Fungi identified as a species of the genus Fusarium are found in many plant species and even humans. Fusarium species are defined by their characteristic macroconidia shape. After mating types are identified and resulting sexual stages are seen, most of the Fusarium species are renamed as a Gibberella species. Fusarium verticillioides (previously known as Fusarium moniliforme) is a cause of an ear mold. The sexual stage of this fungus is Gibberella fujikuroi.
Gibberella zeae, one of the fungi associated with stalk rot produces small black structures (perithecia) on the surface of the stalk near an internode of a rotting stalk. These contain the products of meiosis (ascospores) produced after two mating types of the fungus also known as Fusarium graminearum. Although this dual naming system for the same organism is confusing to us, the fungi seem to know what they are doing! There are some Fusarium species named because of distinctive characteristics that specialists have yet to associate with a sexual stage.
This naming confusion is only part of the difficulty of defining the significance of this group of fungi. They often appear to be ‘opportunistic’ pathogenic. The fungus is common in dead plant refuge in most soils, obtaining nutrition from the decaying organic matter. This propensity to feed on dead, or dying, plant cells plus production of an abundance of spores results in it being found on and in corn leaves, seed, roots and stalks, often without clarity of whether it is the prime cause of the plant condition or only an opportunistic occupant taking advantage of weakened tissue that is unable to fend off this invader.
The frequent isolation of a Fusarium species from corn plant tissue causes confusion as to its importance in actually causing the corn disease problem. It is not unusual to see Fusarium growing from what appears to be normal germinating corn seed in germination tests. The fungus is frequently found in leaf tissue killed by an herbicide or insect feeding. Seedlings struggling for growth under stress conditions frequently have Fusarium present and therefore the condition is called Fusarium seedling disease. Fusarium is a common occupant of rotting roots and stalks and often gets implied as the cause of the condition.
It is not unreasonable to expect a range of pathogenic capabilities among the isolates of a Fusarium species, making some association of the species with disease condition very appropriate. Like much of the real world, the actual interactions among the host, environment and pathogen are not easily characterized.
Rating corn leaf reactions to a pathogen carries some ambiguity because of complications of the timing of infection, intensity of the pathogen and relative maturity of the plants. An exception can come with single gene resistance such as the Ht1 gene. In that case, the single gene inhibited the fungus from developing normal lesions and, more importantly, stopped the spore production and consequent spread of the pathogen within the field. The problem, as with most single gene resistance systems, is that within 10 years of widespread use of the gene, a race of the pathogen could overcome the resistance mechanism, returning the effective resistance to the multi-genetic system and its rating system.
We are interested if a leaf disease is likely to cause a performance problem for a hybrid. This involves the tolerance concept- how likely will an intense infection by the pathogen cause a reduction in grain yield or harvest difficulty because of stalk lodging. Tolerance is real but even more difficult to evaluate and express. We participated in an experiment testing the hypothesis that yield loss from heavy gray leaf blight infection occurred only if the plant died early, such as with the sudden wilt associated with root rot and stalk rot. A known susceptible hybrid was inoculated with the fungus and compared to plants of the same hybrid not inoculated. Individual plants that died before normal black layer were noted for date of death. Ears were hand harvested, dried, kernels counted and weighed. Heavily infected plants that died early had lighter kernels and less total weight than those that equal leaf damage but stayed green until black layer. Yield loss appeared to only occur if the plant died early. This hybrid would be classified as very susceptible based upon leaf damage from the disease but at least somewhat tolerant if based upon yield loss or stalk rot. If the season had been one that encouraged higher kernel counts causing more translocation to the ear, the reduction in photosynthesis from the leaf damage could have resulted in more yield loss compared to the potential for that season.
It is probable that a hybrid that tends to maximize the translocation of available carbohydrate to the grain is likely to be classified as less tolerant to a leaf disease than would be indicated by the lesion number or leaf damage resistance rating. It is best to know the hybrid’s tendency to stay alive until completion of grain fill under many conditions (tolerant to late season stress) and its disease resistance ratings as separate concerns.
Rating the tendency of a corn hybrid to get stalk rot is complicated because of the biology of the corn plant interacting with the environment. Describing resistance to corn leaf disease may be a little simpler but it still is with complexities. Fungi causing diseases such as northern leaf blight, southern leaf blight, gray leaf spot and eyespot exist in dead leaf debris between corn seasons. Moisture and temperatures favoring early corn growth also stimulate these fungi in exposed leaf debris to produce spores that are carried by wind to the young corn plants. Most spores require a few hours of moisture to germinate and grow special hyphae for penetration of the corn leaves.
