Cellular features of fungi are not completely different than plants and animals. Their DNA is in a distinct nucleus, they have mitochondria for transformation of energy to forms used for growth and function and ribosomes for protein production. While most fungal growth is separated into distinct cells, there are some such as in the Pythium genus in which the hyphae do not have distinct cells and have multiple haploid nuclei. Most of the life in most fungi, the nucleus has only one set of chromosomes, instead of the two sets found in most plants and animals. The process of sex has slightly different definition with fungi. Joining of two cells, and then of two nuclei followed by meiosis in which new combinations of chromosomes are distributed into haploid nuclei can occur without sexual distinctions between the two individuals, or, in some species, only with distinct mating types. It can result in creation of distinct (to us) morphological structures or within the normal appearing hyphae. The requirement for having two, or more, mating types certainly causes difficulty for us humans to completely understand the sex life of many fungi but genetic variability for characters that we follow in corn pathogens seems strong. Because of the ability to asexually reproduce quickly the unique genotype that has found a susceptible host, a new variant has been witnessed often in agriculture, whether produced by mutation or sexual recombination.
Because the asexual and sexual reproductions are separate events often occurring in distinct places and times, it often has been difficult for mycologists to associate the two structures to the same organism. Naming of fungi accordingly is the next topic of the corn journal.
Fungal survival is dependent upon warding off competitors from potential organic sources of nutrition through the production of antibiotics and by moving to unoccupied new sources. Most of that movement is via spores. Each species has some distinct method. Pathogens gain advantage over those species that only can feed on dead material by attacking the living, dominating the newly killed area and then moving on. This certainly is the tactic of the leaf spot-causing fungi on corn as they usually infect a small area, kill the tissue and, before saprophytic fungi can invade and compete, they move on by producing spores to spread to new leaf material.
The fungi causing most leaf spots in corn belong to a group (phylum) called Ascomycetes. Among their commonality is the structure of the sexual stage in which haploid nuclei fuse and then undergo meiosis. These fungi live most of their life with only one set of chromosomes and thus as haploids. Widespread reproduction of these fungi comes through asexual production of spores, called conidia, dispersed often by wind. In fact, many fungi are only known by their unique conidia, the sexual stage not identified. This leads to confusion about the names of fungi as well because generally the name based upon the sexual reproduction structure according to fungal taxonomy rules should take precedent.
Most fungal leaf pathogens of corn produce huge numbers of conidia after the leaf tissue is wet for a very short time. It is not unusual for several hundred conidia to be produced from a single lesion of northern leaf blight. Within a few hours those that land in a moist area of leaf are able to germinate, produce specialized structures to enter the leaf tissue and within a few days begin feeding on fresh tissue. A few weeks later it produces more conidia and moves on. The fungus also remains in the dead leaf tissue after harvest and if not destroyed by competitors, such as when buried in soil, is ready to produce more conidia when temperatures and moisture are right the next spring.
There are probably more fungal species than of plants. They occupy nearly all space where some sort of carbohydrate is available. Most fungi produce enzymes to digest complex carbohydrate molecules such as cellulose, lignins and pectins, the main components of cell walls. Most fungi produce filaments called hyphae. The term for a mass of hyphae is mycelium. Competition among fungi for any dead plant material must be fierce and thus evolution has favored mechanisms for fight off competitors with production of antibiotics. Those of us who attempt to culture fungi in a petri dish often witness this when two species develop a neutral zone between them, not penetrated by either. The other tactic favored is to produce large numbers of propagules (spores) that can spread the species to other locations and new sources of food. Spore numbers are huge, as is probably necessary for survival because distribution is mostly random. Most fungi produce small light spores that are easily carried by wind, some for great distances. Spores of corn and wheat rust fungi have been collected in high altitude wind currents. Puccinia sorghi, cause of common rust on corn, overwinters in Mexico but its spores are carried to the US Midwest by those winds in the summer. Spores of the gray leaf spot fungus, Cercospora zeae-maydis, easily are moved between corn fields whereas those of the northern leaf blight (Exserohilum turcicum) appear to be heavier and less mobile.
A few fungal species produce only swimming spores. The fungus causing crazy top, Sclerophthora macrospora, produces such spores (zoospores) when in water that literally swim towards corn (and many other grass species) roots, germinate and infect. The hyphae formed from the germinated spore grow within the plant towards the growing points, causing deformities that favor the fungus. Then it has sex and produces thick-walled spores (oospores) that can remain in dry conditions for several years until conditions favor the swimming spores and infection of a new host. This is one of the many strategies that fungi have for survival and success in their complex, competitive world.
Spores of fungi not only allow them success biologically but also, are used by mycologists as among the few structures available to identify many species. That is the topic for next week’s blogs.
We can study the one dead plant to understand the phenomenon resulting in stalk rot. Roots starved because of insufficient carbs reaching them. This was due to some combination of insufficient photosynthesis to meet the demands of the grain fill and root metabolism. But that one plant does not concern the growers. It is the percentage of plants in a field that lodges and become difficult to harvest that bothers the grower and the seed provider. Physiologically, the concentration should be on energy captured by corn in the field (photosynthesis per acre) and harvestable carbohydrates captured.
A comparison of 44 commercial hybrid parents for the carbon exchange ratio (CER) for a small section of leaf tissue was done about 35 years ago. This was done by putting a known area of fresh live leaf tissue in a chamber with appropriate light wave frequencies, with known amounts of CO2 added and uptake in the leaf tissue measured. The result showed very large range in CER among the inbreds. It was interesting, and probably meaningful, that highest CER occurred in those that were visibly darker in color. But they were all commercial hybrid parents! Inbreds with highest CER also tended to have the more narrow leaves. Of course, the hybrids were selected based upon good grain yields and stalk quality (among other things) in field trials. They must have attained this not only through CER per area of leaf tissue but whole plant photosynthesis at the plant densities and other environments in the field. Those with more leaf area or canopies that allowed light penetration to lower leaves, or better uptake of minerals to support photosynthesis, or more plants per acre were the successful ones. My take home from the CER experiment is that we still have lots of room for selection of more photosynthesis per acre but it is too complicated to concentrate on only one factor.
