Two related corn pathogens (part 1)
Prior to 1940, the cause of a small leaf spot in the northern USA and of a larger leaf spot in the southern USA were considered one species, Helminthosporium maydis. Both had similar conidia with dark pigments. Upon further study, and especially when a variant of the one with shorter and darker spores caused considerable leaf death and ear rot on a few corn inbreds, the two variants were categorized as separate species. Helminthosporium carbonum became associated with northern leaf spot and the race associated with the extreme damage as Race 1 of this species. Race 0 was associated with the much smaller lesions on most corn varieties. The other species remained as Helminthosporium maydis and was more common in the warmer environments of the southern USA caused the disease called southern corn leaf blight. H. carbonum had less curved and darker conidia, as viewed with a light microscope and was generally considered less pathogenic when race 1 was not effective.
Further study of these two species revealed that both had two mating types resulting in separate sexual recombination within each species. Rules for fungal names therefore caused Helminthosporium carbonum to be name Cochliobolus carbonum and H. maydis as Cochliobolus heterostrophus. Asexual stages of these two are currently named Bipolaris zeicola and Bipolaris maydis.
Like many leaf pathogens, both enter the leaf by setting us an appresorium on the epidermal surface from which it penetrates into the mesophyll of the leaf. A combination of toxins and enzymes allows the fungus to kill some tissue, from which it asexually produces conidia to spread the pathogen to new leaf tissue. Race 1 of B. zeicola produces slightly different toxin (HC-toxin) that is destroyed by an enzyme (HC-toxin reductase) in corn plants with a dominant gene called HM. Consequently, the toxin is only effective in varieties that are homozygous recessive (hmhm) for this gene. Fortunately, the dominant version of this gene is found in most corn inbreds. Unfortunately, the fungus B. zeicola is a common pathogen of many grass species including those frequent near corn fields. The rarity of severe damage to corn from this pathogen allows it to be overlooked in corn breeding nurseries. I recall finding Race 1 in one entity of a breeding project in Nebraska in 1988 but if that natural infection had not occurred that susceptibility of that new inbred may have been advanced to hybrid seed production. This apparently did happen a few years ago when seed fields in South Dakota, Central Illinois and Central Ohio showed significant leaf lesions, ear rot and dead plants from Race 1 of B. zeicola even after fungicide application. The fungus, including race 1, perhaps maintained on other grasses in several areas of the corn belt or perhaps spread unnoticed with seed from a seed increase found an inbred with hmhm gene. It is unlikely that it ever showed on hybrids made with this inbred because the dominant gene HM would come from the other hybrid parent.
Another pathogenic variant of B. zeicola was identified in the 1960s giving slightly larger leaf lesions and some ear rot. It was mostly limited to northern corn belt and specific genetics especially those related to the inbred W64A. This was designated as B. zeicola race 2.
Things between these two corn pathogens became even more interesting after Biploaris maydis race T evolved. This is the subject of the next blog.
Single gene resistance and fungal races
Northern leaf blight damage is controlled by two distinct systems. Neither system appears to limit the ability of the fungus (Exserohilum turcicum (=Setosphaeria turcica) to penetrate the leaf surface. Once inside, the fungal hyphae are detected by the nearby cells resulting in production of chemicals to slow the growth toward the vascular bundles. This resistance system involves at least 3-4 genes and is regarded as stable. Fungal variability and intensity causes some range in lesion development but varieties with best multiple gene resistance will show under nearly all levels of fungal pressure by having fewer lesions than more susceptible ones.
The complexity of evaluating and selecting genotypes with the multiple resistance genes, especially when appropriate disease pressure is not present has made it attractive to select genotypes with single gene resistance systems. Such a system was found in a popcorn variety in the 1960s and soon incorporated in many USA corn breeding programs. This HT1 gene did not limit the fungus from reaching the vascular system but took effect after it reached. The gene products limited the fungus, inhibiting from wilting the host tissue and, more importantly, inhibited the fungus from producing spores and thus spreading within the field. Although isolates of the fungus in Philippines and Hawaii were overcoming this resistance earlier, it took 10 years of widespread commercial corn use in mainland US corn belt before this single gene was found to be overcame with a variant of the fungus. Such variants are commonly referred to a new race. Because it may only involve a single gene difference in the pathogen, I prefer the term pathotype, but certainly the race term is commonly used to describe the newly describe mutant. In this case, the E. turcicum variants detected and controlled by the Ht1 gene is called race 0. Race 1 is given to the fungus not controlled by the Ht1 gene.
Three other single genes for severe limitation of this fungus have been identified in corn. These genes are designated as Ht2, Ht3 and HtN, but consequently fungal variants have been found that overcome each of these single gene systems. The races of this fungus are noted as combinations of race 0, race 1, race 2, race 3, race N or combinations such as race 1,2,3 or 2,3,N.