Resistance to the pathogen begins with the recognition of the pathogen, often associated with a protein component of the fungal cell wall. Corn genetics are involved in efficiency of that process. After getting established in the leaf, the pathogen attempts to grow to surrounding cells. Host genes are turned on to produce anti-fungal products to limit the pathogen. Hybrids vary in the efficiency of limiting the number of lesions that develop from initial invasion by the pathogen.
We attempt to characterize the differences among hybrids for their successful limit of damage by each leaf disease. Researchers have used number of lesions, leaf area damaged, number of dead leaves and size of lesions in efforts to quantify the resistance to a leaf disease. Every method requires some control of the pathogen such as virulence of the pathogen isolate, number of spores as well as environmental influences such as moisture. None of these methods are perfect in terms of determining the expectation of the hybrid performance in many environmental interactions that a hybrid may actually face during its commercial life. Disease pressures vary each season in timing and intensity. Fungal isolates vary in their genetics affecting their spore production and virulence. Leaf resistance also is influenced by the biological state of the leaf, often reducing with senescence beginning a few weeks after pollination as sugars are moved to the ear.
We at PSR inoculate plants with spores in a water suspension into the whorl of corn plants at the V8 stage. This attempt to give each corn genotype about the same intensity of pathogen initial pressure. Hybrids vary in the number of lesions successfully established in these leaves a few weeks after inoculation. Further spread of the disease will be influenced by host genetics and environment. Resistance ratings determined reaction to this primary infection can be a fair indication of the relative resistance level expressed on more mature corn plants, at least according to a study with northern leaf blight that I did many years ago (unpublished). It is more assuring, however, if the season favors spread of the disease to more leaves. Resistance ratings always carry some influence of relativity to resistance expression by other corn varieties and to disease pressure.
We prefer that things are simple and easily characterized but there are always qualifiers.
Resistance to stalk rot fungi involves so much of the corn plant’s biology and the environment that it does not become an easy trait to express in hybrid descriptions. On the other hand, there are differences in the tendencies to develop stalk rot when the plants are under certain environments. Hybrids differ in reaction to favorable pre-pollination conditions, some committing to greater movement of carbohydrates to the grain at the detriment of carbohydrate availability to the roots. Reactions to late season photosynthetic stress also varies among hybrids.
The gradations of these variables and hybrid reactions do not allow absolute stalk rot resistance ratings possible. Expression of stalk rot rating is much like expression of corn yield- absolute values are not appropriate but are only meaningful in relation to other hybrids or acceptable performance. It is in this regard that evaluation of stalk rot vulnerability of experimental hybrids by plant breeders needs to be done in hybrid yield tests. Stalk rot vulnerability is a hybrid phenomenon that may be influenced by the inbred parents but it is mostly the product of heterosis, with the combination of the parent genetics affecting the probability of stalk rot problems. Consequently, it is evaluation of the hybrid that is critical. Also, just as with yield testing, commercial seed breeders are interested in predicting the stalk rot vulnerability in the field where the hybrid will be used.
Plot yields can be taken with accuracy but evaluation of stalk rot requires human observation. Counting lodged plants is relatively easily done from the harvest combine but this method does not consider the rotted plants still standing but ready to lodge with the next strong wind. An alternative is to rate the stalk condition by walking each plot, giving it an acceptability rating. If all plants are strong and with green lower stalk color, the plot is scored as excellent. If too many plants are weak and ready to lodge, making it unacceptable, then it can be scored as extremely unacceptable. Of course, some plots would be scored as intermediate. This method should be applied at all plot locations. The final summary can be expressed as the percentage of plots or location in which a hybrid had acceptable levels of stalk rot. This should allow an estimation of the frequency of stalk rot problems expected for each hybrid. Test plot locations will not represent all the environments that a commercial hybrid will need to balance yield, stresses and stalk rot but growers will evaluate annually in their field conditions.
Stalk rot resistance ratings should not be considered as absolute resistance by hybrids against a specific late season fungus but more of the balance of photosynthetic stress and translocation of carbohydrates under most field conditions.
Resistance to fungi causing deterioration of corn stalks late in the season is complicated. Fusarium, Gibberella and Diplodia species usually identified with rotten stalks were on and nearly all the plants in the field all season, successfully stopped by the corn plants from destroying the plants. Even if artificially inoculated into the stalks by puncturing the lower stalk and inserted into the stalk pith tissue, these fungi rarely can kill the plant in any manner suggesting simple resistance genetics. Living vigorous cells in the pith react to the presence of these fungi by limiting their growth within the tissue.