Just as with selection of best commercial grain yields, selection of stable stalk quality is dependent upon performance in the real world of dynamics of growers’ fields. Selecting single traits might be useful but the final proof is in testing the whole plants and not only single components of performance. Ultimately, it is the combination of hybrid genetics, the season’s environment and the culture of the crop that determines the occurrence of stalk rot, just as it does the yield.
For more details about stalk rot, consider the stalk rot booklet in the tab above.
We have established that plants dying early as a prelude to stalk rot, actually wilt because roots have rotted to the point that water transport to the upper part of the plant has stopped. This was caused because there was insufficient sugar to the root tissue. One difference between this plant and adjacent plants was more kernels on the dead plant. In our survey comparing dead and alive we noted one other difference. If a corn borer had caused the plant to break above the ear, that plant frequently had stalk rot. We eliminated those plants from the data because we wanted to emphasize the kernel number variable. Some would say that the corn borer allowed the stalk rotting fungus to enter the stalk but then why was the stalk broken below ear remaining solid? Ear was on ground but stalk penetration by fungi did not seem to destroy the remaining intact stalk.
If distribution of sugars affected early death of roots, wouldn’t reduction of the production of sugar after pollination do the same? Removing leaves after pollination did increase stalk rot in an experiment published in 1980. Five commercial hybrids in plots were included in the study. Every other plant in the plots was cut with at different nodes above or below the ear, or left uncut. All leaf removal was done at 2 weeks after pollination. 100% of plants with all leaves removed above the ear node developed stalk rot. Progressively few plants got stalk rot as fewer leaves above the ear were removed. On the other hand, no plants developed stalk if the plant was cut at the ear leaf including the ear. Very few had stalk rot if cut just below the ear. In a later experiment done with Herman Warren, then at Purdue, with plant stalks inoculated with the anthracnose fungus, Colleotrichum graminicola, showed that the disease increased with cut stalks also.
The plant that died early actually wilted. It is frequently a plant that had more kernels than adjacent plants, Kernel numbers influence the distribution of sugars between the living tissue of the roots and the kernels. Water is constantly being lost from the corn plant through the leaves via transpiration. This needs to be replaced by water moving by osmosis into the roots from the soil. This happens mostly in the young root tissue and then progresses up the xylem tissue in the vascular system because of the cohesion of water molecules.
Meanwhile the roots are existing in an environment full of microbes dependent on organic materials for energy. A few species have the capacity to invade the root cells in a symbiotic relationship in which the root cells benefit from extra mineral uptake, especially phosphorus, while the corn root cell provides carbohydrates to these mycorrhizal fungi. There is some evidence that this relationship actually increases root length and more water uptake.
But it is complex down there and the plant cells need to ward off the other fungi that feed on any tissue. Living cells do respond by producing anti-microbe chemicals. Growing root tips exude proteins but also some anti-fungal agent that wards off invasion. It is difficult to study the full ecological interaction in roots but it is known that many different fungi eventually invade the roots. The chemical defense and the growth of new root tips does take energy. As available sugar supplies wane, the fungi invade. Eventually fungi destroy young tissue and plug up xylem, causing insufficient water uptake to replace the transpiring water. Then the plant wilts. Now fungi below and above the soil have free rein to invade the dead cells. Some fungal species are mostly easily identified, and perhaps are dominant, in the root tissue and/or in the stalk, and consequently we give them the credit. Gibberella, Diplodia, Anthracnose (Colletotrichum) and Fusarium stalk rots are common names but in actuality these fungi invaded dying and dead plant tissue.
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 every one 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.
My first plant physiology course was taught by Professor Loomis in 1960 (oops, I just revealed my age). He told of having a debate with his wife while in the car in the Rocky Mountains as to whether they were going uphill or not, the surrounding terrain causing confusion. He claimed that he got out of the car and poured water on the road to prove the direction of the hill. “Water runs downhill” humph- he had the habit of loudly clearing his throat when he was making a major point. Water moves through cell membranes from a high concentration of water towards the lower concentration of water. High concentration of water equals less concentration of solutes. This process of osmosis is passive transport and is the main means in which water is moved from the soil through the living cells of young root tissue. Many of the minerals, however, require energy to move through the membranes to reach concentrations greater than that in the soil.
The living cells of the roots surround the xylem cells that form a fine tube to the stem tissue. These cells are ridged and dead when mature. Water is imported through the living cells into the xylem because the water moves from high to low concentration. Water molecules attract each other by sharing hydrogen bonds, causing the capillary action and movement of water from the root xylem to stem and leaf tissues. It spreads throughout the plant’s xylem tubes and into all living cells, keeping them turgid and allowing metabolism to occur. Stomata on the leaf surface that open to allow CO2 intake (and photosynthesis) also allow water vapor to escape. For each molecule of water transpired, another water molecule is pulled up the xylem. The corn plant is in need for constantly pulling in more water and the need increases as transpiration increases during dry windy days. It is also important that the chain of water molecules in the upper root tissue and lower stem is not broken to ruin the capillary action within the xylem tissue. Action above the ground is very dependent on the ability of the root tissue to absorb and transport water. Function of the root tissue is dependent upon the energy transformed in leaves and transported to roots.
Water does run downhill and is pulled up hill. Humph. I am sure Professor Loomis said that too but it was a long time ago and I forgot that part.
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.