Disease ManagementSteve Koenning and Gary Payne, Plant Pathology Department
Marty Carson, USDA
Each year plant diseases caused by fungi, bacteria, plant-parasitic nematodes, viruses and air pollution result in some losses in corn production in North Carolina. In 1988, the last year statistics were compiled for North Carolina, plant diseases caused by plant pathogens and air pollution suppressed corn yield by about 10 percent. A number of diseases affect corn, with individual fields often suffering severe losses.
All parts of the plant may be attacked -- the ears, leaves, stalks, and roots -- at various stages of development. In many instances diseases result in lower yields, but also reduce the value and quality of the grain and may increase harvesting costs when affected plants lodge.
Plant diseases, with the exception of air pollutants, are caused by microscopic organisms referred to as plant pathogens. There are several categories of these organisms that may be damaging to plants.
Plant diseases can also be classified as to how they move about, where they reside, and survive. Organisms that infest seed are referred to as being seedborne, organisms that typically reside in soil are classified as soilborne, and organisms that move through the air are referred to as being airborne. These distinctions are important since they often determine the management measures that need to be taken to prevent yield loss.
Insect management may aid in suppression of disease caused by bacteria or viruses, but chemical insect control in field corn is rarely economical and resistant hybrids are the most effective means of preventing these diseases. Good cultural practices, including insect management, are especially important in maintaining the integrity of the corn ears, thus preventing losses due to fungal toxins.
For disease to occur three ingredients are essential. You must have a susceptible host, the pathogen must be present, and the environmental conditions for pathogen development on the host must coincide. This concept is referred to as the disease triangle. Because disease is progressive, a fourth factor, time, distinguishes biotic diseases caused by the various organisms versus injury caused by lighting or hail. This results in the disease pyramid depicted in figure 1. The key to managing diseases in corn is the elimination of one of these key factors. The use of resistant hybrids removes the host from the pyramid and thus eliminates disease. Where soilborne plant pathogens are concerned the pathogen may always be present and long-term management strategies must be used such as rotation to reduce the populations of these organisms. When rotation is impractical, the use of a fungicide or nematicide limits development of the fungus or nematode, and the associated disease is removed or suppressed. Various plant pathogens are favored by different environmental conditions. For example: corn leaf, ear, and stalk diseases generally are favored by warm, wet weather. Most root rots, such as Pythium root rot, are more severe in wet than in dry soils, but charcoal rot is favored by hot, dry conditions. Seedling diseases are favored by cool soil temperatures that delay emergence and growth of seedlings. Diseases of corn seedlings tend to be more severe when planting time is unusually wet. Symptoms of nematode damage are more pronounced in sandy soils, that can support large nematode populations. During dry weather the effects of root damage caused by nematodes are apparent as increased plant stress. In some instances we can modify the environment to eliminate the disease. Irrigation may reduce crop stress, which will suppress the amount of disease caused by pathogens that attack stressed plants. Planting on beds places the seedlings in dryer conditions if fields receive heavy rains. Because of the temperature and moisture requirements of different plant pathogens, the arrival of conditions favorable for plant growth may retard or end the progress of the disease in time, and the crop will recover.
There are no completely effective measures for controlling all corn diseases; however, losses can be minimized by following certain recommended practices. Growers should become acquainted with disease problems and factors affecting the severity of the disease.
Disease Management Tactics:
Seed Rots and Seedling Blights (caused by species of Fusarium, Stenocarpella, Pythium, and other fungi). Germinating corn kernels may be attacked by a number of soilborne or seedborne fungi that cause seed rots and seedling blights. The terms "preemergence" and "postemergence damping-off' are often used to specify the affected growth stage. These diseases are more prevalent in poorly drained, excessively compacted, or cold, wet soils. Planting old or poor quality seed with mechanical injury to the pericarp will increase seed rot and seedling blight, as will planting seed too deep in wet, heavy soils. Hybrids differ in genetic resistance to the fungi that cause seed rot and seedling blight. Seed treatment with a good fungicide is an important method for control of these fungi.
Southern Corn Leaf Blight (caused by the fungus Bipolaris maydis [Helminthosporium maydis]). Southern corn leaf blight occurs worldwide, but is particularly damaging in regions of warm, moist weather. Lesions on the leaves (Fig. 2) caused by the fungus are elongated between the veins, tan, up to one inch long, with limited parallel margins and buff to brown borders. The fungus overwinters on corn debris in the field. Thus, rotation and destruction of residue will reduce losses due to this disease. Resistant hybrids are also available.
