Corn virus diseases

Maize dwarf mosaic is a virus disease of corn that is spread by several species of aphids. Although the symptoms of maize dwarf mosaic are highly variable, the most common symptom is a light green to yellow mottling or mosaic pattern in the leaf tissue.

Scattered, individual plants with symptoms of maize dwarf mosaic may be found in most years. Periodically weather conditions favor the large-scale movement of virus-carrying aphids from southern regions of the United States. These aphids may then “rain out” or be deposited in large numbers in fields in Missouri or more northern areas of the Corn Belt. Under these conditions, maize dwarf mosaic virus may be prevalent and serious over a significant acreage.

Maize dwarf mosaic is caused by several strains of the maize dwarf mosaic virus (MDMV). Corn and sorghum are the main crop hosts of MDMV; however, johnsongrass and other wild grasses are also hosts. Some strains of MDMV overwinter in johnsongrass and are spread from the johnsongrass to corn by the aphid vectors. More than 15 species of aphids can transmit MDMV. Many commercial corn hybrids have high levels of tolerance to MDMV.

Maize chlorotic dwarf is caused by a virus that is spread by the leafhopper, Graminella nigrifrons. Again, symptoms of maize dwarf chlorotic are highly variable. The most characteristic symptom of maize chlorotic dwarf is veinbanding or veinclearing, but other symptoms that may occur include reddening or yellowing of leaf tissue, distortion of leaf tissue and shortening of the upper internodes of the plant.

The maize dwarf chlorotic virus can overwinter in johnsongrass. It is transmitted to corn by its leafhopper vector. Although proso millet, pearl millet, sorghum, Sudan grass and wheat are also hosts of the maize chlorotic dwarf virus, corn appears to the principal host in the field.

Management options for corn virus diseases

Plant resistant or tolerant hybrids.
Plant early.
Maintain good weed control, especially of johnsongrass.
In some cases, control of the insect vectors through application of an appropriate insecticide may be warranted.

Crazy top of corn

Crazy top of corn is caused by the downy mildew fungus, Sclerophthora macrospora. The pathogen is a soilborne fungus that causes infection when corn plants are subjected to saturated soil conditions for 24-48 hours from planting to about the five-leaf stage of growth.

The disease causes a deformation of plant tissues, including excessive tillering and rolling or twisting of leaves. On severely infected plants, leaves may be narrow and straplike. There may be a proliferation of the tassel until it resembles a mass of leafy structures. Plants may produce numerous ear shoots. Infected plants are frequently stunted.

In seasons with wet springs, young corn plants subjected to saturated soil conditions may show symptoms of crazy top. Occasionally a band of affected plants may encircle a drowned out spot in a field. Some hybrids may be more susceptible to crazy top. This disease is seldom severe enough to cause significant losses.

Management options for crazy top

Provide good soil drainage.

Stewart's bacterial wilt of corn

Stewart's bacterial wilt of corn is caused by the bacterium Erwinia stewartii and is spread by corn flea beetles. Foliage symptoms include linear, pale green to yellow streaks that tend to follow the veins of leaves and originate from feeding marks of the corn flea beetle. These streaks soon become dry and brown and tend to be irregular and variable in size and shape.

The bacterium that causes Stewart's bacterial wilt overwinters in the guts of corn flea beetles. Adult corn flea beetles feed on corn seedlings in late spring and early summer and contaminate the feeding wounds with the bacterium. Warm winter weather conditions favor the survival of the corn flea beetle and disease development the following spring. Cold winters reduce beetle populations and limit disease development and spread.

Although the foliage symptoms of Stewart's bacterial wilt are common on field corn in Missouri, the damage is seldom of economic significance. Stewart's bacterial wilt can be especially destructive on some sweet corn hybrids and corn inbreds.

Management options for Stewart's bacterial wilt

Plant resistant hybrids. Most commercial field corn hybrids have good tolerance to Stewart's bacterial wilt.
Maintain good weed control in and around corn fields.
Although insecticide applications to control the flea beetle vector may be warranted on sweet corn and corn inbreds, decisions to treat flea beetles on field corn should be based more on direct insect feeding damage than potential damage from Stewart's bacterial wilt.

Common smut of corn

Common smut caused by the fungus Ustilago maydis results in the formation of galls on the aboveground portions of the corn plant. Initially the galls are firm and silver to grayish white in color throughout. As the galls age, the center of the gall turns into a mass of powdery, black spores while the outer covering of the gall remains silver to grayish white.

