Multiple stresses during the 2020 corn growing season likely produced environments encouraging ear smut. The biology of the fungus and the corn plant are closely related to the occurrence of corn ear smut.
Ustilago maydis is the fungal pathogen of corn causing common smut disease. Its biology includes a fungal version of sex, a portion of which involves corn. The fungus produces thick walled spores called teliospores that overwinter in the soil for many years. Teliospores have diploid nuclei (2 sets of chromosomes). When moistened, the teliospore nuclei undergo meiosis, resulting in 4 individual cells, each with haploid (monoploid) with one set of chromosomes. These cells are called sporidia. Sporidia that land on corn tissue may germinate but cannot enter the corn plant until they are united with sporidia of a different mating type. After the two mating types fuse, the two nuclei stay separate but the fungus forms special structures to enter the corn plant cells. Often it is through wounds but also directly through corn cells as they are elongating, as the combination of host cells and fungus form galls. Within this mass the fungal cells, having two monoploid nuclei per cell, now fuse the nuclei, forming the diploid single nucleus stage of the fungus life cycle. These diploid cells form the thick cell walls as they become teliospores to be released as the corn plants are harvested.
Smut galls may form on leaves, tassels, aborted ear shoots at the leaf nodes and the main ear shoots. The ear shoots are the most damaging to the grain yield, of course.
Corn silks that are not pollinated soon after emergence are vulnerable to infection by Ustilago maydis. The fungus grows down the silk channel infecting the cells surrounding the ovule. If pollen successfully reaches the ovule before the fungus, the smut fungus is inhibited and no gall will form. Subsequently, the timing of pollen release and silk emergence becomes a significant factor in formation of galls on corn ears. Rainy days may encourage silk emergence, but delay pollen release or drought may delay silk emergence until there is limited pollen available. Complete absence of kernels but ears full of smut are a sign that the ear failed to receive pollen. These factors contribute also to the occurrence of smut at the tips of ears as these are from the last silks to emerge, perhaps when no pollen was available.
Another Fusarium species, Fusarium graminearum, forms a sexual stage of the fungus when its mating types combine. That sexual stage is identified by the name of Gibberella zeae. This fungus is associated with Gibberella stalk rot and wheat scab. The fungus is common in corn debris, producing huge numbers of Fusarium spores during much of the corn-growing season. Spores germinate on the silks, with the fungal filaments growing down the silk channel towards the ovule. Generally, if pollen tube growth reaches the ovule first, the following collapsing, dry silk tissue effectively stops the fungus. Silks are most vulnerable during cool, wet weather as pollen spread is poor but fungal spore production is high. This results with a prolonged period of silk exposure. Later infection of the kernels appears to be related to kernels physically injured by hail or insects. Husk leaves tightly wrapped around the ear also appears to be related to spread of this mold within the ear.
Early infected kernels will fail to develop completely, will be light in weight and often will not germinate if planted as seed. Later infection can spread to cover much of the ear with a mold as the fungus spreads from the initial infection area. This mold produces mycotoxins including deoxynivalenol (DON). This toxin is associated with severe health problems in swine. The fungal spores produced among the grain also can be detected by swine and cows, causing them to reject the grain. I recall a personal situation in which I was a sent a corn sample from seed dealer who claimed that newly harvested grain was rejected by pigs and calves. Observation of the grain in my lab showed lots of Fusarium spores- so much that I asked to see the storage bin where the grain was stored. A single hand full of the grain from the bin revealed a cloud of spores. The grain was dried in the bin by air being blown from the bottom up through the bin. This air, however was moved over an accumulation of previous year’s debris below the grain. A sampling of that debris revealed that it was heavily infected with Fusarium. The new crop was being inoculated with the fungus as it was being dried.