Corn varieties do differ in the tendencies to get these late-season stalk rots. Although every plant in a field is exposed to the same fungi stalk rots show in isolated plants, in some areas of the field in some years despite all plants within a single-cross hybrid having the same genetics. This distribution pattern strongly supports the interaction of environmental factors with the occurrence of stalk rot.
Plants that develop stalk rot also have rotting roots, causing the plants to wilt. Roots rotted because they received insufficient photosynthates to maintain defense against the many soil organisms capable of digesting root cell contents. Resulting plant wilting caused senescence and death of the cells in the stalk pith. Consequently, the many fungi surrounding the stalk and in the soil moved through the stalk, destroying the cellular strength.
Stalk rot resistance genetics involves those affecting photosynthesis. This must include structures and physiology basic to production of carbohydrates- leaf size and shape, mineral uptake and utilization, stomata function and number.
Stalk rot resistance genetics must also involve how the carbohydrates are distributed and utilized. The balance of transporting sufficient energy to the root cells for maintenance of vigorous defense against soil microbes and the competing movement of carbohydrates to the developing grain involves multiple genetics for structure and physiology.
These genetic complexities and strong environmental influences leaves us with only being able to express the tendencies of a hybrid to get stalk rot. It is complicated!
Corn stalks have a green outer rind color during most of the growing season as the outer cells are pigmented by the chlorophyll. This color continues beyond grain fill as this annual plant matures without wilting, even up to normal grain harvest. If the plant wilts because of root rot, not only do the leaves desiccate, turning from a green color to gray within a few days and then brown but the stalk color changes also. The dark green color becomes yellow-green a few days after the plant wilt. This color change progresses to yellow and a few days later to brown.
As the brown color intensifies, desiccation of the internal pith causes withdrawal of the pith from the outer rind. This changes the stalk structure from a solid rod to a tube, reducing the strength by a third, leaving it vulnerable to breakage. One can access this vulnerability by gently pushing the stalks or pinching the lower stem. Visual inspection of the color of the lower stalks to judge this deterioration also can be used to evaluate the plant’s vulnerability to lodging. Individual plants with green lower stalks a few days after grain ‘black layer’ will remain intact through harvest.
The anthracnose fungus, Colletotrichum graminicola, will cause black streaks on the outer rind even on a green stalk. This color only intensifies, however, if the plant wilts, apparently because the living cells can restrict the fungus. If there remains a green color around the black streaks, the lodging threat is not great. Another interaction with the fungus commonly occurs in the uppermost internode of the corn plant. It is often noted that this internode turns brown when remaining stalk is green. As sugars are moved from leaves to the grain, this upper internode often is depleted first resulting in senescence of this tissue. The anthracnose fungus is often found in the dying tissue. I interpret it mostly as signal that the plant is successfully moving maximum carbs to grain and not necessarily a sign of stalk rot.
Other fungi also become noticeable on the dead, brown lower stalks. Gibberella zeae produces its reproductive bodies near the nodes, Diplodia maydis produces theirs more scattered on the internode tissue and Fusarium moniliforme gives a pinkish discoloration across the internode surface. It may give us some comfort to have a name for the fungus present but it must be remembered that the cause was insufficient carbohydrate to both meet the translocation demands of the grain and the maintenance of root life. These fungi, and the many others also digesting the senescing and dead stalk tissue were not actively killing the plant.
One can see a lot by looking.
Late season stalk rots usually involve several interactions between various fungi and the host plant biology. Most commonly fungi associated with corn stalk rot are Fusarium moniliforme, Gibberella zeae, Diplodia maydis and Colletotrichum graminicola but in fact multiple other fungi can be isolated from dead stalks as well. All of these species feed on dead plant tissue and create spores that are ubiquitous on and around corn plants all season. Part of the natural corn physiology is to defend against successful invasion of corn tissue even if the stem or leaf tissue is damaged by insects including corn borers. These defense systems are triggered when invaded by production of anti-fungal chemistry after detection, a common character of all living plant and animal organisms.