Northern Corn Leaf Blight (caused by the fungus Exserohilum turcicum [Helminthosporium turcicum]). Symptoms of this disease (Fig. 3) are long elliptical, grayish-green or tan lesions ranging from 1 to 6 inches in length, developing first on lower leaves and later causing severe damage to the upper leaves under moderately warm and moist weather conditions. This disease is favored by somewhat cooler weather than southern leaf blight and has been quite severe in the mountain counties. Northern corn leaf blight can cause premature death and gray appearance of foliage that resembles frost or drought injury. As with southern corn leaf blight, control is by rotation, destruction of crop debris, and use of resistant hybrids. There are at least three pathogenic races of the fungus, but moderate to good resistance is available to all of them.
Anthracnose (caused by the fungus Colletotrichum graminicola). Symptoms of this disease vary widely, depending on the hybrid, age of the leaf, and environment. Small, oval to elongate, water-soaked spots first appear on the leaves at any stage of growth. The spots may enlarge up to one-half inch long and become tan at the center with red, reddish-brown, or yellow-orange borders. The lesions may grow together, blighting the entire leaf. Leaf symptoms are most common early in the season on the lower leaves and late in the season on the upper leaves.
Lesions on stalks (Fig. 4) usually appear initially as black linear streaks under the epidermis. On susceptible plants the lesions may develop into large oval, black areas measuring 1/2 to 1 inch, or larger. In severe infections, large areas of the stalk may be blackened. When the infected stalks are split, a mottled brown discoloration may be seen, particularly at the nodes. This discoloration may be present even when lesions are not apparent on the surface of the stalk. It is common with anthracnose for the upper 1/3 of the plant to prematurely die. Anthracnose is a very important cause of lodging in North Carolina.
Anthracnose is favored by warm, moist conditions during the growing season. Plants are most susceptible in the seedling stage and later as they approach maturity. There is a wide range of susceptibility in hybrids. The fungus over-winters on plant debris left above ground. Thus, control of this disease is based upon crop residue destruction, rotation, and use of tolerant or resistant hybrids.
Southern Rust (caused by the fungus Puccinia polysora). Southern rust can be recognized by the bright orange or golden brown, circular to oval pustules, which give a rusty appearance to the leaves (Fig. 5). The pustules are about the size of a pinhead and are filled with powdery masses of orange spores, which can be rubbed off. These spores are readily dislodged and blown about in the wind. The spores can survive and infect plants after being transported hundreds of miles by the wind.
The southern rust fungus has no known means of survival in the absence of living susceptible plants. During the winter months it is limited to tropical areas where corn is grown year round. The extent to which it spreads into temperate areas depends upon weather patterns and the susceptibility of the corn along the path of spread.
Southern rust is favored by the warm, humid conditions found in many lowland tropical areas where corn is grown. However, even in those areas, corn with good resistance suffers little or no damage. In temperate areas less ideal for the growth of the fungus, damage can occur in corn hybrids that lack good resistance.
Since southern rust cannot survive the winter in North Carolina, the initial infections must result from spores blown into North Carolina from the south. The fungus can multiply very rapidly on susceptible corn, and the amount of damage that occurs depends upon how early the first spores arrive. Epidemics may result from unusual weather patterns that cause mass air movements from the tropics where the rust is present.
Common Rust (caused by the fungus Puccinia sorghi). Common rust occurs in temperate to sub-tropical areas. It differs from southern rust by the darker, more reddish-brown color of the pustules. Also, pustules of common rust tend to be longer than those of southern rust and they occur more often in scattered clumps on the leaves. Pustules of southern rust are usually quite uniformly distributed over the surface of the leaf. Common rust is able to survive the winters in temperate areas because it produces teliospores, which are resistant to weathering. These spores germinate in the spring to produce basidiospores. The basidiospores infect wood sorrel (Oxalis spp.) and the spores produced in infections on wood sorrel complete the life cycle of the fungus by infecting corn.
Common rust has been present for many years in all major corn producing areas of the world. It has not been regarded as a major cause of damage in any of those areas. In 1951 in one of the heaviest outbreaks of common rust known in the United States, estimated average losses ranged from less than 1 percent to 3 percent. Resistance and tolerance to common rust are prevalent and effective in corn hybrids throughout the world.