The black, resting spores (known as chlamydospores or teliospores) fall from the smut galls to the soil where they overwinter. The spores may be spread by surface drainage water, farm machinery, insects and wind. Under favorable conditions, the resting spores germinate and produce another type of spores (sporidia) which are spread by wind or splashing water to young, actively dividing corn tissues. Moisture is needed for the spores to germinate and penetrate the host, so rainfall or humid conditions are assumed to be critical during this phase of the disease cycle. The spores of the common smut fungus are able to infect only tissue that has been damaged by insects, hail, blowing soil particles, herbicides, detasseling or other factors, or young meristematic tissues (such as young silks, young cob tissues and young developing kernels). Visible galls may develop within a few days of infection.

Common smut usually causes only small yield losses (less than 2%), but in rare years it may cause yield losses of 10% or more depending on where gall formation occurs and the number of ears infected.

Management options for common smut

Plant tolerant or resistant hybrids.
Avoid mechanical injury to plants.
Maintained balanced fertility. Excessive nitrogen tends to increase disease incidence and severity.

Corn nematodes

Several species of nematodes, or microscopic roundworms, can cause damage on corn. Some corn nematode species spend most of their lives in the soil, while others live mostly in the roots. During feeding nematodes may directly harm plants or they may cause wounds through which fungi and bacteria can enter plants and cause secondary rots.

Corn nematodes
lesion nematode Pratylenchus spp.
stunt nematode Tylenchorhynchus spp.
lance nematode Hoplolaimus spp.
spiral nematode Helicotylenchus spp.
ring nematode Criconemella spp.
dagger nematode Xiphinema spp.
root-knot nematode Meloidogyne spp.
stubby-root nematode Paratrichodorus spp.
needle nematode Longidorus breviannulatus
sting nematode Belonolaimus spp.

The presence of nematodes in a field depends on the soil type and its properties, other soil microorganisms, cropping history, tillage, the use of pesticides and climatic factors such as temperature and rainfall. Although damage can occur in any soil type, corn growing in well-drained soils, especially sandy soils, is most susceptible to damage. In poorly drained soils, nematode populations usually increase slowly or may even decline. The extent of nematode damage is often related to the growing conditions of the plant. Corn that is stressed by poor fertility or lack of moisture cannot withstand the additional stress of nematode feeding and will show more pronounced symptoms.

It is difficult to generalize about the symptoms caused by nematodes because they vary with the nematode species, the number of nematodes present and the soil environmental factors. Aboveground symptoms are due to nematode injury to the roots. Early-season symptoms may include stunting or off-color leaves. Symptoms later in the season include a ragged or uneven appearance to the field, lodging, general unthriftiness and reduced yields. Common evidence of nematode feeding on roots includes root pruning, especially of feeder roots, proliferation of fibrous roots, thickening or swelling of the smaller roots, and slight to severe discoloration of roots. Damage may be localized in one part of a field or spread over large areas of a field. Because nematodes cannot be seen with the naked eye and because symptoms of nematode injury are easily confused with other types of corn production problems, nematode problems should be diagnosed by submitting soil and root samples to a laboratory qualified to run a nematode analysis on the samples.

Management options for corn nematodes

Rotate to a crop other than corn in fields with nematode problems. The length of the rotation may vary with nematode species and population levels.
Maintain good weed control.
Fertilize according to soil test recommendations, because corn suffering from improper fertility is more susceptible to injury from nematodes.
Although several nematicides are labeled for use on corn, economic and environmental concerns limit their use.

Corn stalk rots

Stalk rots are important worldwide and are among the most destructive diseases of corn. A number of different fungi and bacteria cause stalk rots of corn. Although many of these pathogens cause distinctive symptoms, certain general symptoms are common to all stalk rot diseases. Early symptoms, which occur a few weeks after pollination, usually start with premature dying of bottom leaves. Eventually, the entire plant may die and appear light green to gray. Diseased stalks usually begin losing firmness during August. The cells in the interior of the stalk are dissolved, resulting in a loss of stalk firmness and strength. Stalks may then lodge, particularly if harvest is delayed or wind storms occur.

Stalk rots are caused by several different fungi and bacteria that are part of the complex of microorganisms that decompose dead plant material in the soil. They survive from one growing season to the next in soil, in infested corn residues or on seed. Stalk rot pathogens enter the corn plant in a variety of ways. The spores may be blown into the base of the leaf sheath, where they may germinate and grow into the stalk. Spores may enter directly into a plant through wounds made by corn borers, hail or mechanical injury. When fungi are present in soil or infested residue as either spores or mycelium, they may infect the root system, causing root rot early in the growing season and later grow up into the stalk causing stalk rot.

