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Chapter 3

1. Introduction
• Early Season Seed Corn Disease Surveys:
  Table 3-1, 2003
  Table 3-2, 2004
  Table 3-3, 2005
  Table 3-4, 2006
  Table 3-5, 2007
• Scouting Calendar
  Scouting Calendar as text
2. Seedling Diseases
• Table 3-6, Legend ,
  Insect and Disease Control:
  Untreated seed, Pretreated seed,
  Disease Control
3. Corn Leaf Diseases
• Late Season Seed Corn Surveys:
  Table 3-7,2002-2004
  Table 3-8,2005
  Table 3-9,2006
  Table 3-10,2007
• Anthracnose Leaf Blight
• See provincial trend graph
• Northern Leaf Blight
• See provincial trend graph
• Eyespot
• See provincial trend graph
• Bacterial Leaf Blight or Wilt (Stewart's Wilt)
• Table 3-11, Winter Air Temperature Index for Prediction of Stewart's Disease
• Table 3-12, Iowa State Method for Disease Forecasting
• See provincial trend graphs: Stewart's Wilt and Corn Flea Beetle
• Grey Leaf Spot
• See provincial trend graph
• Common Rust
• See provincial trend graph
• Smut, Common and Head
• See provincial trend graph
• Crazy Top Downy Mildew
• Maize Dwarf Mosaic Virus
  Sugar Cane Mosaic Virus
• Wheat Streak Mosaic Virus
4. Stalk Rots
• Anthracnose
• Gibberella, Fusarium and Diplodia
• Pythium
• See provincial trend graph
5. Ear Rots
• Fusarium
• Gibberella
• Diplodia
• See provincial trend graph
6. Other Problems
• Cold, Early-Season Cold
• Heat
• Flooding
• Drought
• Hail
• Birds
• Herbicide Injury

Introduction

There are many diseases that can attack seed corn plants. Distinguishing between disease and other problems in the field can be difficult and time consuming.  The majority of diseases are caused by infectious micro-organisms (pathogens) such as fungi, bacteria, nematodes and viruses while others are caused by environmental factors (non-infectious) such as nutrient deficiencies, temperature, soil moisture, light, pH, herbicide injury etc.  Even though it may be difficult at times to determine the root cause of a disorder, the process is important since the effectiveness of your disease control measures depends on a proper identification.  If for instance the problem was believed to be due to the environment or a chemical but in reality it was a disease, the cost would be in terms of disease control.  This improper identification could cost you time and money, not only the cost of the remedial measures you started, but the continued plant and yield losses from the original disease.

In addition, check for contributing factors since there may be several factors involved.  For example, dry hot conditions could increase a crops susceptibility to many diseases.  Determining which is the primary problem and which are contributing to the problem is important not only for the present situation but even more important for avoiding the same problems in the future.

Regular scouting of your seed corn fields will allow you to determine which diseases are present, at what levels and whether they are increasing.  This will provide you an opportunity to stay one step ahead and initiate control strategies to reduce these diseases.  It also gives you a way of verifying whether you are effectively managing the diseases or not.

See Scouting Calendar
See Scouting Calendar as text

Inbreds are generally more susceptible to certain diseases than commercial field corn.   Knowing that your inbreds are susceptible to certain pathogens (disease causing organisms) will assist in scouting and in effective disease management.  Some of the most important seed corn diseases that can cause substantial damage to susceptible inbreds are the foliar diseases which include northern leaf blight, common rust, grey leaf spot and anthracnose.  Seed corn companies often target fields with susceptible inbreds for increased scouting visits and are aware of potential disease problems.  Inbreds that are particularly susceptible to certain diseases may benefit from a fungicide application even when disease severity is low.

As the Early Season Seed Corn Disease Survey illustrates, there are many pathogens or disease causing organisms that are associated with seed corn plants in Ontario.  Their relative frequency can change from year to year depending on environmental conditions, inbreds planted (susceptiblility), drainage, tillage, rotation, seed treatment, and other factors. Understanding how to distinguish between each of them and how each of these pathogens cause disease are important in order to properly manage them and minimize losses in seed corn production.

