Our exposure to diversity, in environments, plants, pathogens and in our species shows that diversity is basically good even if it causes some temporary stress. And we have the ability to adjust to changes and move on. This blog from Corn Journal on 7/5/2016 illustrates how it works in corn.
One study of a single corn inbred (B73) indicated that it had 30,000 genes. We benefit from a species with a huge genetic potential and a pollination system that encourages new mixes of genetics. This has allowed the species to be used for food in a wide range of environments. Some of those genes are turned on in response to the many microbes searching for the products of photosynthesis for their nutrition. Microbes have genetics too! Some, like rust and smut fungi, survive by attacking living corn cells, drawing carbohydrates to the cell, and then moving on via spores before the host cells respond to the fungus. Many fungal pathogens of corn simply kill a limited area of the leaf tissue, feed on the dead tissue, produce spores and infect new areas. Corn varieties differ in how quickly and strongly they respond to the invasions.
One fungus that I find interesting is Bipolaris zeicola. It was formally known as Helminthosporium carbonum. There are genetic variants of this species that apparently feeds only on dead leaf tissue, often caused by insect damage or simply physical injury. These variants apparently lack the genetics for either penetrating the live corn leaf tissue or overcoming the resistance system of most corn varieties. At least one variant of this fungus produces a toxin that kills corn plant cells but most corn varieties have dominant gene that effectively blocks this toxin. However, very occasionally, a mutation of that dominant corn gene does occur while developing new inbreds. If this mutation, now a recessive gene, becomes homozygous during the inbreeding process, the inbred is vulnerable to the toxin. The result is practically no defense to this variant (race 1) of B. zeicola. The pathogen kills small leaf area of leaf, produces spores and spreads to new leaf tissue and eventually causes the whole corn plant to die early. Because susceptibility is recessive, and the dominant toxin-resistant gene is present in most corn inbreds, this creates a problem for seed producers but not for hybrid growers. Good that corn has genetic diversity.
2019 diverse US corn belt environments was a strong reminder of all the significance of diversity among corn hybrids. This principle applies to all areas of earth in which corn is planted. Following is from Corn Journal on 11/13/2018 attempting to discuss corn diversity.
Corn’s history and biology has resulted in diversity beyond what most of us see in any single season. Advantages of hybrid plant uniformity for yield, harvestability, disease and pest resistance and genetic repeatability requires development of homozygous inbred parents. Each of many seed companies produce multiple hybrids each year and there are about 40,000 genes in each corn plant that are available to influence something, whether needed or not.
Corn researchers in 1920’s became aware of a need to collect and share many of the genetic sources in corn, forming a Maize Genetics Cooperation Stock Center- it’s history is summarized at http://maizecoop.cropsci.uiuc.edu/mgc-info.php. This collection started and continues to emphasize mutants affecting some identified trait, such as those involved in sweet corn, waxy corn or amylose corn and many that may not have a specific economic advantage but are useful in understanding some biochemical pathway in the corn metabolism. Study of these mutants contributed to location of genes on each chromosome and add to growing knowledge of corn DNA codes for many traits.
Despite these efforts, corn’s genetic diversity is large due to selection by humans over diverse environments. Our experiences with ‘new’ diseases as a pathogen such as the bacterium causing Goss’ wilt suddenly appears, with a previously unknown susceptibility gene in corn became part of popular corn hybrids, or susceptibility of race T of southern corn leaf blight associated with mitochondrial gene in t-cytoplasm male sterile corn. Resistance to Maize Chlorotic Mottle Virus was found in corn genetics in USA after it occurred in Kansas in the 1970’s and in Africa in 2016.
Often the strong resistance to these diseases are associated with single genes already present in corn apparently without intended human selection and without known selection pressure in absence of the disease. Perhaps there was exposure somewhere in its history where the gene was favored but also it is likely that randomness of mutations, segregation of genes during miosis, cross pollination and historic diversity of corn’s environments have provided many genes for characters that we have yet to identify. These genes must be influencing multiple internal aspects of absorption of light wavelengths, translocation of carbohydrates, absorption and movement of minerals, water uptake and conservation, and structures of leaves. Among this diversity is the future adaptation needed for changing environments.
Breeders witness diversity within their nursery as they see differences in plant structures and growers see differences among hybrids in performance each year. At Professional Seed Research Inc., we see differences among hybrids in structures of seedlings (Seedling Growout®). Genetic diversity will continue to be an important contributor to this crop as it interacts with changing environments.
Breakage of stalks in the 2nd or 3rd internode above the soil is a big concern during harvest. Multiple studies have been done attempting to sort out the dynamics of physical strength of the stalk and fungi associated with lodged stalks. Multiple fungi capable of digesting the cellulose and lignin of corn cells surround the corn plants in the field. Most of these are warded off by the anti-microbe metabolites of living corn cells. Fungi that successfully attack dying or dead cells, producing recognizable fungal structures such as Diplodia (Stenocarpella) maydis, Gibberella zeae, Colletotrichum graminicola and Fusarium sp. as well as several others that are found in the deteriorated stalk.
Methods of Evaluating Stalk Quality in Corn, published in 1970, (https://www.apsnet.org/publications/phytopathology/backissues/Documents/1970Articles/Phyto60n02_295.PDF ) is a summary of the dynamics crushing strength of lower stalk pith and rind versus intensity of Diplodia maydis. Both pith integrity and rind thickness are significant contributors to the crushing strength. Their study and others point out that the Diplodia fungus grows only in dead pith tissue, and, therefore, correlation of this fungus with weakened stalks is mostly related to death of pith tissue.
When individual plants wilt, usually because of root rot, the pith cells dehydrate, pull away from the rind and lose production of the metabolites needed to restrict growth of the fungi of the stalk. This results in weakening the strength of the stalk by changing the dynamics of pith attaching to the rind plus allowing the growth of fungi that can break down the rind cells.
It is interesting to observe (from the roadside) multiple fields in our area with very little stalk lodging. Plants are obviously dead from maturity and low temperatures. In general, lower stalks that make it to black layer without wilting, maintain strength for a long time. Dynamics involving environment, genetics of response to environment and vulnerability to root rot are significant in corn stalk lodging.
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 that 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 life 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.
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.