We humans are compelled to name what we see. This includes a guy like me whose PHD dissertation involved fungal taxonomy. And certainly it is important to distinguish among corn leaf diseases in order to identify how to defend against the disease in the future.
The fungi that show up on the stalks rotting late in the season, however, do not indicate the actual cause of the problem. The Diplodia, Colletotricum (anthracnose), Fusarium or Gibberella species that becomes the most obvious on the surface or inside the rotted stalk are quick invaders of dead or dying stalk tissue. Knowing the name may seem satisfying because it implies an aggressive pathogen attacking a helpless plant and thus the grower was not responsible. But, in a field or plot of single cross hybrids, all the plants are genetically identical. Surely these fungi are not only concentrated in a small area of the field where the rot occurred. The analysis should concentrate why those plants rotted and not the name of the fungus.
Late season stalk rot begins as a root rot. That occurs when the root tissue senesces because it no longer receives sufficient carbohydrate to maintain production of defense chemicals to fight off the many bacterial and fungi surrounding the roots. This caused the plant to wilt because of lack of water uptake. Wilting caused death of the stalk tissue and invasion by many fungi, the most evident being the Diplodia, Colletotrichum, Fusarium and Gibberella species. Usually there are other species also working on digesting the stalk tissue but weakness was greatly enhanced by simply the pith tissue wilting and therefore pulling away from the rind wall.
Although it may be satisfying to have a name for the most dominant fungus found on the dead stalk, a better analysis for reducing the problem in the future must include what triggered the root rot in the affected plants. Discussion of some of those factors is the topic of future blogs.
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.
Starting to hear reports of Diplodia ear rot as corn harvest begins. This disease is caused by a fungus that was once named Diplodia maydis or Diplodia zeae but now is considered a single species Stenocarpella maydis. It is believed to only infect corn, overwintering in diseased corn stalks or ears. Infection of the ear occurs through silks or the husks. Once inside the ear, the fungus feeds on the developing kernels, cob and adjacent husk. The result is moldy grain that dries slowly while in the tight husk.
Infection usually occurs shortly after silking and especially if it is rainy during that period. The rain not only stimulates the production of fungal spores but also delays pollination, resulting in longer exposure to fungal infection as the un-pollinated silks are vulnerable spores germinating and growing down the silk channel to the ovules. Infection may be more obvious at the base of the ear, perhaps indicating rain at the beginning of the silking or at the tip of the ear, suggesting rain a few days later. This interaction with rain timing and relative dates of silking makes resistance conclusions very tenable as susceptibility can be confused with bad timing for that season. Also, it is probable that late-season ear uprightness is not a factor influencing susceptibility to this disease.
Diplodia ear rot is associated with continuous corn culture, allowing the fungus to build in intensity over years. Grain quality reduction is obvious in heavily infected fields.
Corn plants that did not produce more grain than could be supported by sugars stored in the stalk pith tissue and post-pollination photosynthesis, finish the season with slowly senescing leaves and green outer rind tissue. Enough roots of these plants remain alive to ward off potential invading fungi until the temperatures in temperate zones become low enough to slow growth of these microbes as well.
After black layer forms in the grain, there is little competition for carbohydrates among remaining living, but ageing, cells in the corn plant. Almost all plants that make it 60 days after pollination with green stalks will be standing until harvest. Pith tissue in these stalks will remain attached to the outer rind cells with the structural strength of a rod.
Plants that committed to more grain fill than photosynthesis could fulfill will have brown lower stalk color. This began with a wilt symptom because the roots, deprived of enough carbohydrate to support life in their cells were invaded by microbes. Eventually this reduced water uptake to a point that the plant was losing more water through stomates in leaves than could be met. These plants showed the wilt by all leaves turning gray and then brown. The stalk color turned from green to yellow and finally brown as well. Inside the stalk, the pith tissue collapsed away from the rind, changing the structure dynamics from a rod to a tube. This essentially reduced the strength in these plants by one third. One can judge the vulnerability to lodging percentage in a field by looking at the lower stalk color in a field or in variety plots a few weeks after black layer. Those with brown stalks will be easily broken with a nudge from the hand as well.
Corn is an annual plant, programed to senesce at the end of the season. Humans have selected for the timing in those hybrids that become commercial to have the senescence delayed as long as possible to obtain maximum grain fill, mostly by delaying time to flowering to maximize photosynthesis. Excess sugars are stored in the pith parenchyma cells, until moved to the developing grain. As these cells are depleted of sugars, rate of senescence increases and they become more vulnerable to invasion by those fungi, such as Fusarium species, that produce enzymes capable of digesting the cell walls.
Initial depletion of sugars is from the top and bottom internodes of the stem. As the top internode pith parenchyma cells senesce, the tissue is invaded, possible beginning at the internode. A common invader, especially of the rind epidermis, is Colletotrichum graminarium, the fungus that causes anthracnose. The result is small black streaks in the epidermis. The pith tissue, may be invaded by Fusarium species. The outward sign is the turning of the upper leaf color from green to gray or white.