See Early Season Seed Corn Disease Surveys:
Table 3-1, 2003
Table 3-2, 2004
Table 3-3, 2005
Table 3-4, 2006
Table 3-5, 2007

seedling diseases

Cool or wet conditions that reduce or delay corn germination or seedling development can also lead to early-season seed rots, seedling blights and/or root rots. Poor stand establishment, non-uniform emergence, “gaps” or missing plants are primary symptoms of seed or seedling infection. There are several different fungi that can cause these diseases. The most common are Pythium, Fusarium, Gibberella, Trichoderma and Penicillium, but other fungi such as Diplodia and Rhizoctonia could be involved. All of these organisms are found in Ontario soils. Depending on the year and field conditions, their impact ranges from minor to severe (replant). Low-lying or poorly drained areas of the field are often the first to show disease problems. Seed rots and seedling blights are more severe in no-till or reduced tillage fields since heavy residue will keep soil temperatures cooler and wetter longer than conventional fields. Damping-off will occur in conventional fields when the crop is planted early into conditions that favour disease development or when environmental conditions cause the corn seed to sit in the ground for a prolonged period of time. Other factors that delay germination and emergence such as compaction, crusting, deep planting, etc., can also result in a poor stand. Plant vigour is often reduced in those plants that do survive. It is important to distinguish between seedling diseases and other potential problems such as insects, herbicide injury, soil compaction, etc.

Symptoms

Seed rots are diseases that affect seeds prior to or shortly after germination. In this case, the seed or the developing embryo has rotted and died. Seeds that have been damaged or have poor seedling vigour are the most susceptible to seed rots, particularly when soil conditions are cool (10°C-13°C or 50°F-55°F) and wet for an extended period of time after planting. Seedlings that take a long time to emerge are most susceptible to fungal infection.

Seedling blights or “damping-off” are characterized into two groups, preemergence and postemergence seedling blights. Preemergence seedling blights affect young seedlings prior to emergence. Affected seedlings may die or grow slower than healthy unaffected seedlings. Postemergence seedling blights (damping-off) affect the roots or lower stems of young seedlings from emergence to second- or third-leaf stage.

Root rot causing organisms infect the seedlings’ root system, including lateral roots and root hairs. Affected plants may be stunted, off-colour or lack vigour. Infection can result in seedling death when disease infection is severe, and infected plants may be more susceptible to stalk rots later in the season.

Seed, seedling and roots infected by Pythium are most often soft (wet) and dark coloured as opposed to roots infected with Fusarium, Gibberella, Diplodia and Rhizoctonia, which are firm or leathery. The colour of the roots most often provides a good indication of which organism(s) are present: Greyish-white indicates Diplodia, tan to pink indicates Fusarium or Gibberella, reddish to brown indicates Rhizoctonia and blue-green indicates Penicillium or Trichoderma.

Disease Cycle

Pythium, Fusarium, Gibberella, Diplodia, Rhizoctonia, Penicillium and Trichoderma all live and thrive in the soil. In most cases, they can affect other crops beside corn. Except for Pythium, all of these organisms also have the ability to live on or in corn seed.

Management Strategies

Planting corn seed with good germination and seed vigour will substantially reduce your risk of seed rots and seedling blights. Seeds that are cracked or have been damaged through harvest or handling are most prone to these organisms and should be removed. Seed treatments will provide additional protection to young vulnerable seedlings. It is recommended that all seed corn be treated with a fungicide seed treatment to prevent early season preemergence and postemergence disease problems. For more information on seed treatments, refer to Table 3.1.Use other management tactics to improve soil structure, minimize soil compaction and remove excess soil moisture through tile drainage (create optimum germination and emergence conditions) to help reduce the severity of disease infections.