Diversity of genetics within a fungus is not surprising. Selection pressure for mutants that allow the fungus to reproduce is great. Best to select for the more stable multiple gene resistance in corn.
Resistance to northern leaf blight fungus
After penetration of the leaf epidermis by Exserohilum turcicum (Setosphaeria turcica), adjacent corn cell plasma membrane detects a common protein component of the fungus chitin wall structure. In response, the cell produces products to inhibit or at least slow the growth of this fungus. This basic defense method is common in plants as they ward off many potential pathogens. The only microbes successfully spreading sufficiently to receive nutrition from the plant are those either avoiding detection or limiting the damage from the plants response. This is the definition of a pathogen.
Corn varieties vary in the quickness and strength of the response and E. turcicum isolates differ in quickness of growth as it heads towards the vascular tissue in the leaf tissue. It appears that 3 or 4 corn genes are involved in effectively detecting and slowing down this fungus from reaching the vascular tissue. It is probable that minor genes also influence the effectiveness of this resistance system. This system of resistance effectively limits the number of lesions but can be overwhelmed if the pathogen numbers are intense. Consequently, resistance ratings need to be expressed with some relativity to disease pressure and reactions of other varieties.
There are some single genes in corn that can more drastically limit the disease. HtN1 is a gene that significantly delays development of lesions and, consequently, slowing the spread of the northern leaf blight in a field. This gene was discovered and later isolated from an old Mexican variety. Other single resistance genes named Ht1, Ht2 and Ht3 initially found in varieties of tropical corn, popcorn and Tripsacum. Each of these single genes produce products that limit the fungus’ ability to produce spores but do not restrict the fungus from reaching the vascular tissue. These genes have been crossed into some corn varieties but the futility of use of these genes eventually becomes realized as the selection pressure within the fungal genome results in a ‘race’ of the fungus that is unaffected by the host gene.
As with most corn diseases, resistance systems affected by multiple genes and limiting the success of the pathogen to establish lesions is the most stable.
Biology of Northern Leaf Blight
The disease is caused by the fungus Exserohilum turcicum (Setosphaeria turcica). The fungus lives in infected, dead or live leaves. It asexually produces spores (conidia) when the diseased tissue is moist and temperatures are proper for corn growth. The conidia have 4-6 cells arranged in a row and are light enough to be distributed by air currents within a corn field. After landing on a corn leaf, with a little moisture, in 3-6 hours the cells on both end of the conidia, begin dividing, emerging as germination tubes. These new hyphae quickly form a base on the surface called an appresorium, from which the fungus grows into the leaf epidermis. Within 12-18 hours after the conidia have landed on the leaf, it has successfully penetrated the leaf. There appears to be no difference in time to leaf penetration between the susceptible and resistant corn hybrids.
Chloroplasts near the infection point soon lose pigments as nearly 100 cells die, perhaps because of enzymatic activity of the fungus. This can be observed when small (0.5-1cm) circular, yellow spots show in the leaves in 24-48 hours after infection. From this initial location, the hyphae grow between cells towards the vascular bundles. Penetration of the vascular bundles is followed by plugging the xylem causing further death of surrounding cells dependent upon the water supplied through these tubes.
Resistance systems appear to begin after initial infection and perhaps mostly once the fungus has reached the vascular system. There does seem to be differences in that initial yellow spot in a couple days after infection and I wonder if the brighter color is not associated with greater quantitative resistance. Could part of quantitative resistance be self-destruction of cells near the infection point, depriving the fungus of nutrition? It does seem that initial spot is less obvious and possibly smaller in the more susceptible corn genotypes.
The wilted areas surrounding the plugged xylems eventually are depleted of living host cells. The fungus responds by producing new conidia within 14 days of the initial infection, ready to spread to more live leaves. Exserohilum turcicum remains viable in dry leaves for a number of years in dry environment, ready to produce conidia within 24 hours after moistened. Tillage, crop rotation and hybrid resistance become major factors in crop damage from this disease.
Much is written about this disease. An interesting summary of the morphology aspects in included in this report: Journal of Applied Biosciences (2008), Vol. 10(2): 532 - 537.
ISSN 1997 – 5902: www.biosciences.elewa.org
Corn pathogen diversity
Northern leaf blight is the name usually given to the corn disease caused by the fungus Setosphaeria turcica. The original name for this fungus was Helminthosporium turcicum was given in 1876 and was based upon the pigment and shape of the conidia spores. Taxonomy researchers attempt to group closely related organism into a common genus name. Fungi have limited morphological characters to use with spores being the most commonly as a stable feature. It is indicative of the prevalence of asexual reproduction in this fungus that the genus name changed to different names based upon asexual characters from Helminthosporium to Bipolaris, Drechslera, Luttrellia and then Exserohilum in 1974. The sexual stage of this fungus was not confirmed until the 1950’s, initially named Trichosphaeria turcica and later in 1974 to Setosphaeria turcica. The time lapse between the recognition of this pathogen based upon it asexual conidial stage and identification of its sexual stage indicates the significance of asexual reproduction of this pathogen.