For the most part, this turning of color of the upper leaf is a good sign because it indicates that the grain was pulling available carbohydrate. Plants that have white-colored, senescent upper leaves, drying and open husks and green lower leaves are expressions of maximum yield for that hybrid for that season’s environment. Those plants in which all the leaves are brown, and the lower stalk color is brown, indicate the grain fill demand was too great for the photosynthesis products available, with the roots depleted of sugars and consequently dead before completion of grainfill. The plant wilted because of lack of water uptake through the dead and rotted root tissue.
Corn stalk structure is completed by the time of pollination. Leaves are attached at each node, vascular system established to move water and nutrients from the roots to leaves and stalk tissue and to move photosynthesis products, outer cells forming a strong rind tissue to keep the plant upright and pith tissue cells provide a temporary carbohydrate storage tissue.
The most outer epidermis cells of the stalk are green with chloroplasts, although the surrounding leaf sheaths probably limit light penetration. Several rows of cells in from that epidermis form thick walls composed of complex cellulose compounds providing a hard surface to the stalk, especially between the nodes. These cells being difficult to penetrate by pathogens plus the chemical resistance in the living outer epidermal cells inhibit most pathogens from entering the intermodal stalk. The nodal plate, however, is both more vulnerable to fungal penetration and breakage at midseason.
Pith tissue is composed of many, scattered vascular strands composed surrounded by thick-walled cells. Phloem cells within the vascular bundles are living and require carbs to provide energy but the xylem cells are mostly empty tubes. Remainder of pith tissue is composed of relatively large living cells that function to temporarily store carbohydrates that continue to be produced after all leaves are formed. This reservoir becomes especially important as the grain begins to form after pollination and especially when the grain fill rate of about 2% per day occurs 10 days after pollination. This storage allows the plant to provide sugar to the grain even when photosynthesis is reduced on cloudy days or leaf destruction by disease or insects.
Pith tissue also contributes to the strength of the stalk because it is attached to the rind tissue. This essentially makes the stalk a rod and not a tube. This becomes an important aspect of stalk lodging later in the season. After pollination, the corn stalk functions to support the leaves and developing ear as well as a carbohydrate storage unit allowing a steady daily flow to the developing grain.
Grain fill after pollination was at a rate of about 1% per day for the first 10 days, then about 2% per day for the next 40 days and about 1% per day for the last 10 days. As the seed embryo matures, the ratio of abscisic acid to other hormones tips towards the abscisic acid. This causes the cells at the base of the kernel to form hard walls eventually resulting in the abscission layer (black layer), cutting off the movement of carbohydrates and nutrients into the grain and therefore the displacement of moisture from the grain.
Lack of attraction of hormones to the ear causes similar balances with abscisic acid in the husk leaves resulting in abscission layers to form at their bases, causing the husks to lose the supply of water and nutrients from the roots. Genetics and environments influence these processes.
If the plant wilts early, because of root death, the abscission layer forms almost immediately, blocking the flow of carbs to the grain, resulting in light grain weight.
Water loss from the grain, after black layer is due to evaporation. Looseness and size of the husk leaves, composition of the kernel and moisture retention in the cob are the genetic factors. There are some observations that husk leaves on plants that die early do not separate from the ear at the normal rate expected for the hybrid, thus slowing down the grain moisture loss. When all goes well, the plants remain green at black layer, the husks turn white and loose as the moisture level in the grain drops rapidly from the 35% at black layer to an economically feasible level for harvesting.
Remaining leaves on the maturing corn plants slowly form abscission layers at their bases as photosynthesis rates slow. Color changes to a yellow and eventually brown. The outer rind of the stalk, however, remains green and the pith tissue solid until freeze if the plant made it too black layer without wilting.
There have been several news releases concerning the occurrence of bacterial leaf streak on corn in the US Midwest. Although it was noted in Nebraska in 2015, it was reported in 5 states in 2016. In no cases has damage to yield been projected but there has been enough notice, I think, to everyone involved in the crop to be aware of the disease. This is a proper response when a new disease distribution is identified.
The photographs of symptoms show an early appearance similar to gray leaf spot (rectangular lesions between veins about ½ inch long) but later becomes as long as 2 inches, unlike gray leaf spot lesions. Many corn diseases have distinctive symptoms that can make diagnosis easy by comparing with photos but this one, at least with early symptoms, is not easy. It is best to have a public or private plant pathologist culture it to confirm if the pathogen is a fungus like Cercospora zeae-maydis, cause of gray leaf spot, or a bacterial such as Xanthomonas vasicola pv. Vasulorum, cause of bacterial leaf streak.
This has been a humid summer in northern Illinois that is conducive to the gray leaf spot fungus and likely to cause confusion with the new disease. It would be an advantage, however, to get accurate appraisal of the occurrence of the bacterial disease however so seed companies can access the significance of the disease and differences among hybrids for susceptibility. It would be beneficial to know if this is just a mostly harmless disease that is easily controlled or if it is more serious. This is a good time to look at fields while leaves are still green and to be aware of any leaf diseases. Public and private corn pathologists meet annually to discuss new information concerning corn diseases. This will be a topic for the meetings in December 2016.
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.