See Table 3-6 Legend for Table, Insect and Disease Control - Untreated seed, Pretreated seed, Disease Control

See Seedling Disease Images

corn leaf diseases

See Late Season Seed Corn Surveys:
Table 3-7, 2002-2004
Table 3-8, 2005
Table 3-9, 2006
Table 3-10, 2007

Anthracnose Leaf Blight

Anthracnose leaf blight (and the stalk rot phase) have been increasing in seed and commercial corn fields in the province. This is especially true for the southwest. In most years, Anthracnose leaf blight is the first corn leaf disease to develop and may become severe in warm, wet years.  It begins on the lower leaves, working its way up the plant. Symptoms often disappear as the corn plant begins its rapid growth phase. The fungus that causes anthracnose leaf blight is also responsible for anthracnose stalk rot. Producers should record where anthracnose leaf blight symptoms developed early in the season and return to those areas to scout for stalk rots a few weeks before harvest. Tillage systems that leave considerable amounts of anthracnose-infected debris on the soil surface may lead to greater severity of the disease and an increased presence in Ontario.  Basically,  bury disease infected debris to reduce the build up of this disease)

See Anthracnose Leaf Blight provincial trend graph

Appearance

Anthracnose may affect both leaves and stalks and the main symptoms are leaf spotting, top dieback and stalk rot.  Early Anthracnose leaf blight symptoms are evident on the lower leaves as small, oval or elongated semi-transparent (water-soaked) spots. These spots may form larger lesions (up to 15 mm long) with a tan center and a reddish-brown, purple or yellow border.  A general yellowing of the tissue surrounding the infected areas often develops. In severe cases, the entire leaf may become blighted as these larger lesions join, forming streaking along the leaf margin or midrib. Within the centers of these areas, concentric rings may be seen and with the aid of a hand lens, small black spots (ascervuli) can be seen in the centre of these lesions. As the fungus develops and begins to reproduce, dark, hair-like structures called setae can be seen protruding from these spots (view with a hand lens).

Anthracnose symptoms tend to disappear around the 6 to 8 leaf stage which coincides with the arrival of warmer temperatures in most years and rapid corn growth. Top dieback can occur late in the season as diseased leaves wilt and gradually die, taking on the appearance of frost damage.

See Anthracnose Leaf Blight images

Disease Cycle

Residue is an important component in anthracnose development, since the fungus survives (overwinters) as mycelium or sclerotia within corn residue or seed. Rain splashes spores from the corn residues onto the lower leaves and stalk. For this reason, corn fields that follow corn (second-year corn) are the most prone to anthracnose infection, especially when the weather is warm and wet.

Management Strategies

Planting anthracnose leaf blight–resistant hybrids can help to manage anthracnose leaf blight and reduce your risk in subsequent seed corn crops. If you have a  history of severe problems with this disease, notify your seed corn company, as this may help in selecting the best seed corn inbreds for your farm.

Resistance to anthracnose stalk rot is separate from resistance to anthracnose leaf blight. Hybrid resistance to anthracnose stalk rot does not guarantee resistance to early-season anthracnose infections on leaves. In conventional corn fields, removal of corn residues through tillage will lower your risk to the disease especially when corn follows corn. In no-till or reduced tillage fields, management of anthracnose leaf blight is best achieved with rotations (avoiding second-year corn) and planting of resistant corn hybrids. Fungicide applications are not economical in field corn situations because more than one application is necessary to control the disease. However, in seed corn fields, fungicide applications may be cost effective.

Northern Leaf Blight

Northern leaf blight has traditionally been one of the most damaging corn leaf diseases in Ontario and the US corn belt. Use of resistant hybrids has limited yield losses from this disease in commercial corn, however, significant losses continue to occur in seed corn production when highly susceptible corn inbreds are planted. A recent AAFC (Ottawa) and OMAF (Ridgetown) field and seed corn survey has found higher levels of the disease in recent years.