Setosphaeria turcica sexual reproduction occurs when hyphae of two mating types (MAT1 and MAT2) fuse, followed by the combining of the nuclei chromosomes, recombination and eventual segregation into new haploid nuclei. These form 4-6 individual spores within a sack called an ascus. Sexual reproduction does assure new genetic diversity but the rarity of finding the two mating types perhaps indicates that this is not the most significant source of genetic diversity of this species.
Those of us that have grown isolates of this fungus in artificial culture media frequently see differences in growth patterns, pigments and sporulation among the isolates. This pathogen is widely distributed on corn. Although it usually prefers cooler environments of 15°-25°C (59°F-77°F) for infection it is adapted to most temperate and semitropical environments. The higher frequency of both mating types in more tropical environments, especially in Mexico, suggests that it originated along with the early development of corn and perhaps was distributed with the crop.
Distribution to many geographic locations, exposure to multiple corn genotypes, haploid hyphae producing huge numbers of conidia has resulted in diversity within the species, whether we call it Setosphaeria turcica, Exserohilum turcicum or even Helminthosporium turcicum.
Corn reproduction is relatively easy to understand when compared to most corn pathogens. Most corn cells have a single nucleus with 10 pairs of chromosomes with only the exceptions of the triploid cells of the endosperm and the haploid cells in the pollen and egg cells. The hyphae of most corn leaf pathogens are filaments composed of multiple cells separated by cell walls called septa that have pores allowing exchange of cytoplasm and even nuclei. The nuclei are composed of a single set of chromosomes. Furthermore, a single cell of the fungus causing northern leaf blight may have up to 30 nuclei, all haploid.
Genetics of diploid cells are affected by both sets of the pair, generally with the dominant form of a gene on one set of the chromosomes resulting in the amino acid components of the protein and therefor the trait expressed. Recessive genes get expressed only when both sets have the same recessive i.e. homozygous recessive for that gene. Mutations resulting in new recessive genes are often ineffective because of the dominant form in the other member of the paired chromosome.
In a haploid nucleus, there is only one form of a gene, allowing a recessive gene to be fully expressed. Resistance, especially those with single gene inheritance, in corn to a pathogen such as Exserohilum turcicum, cause of northern leaf blight, is initiated when the host cells recognize the presence a pathogens product (effector). A mutation in the pathogen’s gene for this effector, negates the plants ability to recognize the pathogen and therefore this single gene for resistance is not initiated. This is the weakness of the single gene system for resistance to any pathogen.
Fungal pathogens have the statistical advantage of producing new races because of the haploid nuclei in most hyphae. For example, if a homozygous recessive form is crossed with a homozygous dominant, the first generation of a diploid organism would only produce the dominant gene product. In the haploid hyphae, half of the resulting cells would produce the recessively inherited product. This explains why some pathogens can successfully overcome the single gene type of resistance. Being able to asexually reproduce with thousands of spores with haploid nuclei from the successful pathogen allows quick spread of a new genetic type.
Fungi and corn
One of the intrigues of our grandchildren has been to put their fingerprints on the surface of the sterile culture agar in petri dishes in our lab. After a few days, something always is growing on the surface of the agar. Even after washing their hands, there is growth. If they leave the petri dish lid off briefly, something contaminates and grows. We are generally unaware of the prevalence of fungal spores around and on us. We, like most other living organisms, successfully limit any potential harm from the fungi.
The majority of fungal species surrounding a corn plant feed on dead tissues. Most grow within organic tissue by thin strands of filaments called hyphae. A single hypha is visible to us only by a microscope but a mass of hyphae, also called mycelium, is more easily seen growing from dead kernels. Hyphae of most fungi are composed of a string of single cells, each with its own nucleus. Growth is mostly accomplished by cellular multiplication. The single nucleus of a cell, which usually is haploid, divides by replicating the genetics, mitochondria also replicates, cell wall develops separating the new cell from the originating. These cell walls, called septa, separating the individual cells have holes allowing the movement of ribosomes, mitochondria and, in some species, even nuclei to move between the cells. These pores in the septa allow transport of nutrients absorbed by the cells at tips of the hyphae to older cells within the strands.
