Integrated Crop Management (ICM): A management approach that recognizes that all crop production practices have effects on each other, and on the interaction of crops with pests and diseases. Risks and benefits of each practice need evaluation.

Host

Crop selection
Resistant cultivars
Adapted cultivars
- maturity
High quality seed
Proper fertility

Pathogen

Crop rotation
Pathogen-free seed
Control volunteers
Weed control
Fungicide
Biological control

Tillage
Reduced tillage
No tillage

Environment

Proper fertility
Planting date

Stand density
Seeding rate
Row spacing

Hybrid or Variety Selection
- Most economical and effective way to control or reduce most diseases
- Genetic concepts relating to disease management:
  1. Single-gene resistance
  2. Partial resistance
  3. Tolerance
  4. Physical attributes
  5. Maturity
Rotation
- Brown stem rot in corn/soybean rotations
Tillage and Crop Residue Management
- Tillage increases soybean cyst nematode
- Residue increases survival of pathogens on the soil surface
Cultural Practices
- Row spacing and white mold
Fertility Interactions
- Corn rust, Gray leaf spot, and common smut and high N levels
Insect Interactions
- Flea beetle and Stewart's wilt
- Aphids and barley yellow dwarf
Weed and Herbicide Interactions
- White mold and weed hosts
Other Methods for Controlling Diseases
- Seed Protection
  - Seed testing and Cleaning
  - Seed Treatments
- Foliar and Soil Fungicides and Nematicides
  - Efficacy of Foliar Fungicides
- Biological Control
- Biotechnology

Common Diseases in Wisconsin

Corn Disease Management National Corn Handbook - 4

SEEDLING DISEASES - Pythium (downy mildew) disease is most common

Problem in cool, wet springs

LEAF DISEASES - Usually not a major problem

Leaf rust - Usually not a problem in dent corn
SCLB - Last major yield-reducing leaf disease; it occurred in 1970
leafspots and blights (anthracnose leaf blight, eyespot, gray leaf spot, helminthosporium leaf spot, northern corn leaf blight)

STALK DISEASES usually the main corn disease problem

Diplodia Stalk Rot- Nodes are infected
Gibberella Stalk Rot - internodes are infected
Charcoal Rot

Corn Stalk Rots

Any factors which stress corn during the growing season may contribute to an increase in stalk rots that season. And this has certainly been a season of stresses for corn in Missouri with late planting due to wet soil conditions, flooding, cool temperatures, high temperatures, high night temperatures, cool night temperatures, overcast days, moisture stress, heavy rains, hail, some foliage diseases, etc. Therefore, it would be wise to scout fields for corn stalk rots and to harvest fields with stalk rot problems as quickly as possible.

A number of different fungi and bacteria cause stalk rots of corn. Although many of these pathogens cause distinctive symptoms, there are also general symptoms which 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.

Fusarium stalk rot and Gibberella stalk rot can be difficult to distinguish in the field. Both can cause a pink to reddish discoloration of diseased stalk tissue. Tufts of white mycelium may be evident at the nodes of diseased stalks. When stalks are split open the pith is usually shredded and discolored.

Anthracnose stalk rot, caused by the fungus Colletotrichum graminicola, may be most evident at the nodes. Initially lesions are tan to reddish-brown but they become shiny black later in the season. These shiny black lesions may begin at a node and extend out from that node. The lesions may merge to discolor much of the lower stalk tissue. Internal pith tissues may also be discolored and may disintegrate as disease progresses.

Diplodia stalk rot may begin as a brown to tan discoloration of the lower internodes. Stalks become spongy. The pith disintegrates leaving only the vascular bundles. Mats of white fungal growth of Diplodia maydis may be evident on affected tissues. Diplodia also produces fruiting bodies which may be seen as small black specks embedded in the white fungal mat. Diplodia also causes an ear rot of corn. Diplodia ear rot has been found in scattered fields across the state so Diplodia stalk rot could also occur this season.

Charcoal rot may begin as a root rot and move into the lower internodes of the stalks. Pith tissues will be shredded and plants may break at the crown. The charcoal rot fungus, Macrophomina phaseolina, produces very small survival structures called microsclerotia which may be visible as very small, black flecks just beneath the stalk surface or on the vascular strands remaining in the interior of the shredded stalks. Charcoal rot is usually more severe under hot, dry conditions, so this corn stalk rot could be a serious problem in areas of the state which suffered from drought conditions the latter part of the growing season.