Living root tissue use this system to ward off the many microbes surrounding them in the soil. Root cells require energy to drive all of its physiology including the defense system. As an annual plant such as corn matures with energy in the form of carbohydrates drawn to the developing grain the roots need to compete with the grain for available stored and freshly produced carbohydrates. Corn root mass begins a slow deterioration a few weeks after pollination, perhaps because of the competition with the grain. Microbes quickly take advantage of the weakened defense system, digesting the dead tissue. If sufficient root tissue can function for the 55-60 days after pollination to absorb and transfer sufficient water to the leaf tissue to meet the loss of water through stomata from transpiration, the plant will maintain turgidity.
If the root rot causes the plant to not meet the transpiration demand, the plant will wilt. This becomes evident very quickly with a whole-plant symptom of gray leaves. Desiccation occurs in all plant cells, stopping movement of carbohydrates to all cells including those in the grain. This also stops ability to ward off the abundant fungi ready to digest the dying and dead cells in the stalk tissue. The more aggressive of these fungi are the ones that we associate with stalk rot.
Corn stalk rot has been studied by many researchers for a very long time. The dynamics of whole plant biology, biology of the fungi and environment interactions are complex. The photosynthetic stress-translocation balance concept of corn stalk rot was first proposed in 1975 to a group of corn pathologists in an attempt to unify previous research on aspects of the problem done by many researchers. That literature and further published information supporting this concept can be sent by postal mail with a request sent to firstname.lastname@example.org.
Heat is a major energy factor influencing the development of corn plants and the ultimate grain yield. Cellular respiration rates increase as temperatures go up. Photosynthesis rates also respond to increased heat as well. It seems reasonable to assume that practically every physiological function in the corn plant is affected by heat energy.
This includes the transformation of the apical meristem from producing leaf buds to production of the tassel. This happens in corn plants at about the V6 stage. Many, many years ago, I dissected young corn plants of hybrids of nearly all maturities sold by a major seed company looking for this change in the apical meristem. The change visible under a microscope, was nearly perfectly correlated with our final classification of the relative maturities of the hybrids. This is consistent with the view that the first influence of temperature on corn maturity occurs early in the season. It is probable that temperatures further affect further development of the differentiated apical cells into mature tassels. We attempt to express the daily temperatures that could affect the timing of pollination with averaging high and low daily temperatures but accurately depicting the duration of a high or a low temperature is difficult. We know that it does affect, but like much of growing crops, we know of the principles but not all the specifics.
Grain fill period seems mostly fixed to about 55 days but there are studies that show low night temperatures can extend the period to formation of the abscission layer, thus increasing grain yield (Elmore, R. 2010. Reduced 2010 Corn Yield Forecasts Reflect Warm Temperatures between Silking and Dent. Integrated Crop Management. Iowa State University, 9 Oct. 2010). It is likely that each hybrid differs in its reaction to temperature during this period.
Given the difficulty of accurately measuring the specifics of temperature interactions of corn plant morphological development, cellular function such as photosynthesis, respiration rates and translocation rate of sugars It is best that we simply compare hybrids for their usual time to harvest moisture. It is all relative.
Comparisons of corn hybrid relative maturities is complexed with flowering dates, black layer timing and harvest moistures. Temperate zone hybrids have mostly shed the day length component affecting time from planting to pollination, as this is mostly determined by accumulation of heat. Earlier hybrids require less heat to reach this stage. After flowering, time to formation of the abscission layer at the base of the kernels and stopping the translocation of carbohydrates into the kernels is about 55 days for all hybrids. Maturity, consequently, as defined by time from planting to completion of grain fill is mostly determined by needed heat from seed germination to formation of tassel and ear shoot meristems.
Grain storage requires moistures close to 15% instead of the 30% when the abscission layer (black layer) forms. Decrease of moisture as the sugar was transported in and starch formed in the developing kernel causing the consistent dropping of percent water down from 85% if the plant remains viable for the 55 days after flowering. Further drying in the field now is through evaporation through the kernel pericarp. Hybrids differ in morphological features affecting this evaporation rate. Husk leaf numbers, length, thickness and tendency to separate after their abscission layers have formed have a great effect on evaporation of water from the kernels. Pericarp thickness also affects loss of water through kernels.
There are several methods used to compare relative maturity of hybrids but practicality demands the comparison made at desirable harvest moistures. That involves the physiology of the young corn plant as it translates heat needed to transform the meristems into forming the male and female reproductive parts. It also involves the morphological structures of ear. The most practical and economic comparison comes when comparing harvest moistures of hybrid growing in same environments such as yield plots.
We probably express relative maturities of hybrids with more definitiveness than we should.
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.