Common Smut (caused by fungus Ustilago maydis). Common smut occurs wherever corn is grown. Losses to smut are generally light, but may be important in some situations, particularly sweet corn. Young actively growing parts of the plant are susceptible to infection. Large galls may appear on stalks at the nodes, on ears (Fig. 6), or rarely on tassels. Leaf infections may result in small inconspicuous galls. On ears or stalks the galls expand rapidly and are covered with a thin greenish-white or silvery-white tissue. As the galls mature, the covering ruptures exposing masses of black spores within. Individual galls on stalks may be up to 6 inches in diameter. On infected ears, a large number of galls originating from individual infected kernels may combine to form the compound gall mass that replaces most of the ear. Some corn plants may form ears in the tassles.
Smut is usually more severe on plants heavily fertilized with nitrogen. The severity is increased by injury from hail, cultivators, etc. Control involves avoiding highly susceptible varieties, avoiding mechanical injury to plants during cultivation and spraying, and providing well-balanced soil fertility.
Gray Leaf Spot (caused by the fungus Cercospora zeae-maydis). The fungus can infect leaf blades and, to a much lesser extent, leaf sheaths. The gray or pale brown lesions are long and narrow with parallel sides delimited by leaf veins (Fig. 7). The ends are usually blunt, giving the lesions a long rectangular shape. Lesions commonly are about 1/4 inch wide by about 1 inch long. When the disease is severe, lesions merge into long stripes. Eventually the entire leaf may be killed.
Gray leaf spot has caused moderate to severe damage to corn in the mountain valleys of the Appalachian region. In North Carolina, the disease is most severe in the mountains and western piedmont, but it has become common in the coastal plain and tidewater in recent years.
The gray leaf spot fungus survives the winter as resistant mycelium in corn debris left in the field. The disease is usually more severe in no-till planted corn without rotation. Thus, rotation, debris destruction, and resistant hybrids offer the best methods of controlling this disease.
Brown Spot (caused by the fungus Physoderma maydis). Brown spot is favored by high temperatures and high humidity. It attacks leaf blades, sheaths, and stalks, producing small, reddish-brown to purplish-brown spots which may merge together to form large brown blotches (Fig. 8). Weakened stalks frequently lodge and leaf sheaths may be reduced to shreds. Good cultural practices and the use of tolerant varieties offer the best control.
Stalk Rots (caused principally by the fungi Stenocarpella zeae and species of Fusarium as well as Colletotrichum graminicola). Stalk rots are present each year and may cause considerable damage, particularly if abundant rainfall occurs during the latter part of the growing season. Stalks previously injured by cold, leaf diseases, or insects are especially susceptible to attack by these fungi. Diseased stalks ripen prematurely and are subject to excessive stalk breaking (Figs. 9-10). Stalk rots not only add to the cost of harvesting but also bring the ears in contact with the ground, increasing their chance of rotting.
Charcoal rot (caused by the fungus Macrophomina phaseolina). Charcoal rot is a destructive disease of corn, soybean, cotton and many other crops. Charcoal rot becomes most evident with the onset of hot dry weather. It may cause a stalk rot, stunting, and death of the corn plant. Symptoms are a silver to black discoloration of the stem tissue when the stalk is cut open (Figs. 10 and 11). This disease is often considered to be a stress related. Typically when this disease occurs in North Carolina soil fertility and pH are at very low levels. Although the fungus typically survives in the soil, rotation is not generally an option since most crops are susceptible to this disease. Reduction of nutrient and water stress are the principle means of control. Hybrid resistance has not been documented.
Ear and Kernel Rots (caused by species of Stenocarpella, Fusarium, Aspergillus, and many other fungi). Some of the same fungi that cause stalk rots of corn also (Fig. 12) also cause ear and kernel rots (Fig. 13). Ear and kernel rots are most serious with warm, wet conditions at harvest time. Severe infection not only reduces yield but also lowers the quality and grade of the grain produced. The two principle ear and kernel rot fungi found in North Carolina are Aspergillus and Fusarium.
Red ear rot is caused by the fungus Fusarium graminearum (Gibberella zeae), and also causes a stalk rot of corn and head scab in wheat. The fungus may cause a reddish discoloration of the cob and kernels. Red ear rot caused by F. graminearum is favored by warm wet weather after silking. Disease tends to be worse when corn is grown without rotation or after wheat, as this pathogen also infects wheat. It may be worse when corn is grown in reduced tillage situations.
Fusarium moniliforme is another species of Fusarium that causes a kernel rot, but the mycelium of the fungus is typically white to salmon color. Kernels infected with Aspergillus usually have a greenish to gray color.