DON has been shown to accumulate in grain stored at moisture higher than 20.5%. One should assume that the fungus is ubiquitous and that monitoring the grain drying and storage is important to avoiding this toxin problem. (Corn Journal 03/01/2018)
Nutrition and moisture in corn silks allow the fast movement of the pollen tube towards the ovule and contribution of the male genetics to the next generation. Those same favorable silk characteristics also can be used by invading fungi. Rapid deterioration of the silk tissue after pollen tube growth offers protection within a few days after pollination, but environments and genetics can have a drastic effect on the time of silk vulnerability and the biology of potential invaders. Aspergillus flavus gets much attention because of its dangerous toxin produced on infected corn. Fusarium verticilloidesis, another common invader of corn kernels through silk infection that can produce a mycotoxin (i.e. fumonisin). Others such as Diplodia maydis and Gibberella zeae also can utilize the silks and initial entry into the ear.
These fungi are mostly saprophytic feeders on plant debris and intensity of their spore production is greatly dependent of corn debris from the previous season near the new crop plants. Their biology also is influenced by the environment affecting competition with other saprophytes feeding on debris and production of spores when the silks are exposed.
Duration of silk vulnerability is also associated with environment. Cool, moist weather a few weeks before normal pollination may cause silks to be exposed before pollen is produced- and may favor Diplodia(Stenocarpella) maydis. Extended dry, warm periods during the pre-pollination time, may cause pollen production before silk elongation and exposure but favor Aspergillus flavussporulation and distribution by the time the un-pollinated silks do emerge. Fusarium species (including Gibberella zeae) produce massive numbers of spores under most environments.
Plant pathologist have shown that one can induce ear infection by directly spraying the silks with the spores of each of these pathogens. These studies have shown evidence of resistance variance among genotypes but usually only on a scale and not of absolute absence of disease. Evaluation for resistance from natural infection is not easy. One can record occurrence of infection within plots, but each genotype may not be exposed to the same environments, including time of silk exposure. One does need to use care before drawing conclusions about ear rot susceptibility based upon single location observations.
Ear rots are prime examples of the complex biology of host and pathogens interacting with environments. Ear rot may not be noticed until harvest, but the problem involved the dynamics occurring at pollination time of the season.
This Corn Journal blog written in 2016 pretty much applies to USA corn in 2020.
There have been numerous studies comparing inbreds and hybrids for the fiber strength of stalks. Perhaps these results are somewhat helpful to predicting standing corn at the end of a season, but I suspect that it ignores the biggest factors affecting standability at harvest time. When the stalk collapses in the lower internodes preharvest, the pith tissue has pulled away from the rind of the stalk. This reduced the structural strength by 1/3 as what was once a rod now becomes a tube. This happened because the root died earlier, reducing water uptake followed by wilting of the plant. This dessication of pith cells caused withdrawal from the rind. The dead cells, now having limited resistance to fungi readily invading the tissue and digesting the cell walls, further weakening the strength of the stalk.
The time between black layer and harvest level grain moisture is the best time to evaluate stalk quality. A simple push test of several plants in many areas of a field can give one a good idea the crop’s vulnerability to lodging. Basically, those that are strong, soon after black layer, will not decrease in strength during the rest of the season. One should never forget, however, the stalk rot of 2016 in one field is not necessarily an indication of the behavior of the same genetics in the same field next year. Evaluations across several environments is critical to predictions of yield and stalk quality.
The individual plant that wilted and prematurely died because of root rot but is surrounded by living corn plants does not necessarily have drier grain at harvest time. Grain moisture replacement by starch formation in grain stops when the abscision layer (black layer) forms at the base of the kernel. In most corn plants this happens between 55-60 days after pollination at about 30% moisture in grain. Those prematurely dead plants that wilted earlier form abscision layers about a day after the wilting, at a higher moisture percentage.
Grain drying in the field after this time is an evaporation process. Moisture must travel through the pericarp of the kernel at a rate determined by the relative humidity surrounding the grain. Pericarp thickness must be a factor but also plant structures such as cob volume and husk leaves length, thickness and adherence to the grain are major factors.