See Northern Leaf Blight provincial trend graph

Appearance

The disease forms long, elliptical (2-15 cm (1-6 in.)) greyish-green or tan streaks. Lesions most often begin on the lower leaves. As the disease develops, individual lesions may join, forming large blighted areas. In some cases the entire leaves may become blighted or “burned.” Losses due to northern leaf blight are most severe when the leaves above the ear are infected at or slightly after pollination. Later infections, in most cases, has little impact on yield. The disease is often confused with Stewart’s wilt (see the section Bacterial Leaf Blight or Wilt (Stewart’s Wilt), below).

See Northern Leaf Blight images

Disease Cycle

The fungus survives in corn residue as either spores or fungal strands (mycelium). The spores of the fungus are spread from the ground residue to the developing corn plant through wind or rain “splashing.” Although the fungus does overwinter in Ontario, a major source of spores remains the U.S. Midwest corn belt and surrounding Great Lakes states. Plants that become infected act as a secondary source of infection and may spread to other fields. Disease development is favoured by moderate temperatures (18°C-27°C or 64°F-81°F) with prolonged periods of humid or rainy weather.

Management Strategies

Northern leaf blight exists as four races, and most of the commercial corn hybrids have resistance or tolerance to the most common races that occur in Ontario. An increase in northern leaf blight symptoms in an area could indicate the potential for a new race developing and should be reported. Crop rotation and tillage will reduce inoculum levels in surface residues. In reduced tillage systems, rotation and resistance are necessary. Chemical control is not usually economical in field corn but may be  in seed corn depending on inbred susceptibility and environmental conditions.

Eyespot

Although eyespot normally causes minor losses in corn, the disease has been increasing in Ontario with the shift to reduced tillage systems and leaving more corn residue on the field.

See Eyespot provincial trend graph

Appearance

The disease produces characteristic round or oval spots (1-4 mm (1/16-1/8 in.) with a tan/brown centre and a brown or purple margin. A translucent yellow halo forms around the margin, and when held to the sun, the lesions resemble an eye. Leaf blighting may occur when these lesions join, killing large portions of leaf tissue. The disease may be confused with non-infectious physiological leaf spots or insect damage.

See Eyespot images

Disease Cycle

The disease is more prevalent under continuous corn and reduced tillage systems, since the fungus overwinters in corn residue. Disease development is favoured by cool, wet conditions.

Management Strategies

Resistant varieties, crop rotation and clean plowing of crop debris help to reduce disease severity. Chemical control is not economical in field corn and usually not in seed corn, as well.

See Foliar Fungicide:
Propiconazole - Tilt 250E

Bacterial Leaf Blight or Wilt (Stewart’s Wilt)

Stewart’s wilt is an important disease of seed corn production and many countries have quarantines in place against it. Stewart’s wilt or bacterial leaf blight of corn is caused by the bacteria Panteoa stewartii and although the disease occurs throughout Ontario and can infect commercial hybrid corn, it is a particularly significant disease of seed corn in southwestern Ontario.  The counties of Essex and Chatham-Kent, where the majority of seed corn production fields are located, tend to be most affected.  Although the disease is sporadic or varies from year to year, it is most damaging when the growing season follows a warmer than normal winter.  The disease is spread or vectored by the corn flea beetle insect, which will survive in higher numbers during a favourable winter.

See Stewart's Wilt provincial trend graph
See Corn Flea Beetle provincial trend graph

Appearance

There are two distinct phases of the disease that occur in Ontario, the late phase and the wilt phase. The wilt phase affects primarily highly susceptible seed corn inbreds and sweet corn hybrids early in the year (V2 to V4). The first noticeable sign of the disease appears as long, yellow streaks that extend along the length of the leaf. These streaks will take on a water-soaked appearance and eventually become brown, dead streaks (necrotic).

The leaf blight phase or late-infection stage often occurs after tasselling and is the most common. Symptoms include pale green to yellow streaks with irregular or wavy margins that run parallel to the veins. These streaks may run the full length of the leaf. Infected leaves eventually become dry and brown. Often corn flea beetle-feeding marks are visible within the lesions.

Premature leaf death can result in reduced yield and an increase in stalk rots since weakened plants are more susceptible to stalk rots.