Most of the nutrition of a fungus is dependent on hyphae growth within the dead or live tissue. Survival of the species, however, depends upon spreading to new nutrition sources as the current source either is destroyed or, in the case of a pathogenic fungus, the host resistance limits the growth. Spread to new nutrition sources is usually done through production of spores. Fungi such as Exserohilum turcicum, cause of northern leaf blight of corn, kills corn leaf cells mostly be plugging vascular tissue within a few inches of leaf tissue, and receives nutrition from the dead leaf tissue. With right temperature and moisture, the hyphae are stimulated to produce aerial structures called conidiophores terminated with spores (conidia) which are excised and carried by wind. This fungus in a single northern leaf blight lesion can produce more than 100,000 conidia within a few days. Survival of the fungus is dependent upon this large production because distribution is mostly random, although many will land on another corn plant within the corn field. Even with this large production from a single lesion within a corn field, successful infection is infrequent. We at PSR, Inc. artificially inoculate with this pathogen by placing about 1000 spores in the whorl of growing corn plants. The more susceptible hybrids usually show 5-8 lesions developing from the inoculated tissue and more resistant hosts may show only 1 or 2 lesions. Although most of the conidia germinate in this moist environment, successfully penetrating the leaf and overcoming the resistance systems of the plant limits the fungus. Most fungi are dependent upon production of large numbers of conidia to overcome the environment limits and host plant resistance for continuation of the species.
Fusarium verticillioides in corn
This fungus, previously known as Fusarium moniliforme, is commonly found in corn seeds. It can be found growing from seeds that germinate normally on germination paper. One study showed that inoculated seeds produced seedlings of reduced size if grown in sterile soil in a reduced light, but not if growing conditions were normal (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC150081). The authors also showed that this fungus would infect the seedlings from soil infested with this species. F. verticillioides is common among dead and weak plant tissue of many plant species including the previous season’s corn debris. Spores (conidia) are prolifically produced on moistened infected tissue and spread within the corn tissue. Any openings in the corn plant is vulnerable to infection by this fungus that easily gets established in the nodal tissue. It is probably not as aggressive as another Fusarium species (F. graminearum) that is better known by its sexual stage (Gibberella zeae).
Fusarium verticillioides is often blamed for causing Fusarium stalk rot, mostly because it is nearly always present and none of the other potential stalk rotting fungi such as Stenocarpella maydis (Diplodia maydis), Gibberella zeae or Colletotrichum graminicola are present in the dead stalk. Because the dead stalk was predisposed to a root rot because of the plants reaction to growing conditions leading to vulnerability to many soil organisms, the identity of the stalk-inhabiting organism may be convenient but the future solution should concentrate on the conditions leading to the root deterioration.
Fusarium verticillioides also infects the developing kernels. Some of this occurred with the spores germinating on the exposed silks and growing into the freshly pollinated kernels. This and other Fusarium species also can reach the kernels within the developing ear through physical damage from insects and birds. Hybrids and environments differ in vulnerability to Fusarium infection of the kernels. F. verticillioides has the potential to produce a toxin called fumonisin in grain.
This fungus is among the occupants of corn plants that are mostly limited by the living corn tissue but ready to digest the senescent and dead tissue.
Fusarium in the stalk
Fusarium species that are often found in dead and live corn plant stalks initially grow between cells, producing enzymes to digest the walls of the pith cells and fibers. These walls are composed of complex cellulose compounds combined with proteins and basic carbohydrate molecules, formed by biological activity within the living cells and secreted through the cellular membranes. These compounds protect the cellular contents by structure and some active resistance products as well. Cell wall components include cellulose, arabinoxylans, hydroxycinnamates, pectins, glycoproteins and lignins.
Fungi initially grow between the cells, secreting enzymes to digest the cell wall structures. Meanwhile living corn cells produce compounds such as Jasmonic acid, abscisic acid and salicylic acid to limit the growth of the Fusarium species. This battle begins with the first incursion of the fungus, perhaps with injury to the seedlings or perhaps with corn borer. Artificially infecting the stalk with any of the fungi associated with corn stalk rot shortly after pollination usually shows infection limited to area near to point of inoculation. As the tissue matures the fungi seem to more rapidly spread, digesting more cell walls. Breaking down the complex compounds allows the fungus to utilize more simple carbohydrates for their own metabolism.
Digestion of the cell wall components weakens the strength of the stalk beyond the sudden decrease of strength that occurred with the plant wilted after root rot. The wilting caused the pith cells to withdraw from the rind, effectively changing the structure from a rod to a tube. This alone is believed to reduced strength by one-third. Corn hybrids that tend to construct more and stronger cells in the rind can have less lodging but the physical change from a rod to tube is difficult to match by cell wall strength alone.
Maintaining sturdy stalks until harvest is a balance among producing sufficient carbohydrates for filling all kernels, avoiding wilt by delaying death of root cells and maintaining stalk cells alive to fend off potential pathogens such as Fusarium species. Acceptable grain yields and standability occurs when the potential stresses from low light, leaf diseases, mineral deficiencies and insect damage are matched by the hybrid producing sufficient photosynthesis to balance these three needs within the plants.
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.