Cercospora zeae-maydis infection is strongly related to relative humidity. Weather in Northern Illinois in 2018 has provided a prime example of the difference between the dependence on moisture from rain for infection by Exserohilum turcicum, cause of northern corn leaf blight, and that of the fungus causing gray leaf spot. Both fungi sporulate on infected corn debris. C. zeae-maydis spores are considerably lighter allowing further distribution within and outside the corn field. E. turcicum requires some free moisture on the leaf to germinate and penetrate the leaf epidermis within a few hours after germination. C. zeae-maydis depends more on humid air than free water to germinate and grow on the surface of the leaf. Growth chamber experiments showed that penetration into the leaf only occurred after 100 hours of 90-100% relative humidity. It is remarkable that the hours do not need to be consecutive but can occur after a series of humid nights, for example. The fungus appears to have the ability to halt hyphal growth until the next high humidity period.
Most of the midseason weather here this summer has featured little rain but warm humid days and nights. Northern leaf blight is not frequent in corn but scattered gray leaf spot lesions are easily found. This example is typical of occurrence of this disease. Although the disease was known in USA more than 50 years ago, it gained notice in the humid environments of Virginia in early 1980s. Wide use of susceptible genetics allowed spread first to areas around river valleys further west until it had spread to the irrigated areas in western corn belt. The pathogen is now present throughout most areas of USA corn belt, becoming especially noted during warm humid seasons.
Weather during corn growing season affects these two corn pathogens in distinctly different ways. Heavy spores of the Northern Leaf Blight fungus tend to not be carried far by wind whereas the lighter spores of the Gray Leaf Spot fungus are easily moved field to field. Northern Leaf Blight fungus is favored by frequent rain showers, but the Gray Leaf Spot fungus is favored by less rain but warm humid weather.
This corn disease caused by the fungus Exserohilum turcicum should have a different name because it occurs tropical, subtropical and temperate zones in all continents. Its occurrence and severity, however, is greatly affected by weather. Spores (conidia) are produced on moist, infected dead or living leaf tissue in humid conditions. These spores are bigger and heavier than those of some corn pathogens such as the cause of gray leaf spot (Cercospora zeae-maydis) and thus tends to move a short distance within a field. The small percentage that are picked up during strong storms do spread the disease longer distances however. There are instances where the disease will be concentrated in a small area of a field apparently initiated with a few early infections and spread mostly to adjacent plants.
Spore production is encouraged on a moist substrate in a humid environment, but new infection requires the spore to be in moisture for a few hours. Condensation on leaves with cool evening temperatures can be adequate, moisture in the leaf whorl while plant is young, and frequent rain showers are usually adequate for the spores to germinate. These elongate, multicellular spores send out hyphae from cells on each end. The hyphae set up clusters that enzymatically drill into the host epidermal cells to absorb nutrition. At his point the fungus is no longer dependent on weather.
The fungus grows within the leaf towards the vascular bundle. As it feeds and grows in the local vascular cells, a small wilt symptom develops about 2 weeks after initial spore germination. High relative humidity that can be associated stimulates the creation of more spores from the lesion and with rain showers the cycle is repeated.
Occurrence of severe outbreaks of this disease in seasons and locations with frequent rain showers and higher humidity has resulted in natural and intentional selection of higher levels of resistance in some corn-growing regions than others. Open pollinated varieties of the early 1900’s with highest resistance were selected in the Eastern United States. Many tropical varieties tend to have higher levels of resistance than those selected in drier environments.
This disease frequently is more severe in a small region that happened to have frequent rain showers during the corn midseason development. Resistance differences will be more evidence in these conditions than when corn is grown under drier and less humid environments, which may occur in the same field the following season.
Bacteria are single cell forms that continually surround corn above and below the soil. Defense systems of corn, and all other forms of life, generally is effective in keeping potential pathogens from invading cells, while tolerating the saprophytic nature of most bacteria species. Bacteria are vulnerable to desiccation and generally most successful in moist environments, including those inside corn plants.
Corn plant defense systems against bacterial infection includes the tight epidermal layer of cells with a waxy covering that inhibits invasion in leaves. Vulnerability to entrance through stomata is limited by the anti-microbial fumes emitted through the stomata. If this outer defense is avoided, some pathogenic bacteria thrive on the moist internal leaf environment until other host resistance systems stop its spread.