See Stewart's Wilt images

Disease Cycle

The bacteria overwinters in the gut of adult flea beetles, which hide through the winter in protected areas. Mild winters can result in higher beetle numbers. Overwintering adult flea beetles feed on corn in the seedling-to-whorl stage, and susceptible varieties will develop a stem wilt resulting in complete plant loss. This occurs rarely in hybrids but occasionally in susceptible seed corn parents. The next generation of adult beetles emerges after corn silking and causes leaf wilting symptoms, which are commonly seen in many hybrids. Seed transmission is rare. Most often, late infections after silking are associated with high beetle populations. Sweet corn is often more susceptible than field corn and can serve as a reservoir for the bacteria. The disease is often found in the best fields, and fertility seems to play a part. Susceptibility to the disease increases in fields that have high nitrogen and phosphorous levels.

Management Strategies:

1. Genetic Tolerance :  Field corn has good tolerance to Stewart’s wilt and therefore no control is required, but certain seed corn inbreds are susceptible and are rated for disease tolerance. Check with your seed company for the ratings of your particular inbreds to the disease.



2. Insecticides: The disease is also managed by controlling the corn flea beetle. Not much can be done once seed corn is past the five leaf stage.   Little work has been done in Ontario on chemical control of corn flea beetles. In the U.S., insecticide applications have only been used to control flea beetles spreading the wilt phase or early season Stewart's wilt disease. The thresholds used in the corn belt states are for corn plants that have NOT reached the V5 growth stage:



Corn Flea Beetle Thresholds:
Seed Corn (Prior to five leaf stage or V5 growth stage) - 10 percent of the plants have severe feeding injury and 2 or more beetles per plant found.
Field Corn (Prior to five leaf stage or V5 growth stage) - 50 percent of the plants have severe feeding injury and there are 5 or more beetles per plant found.

For further management information, see the section on Corn Flea Beetles in Chapter 4 “Seed Corn Insects” for more information.

3. Disease Forecasting: The ability to accurately predict Stewart's wilt of corn will allow for better management of the disease (resistant hybrids or inbreds) and the corn flea beetle (insecticide seed treatments or foliar insecticides). Stewart's wilt is the most important disease of seed corn production in Ontario. Many countries have quarantines in place against it.  Two weather based computer prediction models are being evaluated and developed for Stewart's wilt in Ontario.
   
   The original Stewart’s wilt prediction model (Stevens-Bowes) involved taking the average monthly air temperature for December, January and February and adding each months average air temperature together. The total was called a winter temperature index (WTI). A low WTI (< -3.3° C = cold winter) indicates reduced winter survival of the beetle vector and therefore, low risk for Stewart's wilt. A high WTI (> -3.3° C = mild winter) can result in a large population of beetles surviving the winter that can potentially start infections and spread the disease (Table 3-11). Using this model, observations in Ontario indicated that the risk for Stewart’s wilt was high in 2002 and low in 2003 and 2004.
   Although this model works relatively well, it is not 100%.  For this reason, other models are being evaluated.  One such model, developed by Iowa State University researchers, also uses air temperature but the interpretation is different.  They have taken each month’s average winter air temperature and gave that month a point if it’s average was below - 4.4°C (°24 F). If over the winter 3 points are accumulated then your risk of Stewart’s wilt is minimal (Table 3-12). If you accumulate fewer points, the risk of Stewart’s wilt increases.

Grey Leaf Spot

The ability to accurately predict Stewart's wilt of corn allows for better management of the disease (resistant hybrids or inbreds) and the corn flea beetle (insecticide seed treatments or foliar insecticides) by the seed corn companies.

