows after plants wilt.
 
Deterioration of stalk quality follows plant wilting during the grain fill period. Wilting is caused by root tissue unable to absorb enough water to be transported to leaves.  Loss of water from leaves occurs through leaf stomata via evaporation.  Dry, windy environment around leaves causes more rapid transpiration.  Early season environment affects root growth and mid-season environment affects the size of grain sink.  Genetics determine how the plant reacts to these environments.  These multiple factors determine whether the grain successfully completes normal grain fill on plants with green stalks or not. 

Individual corn plants that have brown lower stalks are invaded with several fungi that can be identified in the dead stalk tissue.  Some, such as Diplodia (Stenocarpella) maydis, Gibberella zeae, Colletotrichum graminicola and Fusarium verticilloides, become obvious in the deteriorating stalk tissue and therefore the naming of the stalk rots as Diplodia, Gibberella, Fusarium and Anthracnose. Although these are the most frequent and most easily identified fungi found in those plants with rotted stalks, the underlying cause of the plant death involved more complicated biology of carbohydrates to the root tissue as the plant moves sugars to the grain.  If the movement to grain is too great for the supply from the leaves and stored carbs in the stalk tissue, roots suffer without sufficient energy to meet the root metabolism needs.  Eventually deteriorating root tissue succumbs to destruction by soil microbes, resulting with the plant wilting.  This allow many fungal species to advance into the dying and dead stalk tissue, destroying the structural strength of the stalk.
 
Beyond naming the dominant fungus present in the dead stalk, it is important to identify the cause of insufficient supply of carbohydrates.  Potential causes are shading from other plants, leaf disease destroying leaf tissue, insufficient sunlight, leaf removal from corn borer, or hail damage.
 
Stalk rot of corn is a problem in some fields somewhere each year. Complexity of environments and genetics makes conclusions of causes very difficult. Predicting whether this year’s performance is likely to reoccur is not easy.
 
More perspectives on corn stalk rot can be found by using the search on this page in Corn Journal.  It is a subject that I (and others) have studied for a long time.

​That wilted corn plant, often surrounded by green plants with carbohydrates still being transported to the grain, did not have sufficient supply of carbohydrates available to meet the draw to the ear.  This may have been due to reduced photosynthesis because of leaf tissue destruction from leaf disease or hail, shading from adjacent plants, insufficient potassium available or dark cloudy days. 
 
Movement of carbs to the grain is directed by hormones produced in growing points, each embryo having an apical meristem producing auxins to direct the flow.  Genetics affects the amount per day and number of meristems affects the total flow per day.  If daily photosynthesis in leaves is insufficient to meet this demand, reserves stored in the corn stalk are drawn upon.  Sugars that are being stored in grain are also required to maintain life in the root cells.  Depletion of stalk sugar reserves available to roots, eventually weakening those cells ability to resist invasion by soil microbes. 
 
Eventually, root rot reduces water uptake and transport that water availability to transpiring leaves causes desiccation of all plant tissue.  All leaves on this plant show the wilting symptom by a gray appearance and pointing downwards.  Wilting also causes the pith tissue, previously attached to the inner layer of the rind tissue to shrink away from that attachment.  Stalk cells die.  Chloroplasts in the outer rind cells die, resulting in the lower stalk tissue turn from a green color to yellow-green and eventually brown, while adjacent plants continue to have a green color.
 
Abscission layers form at base of all leaf tissues immediately after tissue wilting.  That includes the formation of black layer at base of each kernel.  Consequently, these kernels will not have as complete grain fill as the adjacent green plants that continue to receive the flow for the 55 days after pollination.  If the major cause of the early wilt was from producing more kernels than the adjacent green plants, the total grain weight on the wilted plant may not be much different although the weight per kernel will be less.
 
Cause and effect of wilting corn plants is dynamic with multiple interactions between the corn biology and environments. Assessment of these potential causes when they occur could be useful in preventing significant yield and harvest problems in the future. 

​As the corn plants approach completion of grain fill about 40-50 days after pollination, some individual pants will change in color.  These changes can indicate the effect of the season on the plants.
 
Plants in which all leaves are gray, then brown, pointing towards the ground, have wilted because the roots could not provide sufficient water to meet transpiration needs.  These have root rot caused by lack of sufficient sugars moved from the leaves, allowing soil microbes to invade and destroy the root tissue.  Movement of limited supply of sugar moved to the grain was preferred over movement to roots.  That individual plant did not produce sufficient photosynthesis to maintain both the root life and grain fill.  Such plants will develop hollow stalks and be invaded with fungi.   These plants are likely to lodge by harvest time.
 
Some individual plants may turn red gradually during this time period.  These plants usually have only a few kernels.  Sugars accumulate in the leaves because not enough hormone driven transport is causing the sugars to be moved, or perhaps because of interference of movement because of insect damage to the vascular system. Chemical processes in the cells with accumulating sugars transpose the sugar molecules to anthocyanin, providing the red color in the leaves. This is usually an indication of poor pollination, perhaps because this individual plant emerged late as a seedling and therefore missed much of the pollen from other plants.
 
Nitrogen deficiency is indicated when several plants have yellow lower leaves while upper leaves remain green.  This is common in areas of fields that were water-logged.
 
Desired color of maturing plants indicating a successful yield has white ear husks while other leaves remain green and turgid all the way to completion of grain fill, about 55 days after pollination.  The occasional wilted plant can be a sign that the field got maximum grain allowed by that season’s environment.

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.
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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.