‘Water runs down hill’ as Professor Loomis in my plant physiology class of 1960 would say to emphasize that it goes from a high concentration to a low one. Grain evaporation rate is very much dependent on relative humidity immediately surrounding the kernels. Transpiration from the senescing, but green, leaves in plants for a time after kernel black layer contribute to higher humidity in the field, including the area surrounding that single dead plant. Eventually, senescence of these plants halts transpiration, leaving the ear structures as the only barrier to response to atmospheric factors such as relative humidity and wind affecting the dry-down of the grain. The individual plant that wilted and prematurely died because of root rot but is surrounded by living corn plants does not necessarily have drier grain at harvest time. (Corn Journal, 9/22,2016)
As harvest season approaches, standability in the corn field becomes a primary interest to the grower. Appearance of the lower stalk is an indication of the plant’s vulnerability to lodging from stalk rot. Those stalks that remain green 60 days after pollination are likely to have solid interiors of the stalk and will not break before harvest. These plants did not have premature wilting and thus the cells within the stalk did not withdraw from the rind. One can judge this by observation alone in plots. Plants with deteriorating stalks show discoloration beginning with a slight pale yellow, and then a darkening yellow that eventually turns to brown.
Various fungi invade the dead interior tissue of such a corn stalk. Multiple species can be present in such stalks although a few get the most attention because of their distinct features on the stalk. Colletotrichum graminicola, the fungus associated with anthracnose is distinguished by narrow black streaks on outside of rind but is also present inside the infected stalk.
Gibberella zeae, sexual stage of the fungus Fusarium graminearum, shows distinct black ‘perithecia’ on the outside of the rind near the stalk nodes. Ease of rubbing these off the stalk is diagnostic of Gibberella stalk rot. Inside such a stalk usually has a pink discoloration caused by the asexual stage of this fungus that is known as Fusarium graminearum.
Another common fungus found in rotted stalks is Diplodia maydis, also known as
Stenocarpella maydis. This fungus frequently is found on the outside of the stalk nodes as a black sticky structures known as pycnidia. The fungus in only known to be present on corn and overwinters in the soil.
Nearly all corn plants are exposed to another Fusarium species, Fusarium verticillioides (also known as Fusarium moniliforme. Some think it exists in corn plants mostly as a nearly saprophytic occupant rather than an aggressive pathogen, but it is easily found in nearly all rotted stalks.
There are several other fungi that have been isolated from deteriorated stalks, but we like to name a stalk rot as caused by the most easily identified. Ultimately, however, the actual cause is the physiology of that plant. That plant had insufficient carbohydrate to complete the fill of grain and maintain root life. The plant wilted, cells in stalk die, withdraw from rind (reducing strength by 1/3) and lose resistance to invading fungi. It is most important to analyze the cause of this lack of the photosynthetic stress and translocation balance in these rotted plants
Summer of 2020 in Midwest USA corn-growing areas has been unusually stressful. Early season rain caused delayed planting in many fields, extreme winds in Iowa resulted lodging in healthy plants, absence of rain in many areas in northern Illinois resulted in parts of fields to have plants with yellow leaves long before normal maturity. This is not the usual precursor of stalk rot.
Nitrogen is a major component of chlorophyl and thus contributes to the green color of leaves. This mineral, along with others, is absorbed through the roots along with water and transported to the leaves through the vascular system. Lack of water in soil makes the nitrogen less available in the soil and thus less available for this transportation. The result is the plant leaves slowly turns from dark green to yellow. This also affects the physiology of the developing kernels resulting in abnormally incomplete transport of carbohydrates. Symptoms of late season drought in multiple plants in areas of a field have all plants with completely yellow leaves.
Premature plant death leading to stalk rot, on the other hand, usually occurs with individual plants turning gray, not yellow. These plants wilted from inability to withdraw sufficient water from soil to match the water loss from translocation. The lack of water for leaves was caused by roots dying because of insufficient carbohydrate supply. Contributing to the lack of carbohydrates was the competition with the grain. Individual plants with excessive movement to grain, depleted the stored carbohydrates of the stalk and thus the source of energy for life in roots. Insufficient root life in root hairs resulted in reduction of water absorption and thus wilting of that individual plant. These plants frequently show a slight gray appearance a few days before complete wilting and then all leaves become gray. Loss of turgor results in all leaves to turn downwards. The ear likewise turns down. Adjacent plants may remain green with upright ears until completion of grain fill, normally about 60 days after pollination.