In Ontario, the disease has not traditionally been of major concern but  grey leaf spot has been increasing in the surrounding Great Lakes states and has recently been developing in Southwestern Ontario. Significant losses can occur from this disease under warm, wet and humid conditions.  Fields most at risk are reduced tillage, continuous corn fields since residue from the previous corn crop stays on the surface. The fungus (Cercopspora zeae-maydis) survives in the surface residue and inoculum (spores) will be blown from field to field

See Grey Leaf Spot provincial trend graph

Symptoms

The disease initially starts on the lower leaves soon after tasselling, as small watersoaked or orange-red spots that have a narrow yellow halo surrounding the lesion. The lesions can expand in length from a ½ to 2 inches. The rectangular lesions often have straight sides since lateral expansion is inhibited by the major veins of the corn leaf. That is why lesions are not usually more than a ¼ inch in width. As the lesions mature, they become tan and the thin yellow halo surrounding the lesion may not be obvious. If you hold the leaf up to the sun or a light the yellow halo should become apparent. Spores produced from lesions on the lower leaves will infect the upper leaves, husks, sheaths if warm, humid conditions remain.

See Grey Leaf Spot images

Disease Cycle

Grey leaf spot is most problematic when corn follows corn in fields with a considerable amount of corn residue. The fungus survives as fungal strands (mycelium) in corn residue. Spores produced on the residue are dispersed by wind and rain splash. Warm, humid weather helps spore and disease development.

Management Strategies

As mentioned earlier, the disease is increasing but still has not resulted in the same level of damage as seen in some of the surrounding great lake states.   Occasionally damage does occur on seed corn and popcorn in the province. Field corn hybrids vary in susceptibility to gray leaf spot whereas most seed corn inbreds are very susceptible to this bacterial disease. If warm, humid weather persists following tasselling gray leaf spot development is favoured. Unlike many other leaf diseases, prolonged rainfall is not necessary for disease development. Winds along with 13 hours of leaf wetness (from high humidity, heavy dews , fog or light rains) are sufficient to spread the disease. Good crop rotation and tillage will reduce inoculum levels in surface residues. Chemical control is not usually economical in field corn but can be beneficial in seed corn especially if other leaf diseases such as common rust, northern leaf blight or others are present.

See Foliar Fungicides:
Propiconazole - Tilt 250E
Pyraclostrobin - Headline EC

Common Rust

Common rust does not overwinter in Ontario but instead originates from infected corn in the southern United States, Mexico and the Caribbean. Rust spores are blown into Ontario from these infected corn plants. In most years, rust is of minor economic importance, however sometimes spring storm fronts bring in spores and cause early season infection. The disease is favoured by high humidity with cool evening temperatures (14°C-18°C) followed by moderate daytime temperatures.

See Common Rust provincial trend graph

Symptoms

Early symptoms of rust infection are yellow flecks or spots on either side of leaf. These develop into small, brick-red pustules that break through the surface (epidermis). The brick-red colour is the result of spores being released from these oval or elongated lesions (2–10 mm (1–4 in.) long). Yellowing of the leaf occurs around these lesions. Dead, brown (necrotic) areas of the leaf develop, and in severe cases the entire leaf dies. The brick-red spores mature and turn black as they mature, causing the lesions and leaf surface to appear black.

See Common Rust images

Management Strategies

Since common rust does not survive in Ontario, cultural practices such as reduced tillage and crop rotation do not influence disease development. Commercial corn hybrids have good tolerance, whereas many seed corn inbreds, sweet corn and speciality corn hybrids are very susceptible to the disease. Foliar fungicides in field corn are not usually needed but can be economical in highly susceptible seed corn inbreds or specialty corn hybrids.

See Foliar Fungicides:
Propiconazole - Tilt 250E
Azoxystrobin - Quadris
Pyraclostrobin - Headline EC

For updated Fungicide Information refer to OMAFRA Publication 812 - Field Crop Protection Guide

Smut, Common and Head

Two corn smut diseases, common and head smut, occur in Ontario with common smut occurring most frequently. In severe cases, over 25% of the plants in some fields or higher levels in localized areas can have smut galls. In 2004, one seed corn field that was surveyed, 96% (48 of 50) of the plants in some areas of the field were infected. Young plants are most susceptible.