Goss wilt bacteria (Clavibacter michiganensissp. nebraskensis) was first recognized causing a corn disease in 1969. The disease was strongly linked to physical damage to leaves by hail. Breaking the leaf epidermis allowed the bacterial to enter and thrive in susceptible corn genotypes. Spread elsewhere in the USA corn belt has implied that less obvious damage, perhaps from wind in rain storms provide sufficient injury for the bacterial to enter plants. Structural damage to leaves and moisture are essential to successful invasion by this bacterium.
Stewart’s wilt bacteria (Pantoea stewartii) cause corn disease by avoiding desiccation by surviving in an insect vector, primarily the corn flea beetle (Chaetocnema pulicaria). This insect feeds on corn and other grass leaves, penetrating the epidermis while inserting the bacteria into the leaf. This pathogen multiplies and spreads especially through the vascular system in susceptible genotypes.
Bacterial leaf streak is caused by bacterium (Xanthomonas vasicola pv. vasculorum) that appears to invade through stomata. Appearance of linear lesions mostly limited on sides by vascular bundle cells implies that these bacterial mostly digest the mesophyll cells in susceptible genotypes. There are some indications that it is associated with warm, rainy weather, perhaps allowing the bacteria to increase in the moisture of the corn whorl and eventually penetrating through the stomata.
A few other Xanthomonas and Pseudomonas species have been associated with contaminated irrigation water from ponds with infections causing leaf blights. Bacterial stalk rot likewise is associated with heavy concentration of a bacterium (Erwinia carotovora) in flooded soils, allowing penetration in to the lower shoot area of the plants.
Although most corn inbreds and hybrids have good resistance to most of these bacteria, inconsistent weather patterns, genetic changes in potential pathogens, factors influencing vectors and unexpected susceptibility in new corn genetics have and will continue to allow emergence of bacterial diseases.
Apparent changes in weather patterns during recent years affects corn and pathogen biology. Diseases may be more prevalent in areas that they were virtually absent and nearly absent in areas in which they were frequently damaging because of timing of particular weather.
Diseases caused by viruses such as Maize Dwarf Mosaic (MDMV), Maize Chlorotic Dwarf (MCDV, Maize Chlorotic Mottle (MCMV) are transmitted by insect vectors which are also affected by weather. Virus generally must reach the growing point of the corn plant to damage the plant or even show symptoms. Consequently, the infection must occur before the V4 seedling stage before the apical meristem is pushed upwards by cell elongation. Wet weather can delay planting, allowing increasing populations of the vectors feeding on alternative virus host plants such as Johnson grass (Sorghum halepense). As result of increase of aphids feeding on MDMV-infected plants and delayed corn planting, transmission of the virus into corn seedlings allows the virus to become systemic after it reaches the apical meristem. This becomes especially damaging if the leafhopper (Graminella nigrifrons) picks up the MCDV virus from grass hosts and then feeds on the same young corn plant. Synergistic effect of concurrent infection by these two viruses can cause extreme damage to susceptible corn genotypes.
This synergism between two viruses has even be more damaging when a MDMV or Wheat Streak Mosaic Virus (WSMV) infect the same plant infected with MCMV. The result is Corn Lethal Necrosis disease. WSMV is transmitted by wheat leaf curl mite (Aceria tosichella), that frequently picks up the virus from infected wheat. Maize Chlorotic Mottle Virus is transmitted by beetles, primarily Diabrotica species in the USA and by thrips (Frankliniella williamsi) in Africa. Transmission can be done by larvae and adults. Corn plants infected as seedlings with MCMV vectored by infected rootworms (Diabrotica species) and MDMV or WSMV as vectored by aphids or wheat leaf curl mites will be severely damaged. Weather affecting timing of planting of corn, growth or control of alternate hosts such as Johnson Grass, harvest of wheat all interact in determining the damage from these viruses.
Several other viruses, each with unique dynamics of vector biology, alternate hosts and corn development cause significant damage in specific environments. Weather is significant in each development of each of the diseases. Many perennial grasses are infected with viruses and are vectored by insects that feed on both corn and the grasses. Weather affects the plant and insect biology in relation to timing and intensity of virus infection in corn. Infection by one virus species may go unnoticed but dual infections because of the coincidence of many variables can result in considerable damage to corn.
Cytokinins and auxins are operative during all of the corn plants life, including the movement of sugars to the young kernels. These two kinds of hormones have different roles in origin and effect on corn growth. Cytokinins are mostly produced in root tips in root meristems and transported through the water distribution in the xylem tissue. Auxins are mostly produced in stem meristems and distributed in the phloem system. Cytokinins are associated with increasing cell division in the stem meristems whereas auxins are involved in cell elongation. Apical dominance resulting in the corn plant usually having only one upright stem is because of the interactions of the auxins produced in the apical meristem. Removing that stem tip in early corn development and thus reducing auxin production tips the balance towards more cytokinin and stimulation of cell division in the lateral buds of the corn plant, resulting in branches.