Stalk rots are caused by several different fungi and bacteria which 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, 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 of stalk rots of corn should include the following:

Select hybrids with good stalk strength and lodging characteristics.
Plant at recommended plant populations for that hybrid.
Follow proper fertility practices.
Avoid or minimize stress to corn (especially during pollination and grain fill).
Harvest in a timely manner.

EAR DISEASES - usually minor in importance

Smut - may reduce feeding value
Diplodia and Gibberella Ear Rots
Fusarium Kernel Rot - present in all corn fields, but rarely causes problems

Ear and Kernel Rots of Corn

Corn harvest is beginning or rapidly approaching in many parts of the state. So far we have received only a few samples with ear and kernel rots. Usually Diplodia ear rot, Gibb ear rot, Penicillium ear rot and Aspergillus species other than Aspergillus flavus are associated with wet conditions, especially wet falls and harvests that are delayed by wet conditions. Although much of the state has been unusually dry recently, there have already been fields in which these ear and kernel rots are present. In some cases the ears showing molds had been damaged by insects or hail and the molds had come in around the damaged areas. But in other cases Penicillium and Fusarium are showing up on the tips of the ears. And then in fields in which hot, dry conditions occurred at or just after pollination, Aspergillus flavus and aflatoxin could be problems.

Diplodia ear rot, Penicillium ear rot and Gibb ear rot are common problems year in and year out but the severity varies with weather conditions at pollination or close to harvest. The Penicillium ear rot and Gibb ear rot are particularly evident on the exposed tips of ears, around insect tunnels and on ears that have remained upright. If there are periods of wet weather before corn is harvested, some of the corn plants that died prematurely may show the black discoloration caused by secondary fungi coming in on the senescing plant tissues. Because of the wet conditions early in the season, there was a wide range of planting dates for corn. Unfortunately, there were some fields which were silking and pollinating during the stretch of hot, dry weather about mid-season. These fields might be at risk for the occurrence of Aspergillus flavus and aflatoxin.

Diplodia Ear Rot
Both Diplodia maydis and Diplodia macrospora can cause Diplodia ear rot of corn. The ear leaf and husks on the ear may appear prematurely bleached or straw-colored. When the husk is peeled back, dense white to grayish-white mold growth will be matted between the kernels and between the ear and the husks. Small, black fungal fruiting bodies may be scattered on husks or embedded in cob tissues and kernels. The entire ear may be grayish-brown, shrunken, very lightweight and completely rotted. Diplodia ear rot is favored by wet weather just after silking and is more severe when corn is planted following corn.

Penicillum and Aspergillus niger ear rots
Penicillium rot is usually evident as discrete tufts or clumps of a blue-green or gray-green mold erupting through the pericarp of individual kernels or on broken kernels. Penicillium appears as small, discrete colonies of mold growth with a dusty or powdery appearance. The fungus may actually invade the kernel giving the embryo a blue discoloration. Blue-eye is the term used for this blue discoloration of the embryo.

Gibb ear rot (caused by Gibberella zeae) usually begins as a reddish mold at the tip of the ear. Early infected ears may rot completely with husks adhering tightly to the ear and a pinkish to reddish mold growing between husks and ears. Although mold growth usually has a pinkish to reddish color, it can appear yellow to yellow-orange or yellow-red. Gibb ear rot typically begins at the tip of the ear but under favorable conditions it can move down the ear causing extensive damage. It may also develop around injuries from hail, birds or insects.

Aspergillus niger
Aspergillus niger is also common on exposed ear tips. This fungus will be evident as black, powdery masses of spores on the tip of the ear or around insect tunnels.