Mycotoxins in Corn
Toxic metabolic by-products of fungi, known as mycotoxins, have received considerable attention during the past several years. Aflatoxin, produced by the fungus Aspergillus flavus, has been considered to be the most serious problem in North Carolina in recent years (Fig. 14). The detection of aflatoxin in corn can result in a reduced price for the grain or even rejection. The concentration of aflatoxin in corn for interstate trade is regulated at 20 parts per billion (ppb) by the Food and Drug Administration (FDA). Another class of mycotoxins are referred to as fumonisins. These toxins are produced by the fungus Fusarium moniliforme and are quite common in corn produced in North Carolina. It appears likely that fumonisin levels will be regulated in the near future. Corn shipped to Europe will probably be monitored for levels of fumonisin as well as aflatoxin. Allowable levels of fumonisin in corn have not yet been established at this time.
Mycotoxins are known to cause serious health problems in animals including reduced weight gain, capillary fragility, reduced fertility, suppressed disease resistance, and even death. No animal is known to be resistant, but in general, older animals are more tolerant than younger animals. Mycotoxins have been implicated in deaths from acute toxicoses in young animals, particularly poultry, as well as several animal health problems, including reduced fertility and growth rate.
Both Aspergillus flavus and Fusarium moniliforme are widely distributed in nature and are favored by high temperature. Temperatures ranging from 80 to 100 degrees F and a relative humidity of 85 percent (18 percent moisture in the grain) are optimum for fungal growth and toxin production. Growth of these fungi does not occur below 12 to 13 percent moisture in the grain.
Aflatoxin contamination is higher in corn that has been produced under stress conditions. Thus, drought, heat, insect, and fertilizer stress are all conducive to high levels of aflatoxins. Factors that influence fumonisin production in corn are not well understood at this time. Certainly, insects provide an avenue of infection for both Aspergillus and Fusarium. High rainfall and humidity at silking may increase infection of corn kernels by Fusarium spp. Hybrids genetically engineered to resist insects have been shown to have lower levels of fumonisin. Therefore, in order to minimize the level of mycotoxins, the following practices should be followed:
Nematodes attack corn roots, thereby limiting their development and restricting the uptake of water and nutrients. Thus, affected plants are stunted and appear deficient of nutrients. Since nematodes do not occur at a uniform population density throughout the field, stunted plants likewise are not uniformly distributed. They often appear in roughly circular areas in the field (Fig. 15). Nematode damage occurs most often when the preplant densities of certain nematodes are high and corn seedlings get off to a slow start because of unfavorable growing conditions. Damage to corn from plant-parasitic nematodes is most severe in the coastal plain area. The two most damaging nematodes on corn in North Carolina are the stubby-root and sting nematodes.
Stubby-root (Paratrichodorus minor). The stubby-root nematode does not enter the roots of corn plants, but remains outside the roots and feeds on the growing root tips. Their feeding prevents the further development of the root tip, resulting in short, stunted or stubby roots (Fig. 16). The damage to the root system by stubby-root nematodes resembles that caused by several herbicides. A plant heavily parasitized with these nematodes is stunted, turns yellow, often exhibits magnesium deficiency, and produces a small ear. Since these nematodes are so widespread in the coastal plain area, they may very well be the most damaging nematodes on corn in North Carolina.
Sting (Belonolaimus longicaudatus sp.) - The sting nematode feeds on roots from the outside without penetrating or becoming attached to roots. They feed at root tips and along the sides of succulent roots. Injured roots show blackened, sunken dead areas along the root and at the root tip. These areas may girdle the root causing it to die. Sometimes the damage done to young plants is quite severe and infected plants may obtain a height of only 8 to 10 inches. Sting nematode is found in soils that contain at least 80 percent sand. This nematode, especially when combined with the stubby-root nematode, causes severe yield losses.
Columbia lance (Hoplolaimus Columbus). The Columbia lance nematode can be damaging to corn, especially if numbers are high and poor conditions for early corn growth occur. The Columbia lance nematode is damaging to corn, whereas a related species of lance nematode, Hoplolaimus galeatus, does not generally affect corn. Currently, the Columbia lance nematode is restricted to sandy soils in parts of the southeastern North Carolina coastal plain, whereas the common lance nematode (Hoplolaimus galeatus) is found in many parts of the state. Lance nematodes feed on the root surface but may also penetrate the root system causing internal damage. Columbia lance nematode can be extremely damaging to cotton and soybean, but usually causes only slight-to-moderate damage on corn.