Stalks of the plant death caused by drought and those from wilting also differ. Yellow plants may have sufficient carbohydrate and water in stalk tissue to maintain strength whereas the wilting plant has collapse of pith cells weakening its strength. Both conditions are not good and may ultimately cause lodging but at least it is probably that the additional kernels on the wilted plant can make up for the light kernel weight whereas the yellow plants probably have fewer kernels.
As a corn plant moves carbohydrates to the developing grain from the leaves and storage pith cells of the stalk, the rate of flow may vary from adjacent corn plants. This rate is determined by genetics and nutrients affecting the flow per kernel and the number of kernels forming on each individual plant. Environment of each individual plant in field can have sufficient differences that affect number of kernels and amount of photosynthesis in the plant.
Those individual corn plants that seem to suddenly turn gray during grain fill have a permanent wilt. It is not unusual for it to seem sudden, because the plant looked as green as others in the field just a few days previous. However, closer observation of these individual plants, reveal a few early signs of wilting. The upright ear starts to point downward, the leaves get a sort of faded green color that can be noticed a couple days before all of that plant’s leaves turn gray. This symptom is not just the top leaves – it is all leaves on the plant. The water transportation from the root has stopped. Probably the continual chain of water molecules in the xylem tissue has been broken, ruining the capillary action needed for supplying the rest of the plant for water needs.
Wilting causes cell functions to stop; no more photosynthesis, no more movement of sugars and minerals, and no more movement of sugars into the grain. In fact, the kernel forms an abscission layer at the base of each kernel soon after permanent wilt, cutting off all movement of sugars into the grain- or water from the grain. Loss of potential carbohydrate storage in the grain of a wilted plant is determined by the number of days the filling period was cut short. Grain fill between about 10 days after pollination and day 50 is about 3% per day, between day 50 and day 60 it is about 1% per day.
The contradiction can be that having more kernels on the plant, ultimately caused the roots to die early, resulting in a wilt that cuts off the flow of carbohydrates to kernels. So, there are more kernels than on adjacent plants but less carbohydrate per kernel because of the wilt.
And that is just the beginning of the problem for the grower who needs to harvest the corn.
Pursuit to understand why and how corn stalk rot develops was certainly undertaken by many researchers before my attempts. There were research publications done before corn hybrids were commercialized, when farmers mostly saved seed from the variety in the field. But these plants were not genetically uniform. Superior yields of hybrids and eventually the realization that highest yields came from single cross hybrids in the mid to late 1960’s, it became clear that even when each plant in the field was genetically identical, stalk rot could not be explained by genetics alone. Pappelis at Southern Illinois University showed that the pith cells senesced after pollination and at rates faster in genotypes that tended to get more stalk rot. Mortimore’s studies in Canada indicated that stalk rot is always preceded by root rot. Ullstrup, at Purdue, did studies showing that root tissues start senescing soon after pollination. Foley, at Iowa State, published that when the pith tissue pulls away from the stalk rind as when a plant wilts, the structural strength is reduced by 1/3 by simply changing from a rod to a tube. Other studies showed that one could increase occurrence of stalk rot by artificially shading plants after pollination. Corn borers were associated with stalk rot occurrence. Leaf diseases like Northern Leaf Blight were seen to increase stalk rot. Corn breeders were (and still are) frustrated that some of the highest yielding hybrids tend to be the most vulnerable to stalk rot. Although pathologists usually identified a few fungi, such as Fusarium moniliforme, Gibberella zeae, Diplodia maydis, and Colletotrichum graminicola as frequently found in the dead stalks, actually many other fungi were also present. But if root destruction occurred first, were these only quick invaders of tissue that was on its way to death? And then there was the observation made by many that one often could see, in a corn row along the edge of a field, that the plant that wilted first had two ears, instead of the one ear on most of the other live ones in the same field. Why that plant?
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.