Stalk rot becomes a problem when plants are stressed during the grain filling stage of development. Water shortage, extended periods of cloudy weather, temperature stresses, hail damage, corn borer infestation, low potassium in relation to nitrogen, leaf diseases and other stresses that occur in August and September may be associated with an increase in stalk rot.

Losses from stalk rots vary from season to season and from region to region. Yield losses of 10 to 20% may occur on susceptible hybrids. Tolls greater than 50% have been reported in localized areas. Losses may be direct losses due to poor filling of the ears or lightweight and poorly finished ears or indirect through harvest losses because of stalk breakage or lodging. Harvest losses may be reduced if fields are scouted 40-60 days after pollination to check for symptoms of stalk rot. Stalk rot can be detected by either pinching stalks or pushing on stalks. If more than 10-15 percent of the stalks are rotted, the field should be harvested as soon as possible.

Management options for corn stalk rots

Select hybrids with good stalk strength and lodging characteristics.
Plant at recommended plant populations for that hybrid.
Follow proper fertility practices.
Maintain good insect and weed control.
If irrigating, try to deliver optimum water from silking to late dough stage.
Avoid or minimize stress to corn (especially during pollination and grain fill).
Harvest in a timely manner.

Ear and kernel rots of corn

A number of fungi can invade and cause damage to corn ears or kernels. Field fungi invade the kernels before harvest while the corn is still in the field. These fungi may affect the appearance and quality of kernels. Usually damage caused by field fungi occurs before harvest, can be detected by routine inspection of corn in the field and does not continue to develop in storage if the grain is stored at proper moisture content and temperature. Some of the field fungi on corn in Missouri include species of Alternaria, Cladosporium, Aspergillus, Penicillium, Diplodia, Fusarium and Gibberella. Most of these fungi are more prevalent when rainfall is above normal from silking to harvest. One exception is Aspergillus flavus, which is favored by drought stress to corn during pollination and by warm temperatures as kernels mature. For all field fungi, damage tends to be more severe on ears with insect, bird or hail damage. Ears well covered by husks and maturing in a downward position usually have less rot than ears with open husks or ears maturing in an upright position. Some of these fungi, in particular species of Penicillium and Aspergillus, can also be problems on corn in storage. If grain is not stored at the proper moisture content and temperature, these fungi can cause extensive damage to the stored grain.

Mycotoxins associated with ear and kernel rots of corn

An additional concern with ear and kernel rots of corn is the possibility of mycotoxin production. Mycotoxins are naturally produced chemicals that in small amounts may be deleterious to animal or human health. Three genera of fungi – Aspergillus, Penicillium and Fusarium (Gibberella) – are most frequently involved in cases of mycotoxin contamination in corn. The presence of molds or their spores on or in corn does not necessarily mean that mycotoxins will be produced. Circumstances that favor mold growth may allow production of mycotoxins in some situations but frequently mold growth occurs with little or no mycotoxin production. Once formed, mycotoxins are stable and may remain in grain long after the fungus has died. In general, swine and poultry are more susceptible than ruminants to mycotoxin induced health problems at an equivalent dosage. Where mycotoxin problems are suspected, a sample should be submitted to a qualified laboratory for mycotoxin analysis.

Little can be done to prevent or reduce the invasion of corn by field fungi. However, the following recommendations should help minimize damage from field fungi on corn, especially corn going into storage.

Management options for corn ear and kernel rots

Plant locally adapted hybrids with husks that close over ear tips.
Plant at recommended plant populations for that hybrid and maintain good plant vigor over the growing season.
Use a balanced fertility program.
Select planting dates appropriate for your area.
Follow recommended management practices to limit damage by ear feeding insects.
If irrigating, try to deliver optimum water from silking to late dough stage.
Harvest in a timely fashion.
Adjust the harvesting equipment for minimum kernel damage and maximum cleaning.
Clean the grain and bins thoroughly before storage to remove dirt, dust and other foreign matter, crop debris, chaff and cracked or broken kernels.
Store grain in watertight structures that are free from insects and rodents.
Store grain at proper moisture content and temperature.
Monitor grain on a regular basis throughout storage life to ensure moisture content and temperature are maintained at correct levels.