Pollination of the multiple ovules in the corn ear results in attraction of cytokinins to each developing kernel. Moisture stress during the first 10 days after pollination is known to cause early death to some kernels, perhaps because of reduction transportation of cytokinins to the most immature embryos (my conjecture!). Cell division in the new embryo meristems establishes the movement of sugars through the phloem to the kernels. Much of the sugar is deposited into the endosperm portion where it is changed to more complex carbohydrates and thus allow the osmotic pressure for more sugar movement towards the kernels.
More is known about the effect of these plant hormones on plant growth than all of the mechanisms involved with those effects. Auxins involvement in cell growth involves softening cell walls, making elongation of cells easier. Cytokinins have been shown to prevent protein breakdown and activating protein synthesis.
Cytokinins produced in root meristems are transported to and stimulate the cell division in the kernel embryos. Meristems of those embryos produce auxins. Auxins are associated with production of ethylene which has been associated with formation of abscission tissue as leaves and fruit mature. It is assumed that the auxins are associated with formation of the black layer at the base of kernels, resulting in stoppage of movement of material to the kernels.
We know that these plant hormones are associated with the growth of corn tissues including the formation of kernels but there remains lots to learn of the actual molecular interactions that allows this to happen. Meanwhile, corn breeders, agronomists and growers attempt to coordinate it all by selecting the genetics that maximize grain production.
Water moves from soil into root tissue by diffusion, going from higher concentration in the soil into the cells with water concentration diluted by sugars and minerals. Each mineral likewise is absorbed according to its own concentration inside and outside of the root cells. Movement of water and the mineral solutes flow upwards because of water cohesiveness and the removal of water via transpiration through leaf stomates.
Minerals pulled along with the water are essential to each biochemical process, from formation of cell components to the function of those components. Not only the structures but also manufacture of products integrate the minerals are dependent upon the supply of minerals carried from soil with water.
Stomatal pores are open during the day as the result of photosynthesis and unique shape of guard cells of the stomates (https://www.cornjournal.com/corn-journal/corn-leaf-epidermis). Evaporation of water through open stomata is determined by relative humidity immediately outside the openings. Transpiration is greatest if immediate outside humidity is low. Water movement from roots to leaves is greatest in a dry daytime environment. It follows that mineral movement from soil to leaves that is greatest with drier daytime environments. This becomes especially significant during growth stages of corn as minerals become tied up in cell structures but some elements such as nitrogen, potassium and phosphorus are essential components of enzymes essential to basic photosynthesis and respiration needed continued cell function until completion of the corn plant’s life cycle.
Among the genetic differences in corn varieties is efficiency of water absorption. These must involve structural differences affecting total roots volume and direction of growth. Some have a deeper root and some more spreading than other varieties. Each may be more suitable for specific soil conditions and a season’s weather. Varieties must also differ in vascular structure affecting the efficiency of movement of water upwards.
Varieties of corn also must vary in number of stomates. More stomates may result in ability to absorb more carbon dioxide for photosynthesis and also more movement of water with minerals from the soil but this advantage may be a disadvantage during drought weather. Variety features such as more leaf area is advantageous for total photosynthesis but may result in higher water loss because of more stomates.
Water and mineral movements are affected by a combination of plant structure and environment ultimately expressed at the end of the growing season by grain yield.
Sugar solubility of water favors the initial movement of water into root hairs. This process of movement of water across root hair cell membranes is a physical phenomenon of osmosis, as water moves from a higher concentration outside the root hair to a lower concentration in the sugar (and other molecules) dissolved in water within the cells. This osmotic pressure also promotes water movement into the xylem tubes within the vascular bundles of the roots.
Water cohesiveness keeping the water molecules together along with the removal of water molecules from transpiration through leaf stomata, essentially pulls the water up the plant, carrying with it the minerals dissolved in the water.