Black Corn
Black corn occurs when any of a number of saprophytic or weakly parasitic fungi grow on corn plants in the field. Alternaria, Cladosporium, Aureobasidium and other species are frequently found on these discolored or black plants. Since the affected plants may have a sooty appearance these fungi are sometimes called sooty molds. These sooty molds or secondary fungi tend to develop on plants when wet or humid weather occurs as the crop is maturing or if harvest is delayed because of wet weather. Typically these fungi come in on plants that are shaded, undersized, weakened or prematurely ripened and on senescing foliage. Plants that are lodged or that have been stressed by nutrient deficiencies, plant diseases or environmental conditions may be more severely affected. Although many of these fungi produce dark or black mold growth, the color of the mold growth can range for dark or black to olive green or even pink to white.

These secondary fungi tend to develop on senescing plant tissues, primarily leaf, stalk and husk tissue, but under favorable conditions can cause infection of the kernels. Infected kernels might show a black discoloration.

It is possible that these sooty molds or secondary fungi could contribute to stalk deterioration or stalk rot. Lodging could become a problem in these fields, especially if there are high winds or strong storms before harvest.

Grain from fields with high levels of sooty molds should be treated with care if it is stored. Grain should be thoroughly cleaned to remove lightweight, damaged or broken and moldy kernels. Grain should be stored at the proper moisture content and temperature and checked on a regular basis during storage.

Aspergillus flavus
Aspergillus flavus is evident as greenish-yellow to mustard yellow, felt-like growth on or between kernels, especially adjacent to or in insect damaged kernels. Aspergillus flavus is favored by high temperatures and dry conditions, so Aspergillus ear rot is typically associated with drought stress. The fungus survives in plant residues and in the soil and spores are spread by wind or insects to corn silks where the spores initiate infection.

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. Aspergillus and Gibberella are most frequently involved in cases of mycotoxin contamination in Missouri corn. The presence of molds or their spores 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. In cases where mycotoxin problems are suspected, a sample should be submitted to a qualified laboratory for mycotoxin analysis. Table 1 below gives the acceptable levels of aflatoxin in corn intended for various uses as established by the United States Food and Drug Administration.

Table 1. Present acceptable levels of aflatoxin in corn used for food and feed as established by the United States Food and Drug Administration (FDA) are as follows:

Corn containing no more than 20 ppb of aflatoxin when destined for food use by humans, for feed use by immature animals (including immature poultry) and by dairy animals, or when the intended use is unknown.
Corn containing no more than 100 ppb aflatoxin when destined for breeding beef cattle, breeding swine or mature poultry (e.g. laying hens).
Corn containing no more than 200 ppb aflatoxin when destined for finishing swine (e.g. 100 lbs. or greater).
orn containing no more than 300 ppb aflatoxin when destined for finishing (i.e. feedlot) beef cattle.

Little can be done to prevent or reduce the invasion of corn by fungi in the field. These ear and kernel rots tend to be more severe on ears with insect, bird, hail or other physical 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. However, if ear and kernel rots developed in the field, it is important to harvest the field in a timely manner and to store the grain under the best possible conditions. Both Penicillium and Aspergillus can continue to develop on corn in storage if the grain is not stored at proper moisture content and temperatures. These two fungi can cause serious storage mold problems.

Adjust harvest equipment for minimum kernel damage and maximum cleaning. Before storing grain, clean bins thoroughly to remove dirt, dust and any grain left in or around bins. Thoroughly clean grain going into storage to remove chaff, other foreign material and cracked or broken kernels. Dry grain to 15% moisture as quickly as possible and monitor grain on a regular basis throughout storage life to insure moisture and temperature are maintained at correct levels. Protect grain from insects.

OCCASIONAL DISEASES

rusts (common, southern rusts)
seed rots and seedling blights (damping off)
smuts (corn smut)
stalk and root rots (bacterial stalk rot)
Stewart's disease
virus diseases (corn lethal necrosis, maize dwarf mozaic virus)

NEMATODES (needle, root lesion, sting nematodes)

STORAGE DISEASES - Need to control moisture percentage to minimize storage diseases

Images of common corn diseases

Corn Diseases

Conditions necessary for disease development:

Common disease pathogens in the Midwest:

Managing Diseases (IPM versus ICM)

Hybrid or Variety Selection

Rotation

Tillage and Crop Residue Management

Cultural Practices

Fertility Interactions

Insect Interactions

Weed and Herbicide Interactions

Other Methods for Controlling Diseases

Common Diseases in Wisconsin

Further Reading