Root-knot and Lesion (Meloidogyne and Pratylenchus spp.). Most species of root-knot and lesion nematodes will reproduce on corn. Ordinarily, corn is very tolerant of these nematodes, but may be damaged if populations are very high.
In order to determine whether or not a field should be treated with a nematicide to control nematodes, a soil sample should be collected in September-November and sent to the N.C. Department of Agriculture for an assay. Soil cores collected from the corn root zone need to be taken in a zig-zag pattern across the field and mixed in a bucket. Each sample should cover 5 to10 acres, and sections with different crop histories should be sampled separately. Samples are sent to the North Carolina Department of Agriculture Nematode Advisory and Diagnostic Lab, 4300 Reedy Creek Rd., Raleigh, NC, 27607-6465. There is a $2.00 charge per sample. The Nematode Advisory Service will make a recommendation for nematode management based on the thresholds in Table 7-1.
Based on estimates from the NCDA&CS, about a third of the corn acreage in eastern North Carolina should be treated to control nematodes. Where the population density is high enough to justify treatment, a grower can expect an increase of about 20 to 25 bushels per acre. Nematodes are controlled by use of a nematicide, rotation, and crop destruction.
There are two major viruses of corn in North Carolina, maize dwarf mosaic virus (MDMV) and the maize chlorotic dwarf virus (MCDV). These two virus diseases can cause serious yield reductions, with reported losses ranging from 5 to 90 percent in some fields. Much of the loss due to these two diseases in North Carolina is confined to the piedmont section of the state, although losses in the coastal plain and mountain areas have been reported. This may be due to two factors:
The two viruses are transmitted from infected johnsongrass to corn by insects. MDMV is transmitted by aphids (principally the corn leaf aphid, Aphid maidis and MCDV is transmitted by leafhoppers (Graminella nigrifrons).
Table 7-1. The probability of nematode damage to corn.
* Any detectable number can pose a serious problem.
Maize Dwarf Mosaic Virus. Symptoms of MDMV first appear on the youngest leaves as an irregular, light and dark green mottle or mosaic which may develop into narrow streaks along veins that appear as dark green "islands" on a lighter green background. As infected plants mature, leaves become yellowish-green. Plants with these symptoms are sometimes stunted with excessive tillering, multiple ear shoots and poor seed set. Early infection may predispose corn to root and stalk rots and premature death. Symptoms can appear in the field within 30 days after seedling emergence.
Maize Chlorotic Dwarf Virus. MCDV, previously called corn stunt, causes more severe stunting than does MDMV. Infected leaves become yellow, but no mosaic pattern develops. Such leaves usually develop a deep, reddish discoloration later in the season. The internodes of infected plants fail to elongate, resulting in very stunted plants (Fig. 17). Quite often infection occurs late in the season. Thus, the lower portion of the plant develops normally with the upper portion being red and stunted. Infection can result in severe reduction in ear size if susceptible varieties are grown and infection occurs early enough in the development of the plant.
Losses from both MDMV and MCDV can be avoided by growing hybrids that are resistant, or tolerant, to these viruses. There are several hybrids adapted to North Carolina that are resistant to both viruses.
There are two major bacterial diseases of corn in North Carolina, bacterial leaf blight (sometimes called Stewart's bacterial wilt) and bacterial stalk rot.
Bacterial leaf blight (caused by the bacterium Erwinia stewartii) is more of a problem with sweet corn than it is with field corn; however, it can be a problem with certain hybrids. The symptoms are short to long, irregular, pale green to yellow streaks in the leaves (Fig. 18). The streaked areas, which die and become straw-colored, originate from feeding marks of the corn flea beetle. Sometimes entire leaves die and dry up. When leaves die prematurely, yield is reduced and weakened plants become more susceptible to stalk rots. The bacteria over-winter in corn flea beetles, which also spread the bacteria. Although insect control is important in controlling this disease in sweet corn, it is not a sound practice for field corn producers. Resistance to the disease, which is available in many hybrids, is the preferred method of control.
Bacterial stalk rot (caused by the bacterium Erwinia chrysanthemi pv. zeae) can be a problem where overhead irrigation is used and the water is pumped from a lake, pond, or slow-moving stream. Quite often the infection occurs at about ear height, and the upper portion of the plant breaks over due to a collapse of the stalk (Fig. 19). Often, an unpleasant odor is associated with this disease. The bacteria usually do not spread from plant to plant, so diseased plants are quite often found scattered throughout the field.
This page created by: Alan D. Meijer, Agricultural Research Technician II on 07/24/00 and last revised on 7/26/00