Corn stems and leaves with multiple vascular bundles in the stem contribute to stability of water uptake and distribution throughout the leaves. The veins are parallel to each other in the corn leaves. At the base of the leaf, where it connects to the stem at the node, the system becomes much more complex. The vascular tissue goes horizontal with fusions between the individual veins. Also, the xylem ‘tubes’, (vessels) have end walls, forcing the water moving up from roots and stem through small pores that act as filters. The pores are sufficiently small to filter out any particles being carried upward with the water. Many bacteria and even some viruses are too large to pass through the pores. Each node of corn, even in the small seedling, has this complexity of the vascular tissue. Root vascular tissue connects with the stem vascular system at the first leaf node. Whereas an individual leaf may have up to 20 main veins, the node may have 100 horizontal vascular bundles and with fusion of vessels at the nodes. This redundancy protects the plants from a problem in single vascular bundle or one root branch from blocking transport of water and minerals to the leaves. Likewise, the movement of carbohydrates from leaves to roots gets distributed to all roots. Water soluble substances such as minerals and toxins can move freely up the plant with water through the xylem, but most fungal spores and bacteria are filtered out by the pores.
Movement of water into stem and leaf cells also is physical, water moving from higher water concentration in the xylem tubes through membranes of the living, metabolically- active cells, allowing direct utilization of water in photosynthesis and other activities. Cohesiveness allows more water to follow.
We are apprehensive that corn plants lose water through transpiration but also should appreciate that because of the loss of water through the stomata, not only is CO2 allowed in the plant, and oxygen escapes, the process allows uptake of water and transport of minerals also is occurring.
Water is essential to growth and grain production of corn. It is estimated that current corn hybrids require about 20 gallons of water per plant to produce high grain yields (http://articles.extension.org/pages/14080/corn-water-requirements). Of course, the actual number is variable as affected by weather.
Water is essential for all plants because of its unique molecular structure. Oxygen atom fulfills its need for 2 electrons by sharing an electron from each of two hydrogen atoms. This specific covalent bond with hydrogen is unique in that the single hydrogen atoms are not distributed exactly symmetrically around the oxygen atom, resulting in a water molecule with a slight negative charge on one side of the oxygen component and a slight positive charge around the hydrogen components. This has profound and unique effects on water characteristics essential to plants.
Water molecules are cohesive. They attract other water molecules because each has a slight positive and negative electronic charge. Cohesion of water molecules is essential for the transport of water from the root hairs through the xylem tissue up to stem growing points and leaves. This strong capillary action allows movement water molecules upwards as water evaporates and escapes the plant through the stomata that is replaced by water molecules being pulled upwards.
Water is a universal solvent, again due to its polarity. The negative side of the oxygen atom and the positive side of the hydrogen atoms essentially break apart most ionic compounds. Consequently, it becomes solvent to most mineral sources essential for corn growth. For example, the positive charged potassium ion of potassium chloride is attracted to the oxygen side while the negative side of the chloride ion is attracted to the hydrogen side of the water molecule. This allows the transport of essential minerals for metabolism and structure of the plant.
Oxygen and hydrogen components of water also are essential to the many complex chemical reactions that are within the corn plant cells. Water interacts with the essential storage of energy in carbohydrates during photosynthesis and the release of that energy during respiration.
There are obvious effects on corn when water is deficient but there are also many hidden aspects of the unique water molecules essential to corn growth. An interesting summary of properties of water can be found at https://en.wikipedia.org/wiki/Properties_of_water.
Vascular systems of corn plants include xylem cells that essentially become small tubes allowing the movement of water mostly by then tendency of water molecules to stick together (cohesion). As a molecule is removed from the leaf thru a stomate, one is pulled up from the root. Movement of carbohydrates from the source of photosynthesis in leaves to the various sinks in all living tissues of the plant is multidirectional and complex. Glucose, the immediate product of photosynthesis, is transformed into more complex sugar molecules such as sucrose when it is moved.
Movement carbs between cells can be simple diffusion through those small ‘holes’ in cell walls, the plasmadesmata, as the molecules move from a high concentration to a low concentration. It is a little more complicated with travel through membranes by osmosis, but the basic principle is the same. Water is involved because It is the solvent of the sugar. Greater concentration of water equals less concentration of sugar. Water molecules are also affected by the principles of movement from high to low concentration, setting up dynamics for what is called turgor pressure within each plant tissue.
As the sucrose molecules move into a ‘sink’ such as the newly formed kernels, they are transformed into more complex molecules such as starch and thus maintains the osmotic pressure for more movement of sucrose into the kernels. Other sinks, such as biologically active tissue of all living plant cells, consume the sucrose in cellular respiration and formation of essential amino acids and cell structures. These various sinks are not all in the same direction from the carbohydrate sources where the photosynthesis occurred and consequently flow among phloem cells may not be in the same direction.
Whereas water movement in xylem tissue of the vascular bundles of a corn plant is mostly upwards from the roots, movement of the products of photosynthesis is affected by concentrations of various sugars in the sinks, allowing bi-directional flows.
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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.