See Common Smut provincial trend graph

Symptoms

Common smut overwinters, not only in the soil but in corn residue as well. The spores are spread by wind and rain through splashing. All above-ground plant tissue is susceptible, but infection occurs most often in areas of actively growing tissue. Common smut incidence increases in fields where the plants have been wounded by hail, frost, drought, mechanical injury, detasselling, herbicide injury, insects, sandblasting, and other stresses. High levels of nitrogen and manure promote this disease.  In some cases, increased levels have been reported following sugar beets in Ontario.

Greyish smut galls up to 10 cm (4 in.) in diameter develop on the stalks, ears and tassels, while smaller galls often appear on the leaves .The galls initially have a white membrane cover that eventually breaks and releases dark-brown or black powdery spores. On the leaves, galls develop into a hard, dry growth. Smut galls can replace kernels. Unlike common smut, head smut occurs on the ears or tassels (or both) only

See Smut images

Disease Cycle

Spores released from the galls are well adapted for Ontario conditions. They survive in soil and crop residues for many years. In the spring, these spores germinate to produce new spores that will infect the rapidly growing areas or injured areas of the plant. The resulting galls will release spores that infect other plants. Disease development is favoured by rain showers, high humidity and warm temperatures.

Management Strategies

Seed corn inbreds are more susceptible than most corn hybrids which have enough resistance to smut to prevent serious outbreaks However, some smut is present in most commercial corn fields as well. Reduce your risk by minimizing mechanical and herbicide injury while maintaining a balanced fertility program. Rotation and cultivation have little effect on the disease since spores can survive for a long time in the soil.

Crazy Top Downy Mildew

Although this unusual disease is a sporadic problem in southern Ontario, economic losses have been recorded in some instances although in most cases the disease is more of a curiosity or oddity then a significant problem.

Symptoms

Corn plants are most susceptible after seed germination and before the plants are 15 cm high, especially during flooded or waterlogged conditions.  Typical symptoms are a distortion and/or stunting of the plant and tassel which may be partially or completely replaced by a proliferation (mass) of leafy tissue. Ear shoots may be numerous, elongated, leafy, and barren. Leaves may be narrow and leather-like and there may be excessive tillering. Ears and tassels may not develop.

See Crazy Top Downey Mildew images

Disease cycle

In Ontario, diseased plants are most often found in areas that had been flooded after heavy rains in May.  This fungus overwinters as oospores in the soil or in infected tissues. When the soil is very wet, these oospores germinate into zoospores that can swim to young plants and infect them. Grassy weeds and small grain cereals are also susceptible to this pathogen, so the pathogen can remain in a field even without corn for several years.

Management Strategies

Providing adequate soil drainage is an effective method to control this disease. Remove excess water quickly from fields in the event of flooding etc. Wet spots should be avoided. Plants will not be affected if the growing point is not submerged.

Maize Dwarf Mosaic Virus and Sugar Cane Mosaic Virus

Two strains of this virus have been found in southwestern Ontario. Strain A (maize dwarf mosaic virus) overwinters primarily on Johnson-grass whereas Strain B (sugar-cane mosaic virus),is thought not to overwinter in Johnson grass in southern Ontario.

Symptoms

Symptoms of both viruses vary with time of infection, hybrid and viral strain. Light and dark green spots appear on young leaves giving a mottled or mosaic pattern. These spots can develop into narrow, light-green or yellowish streaks along the veins

Later in the season, plants may have streaks of red on the leaves especially after a cool night. Early-infected plants are stunted, have shorter internodes and are yellowish-green with poor seed set. Plants infected at the10-12 leaf stage may have a 'cattle-tail' appearance in which the new leaves and/or tassels are covered by the old leaves. Plants infected after silking time may appear nearly normal.

See Maize Dwarf Mosaic Virus and Sugar Cane Mosaic Virus images

Disease Cycle

Both viruses can infect over 200 species of wild and cultivated grasses but Johnsongrass is the only important overwintering host in North America.  The virus is transmitted by over 20 aphid species which are blown into Canada on migrating aphids from the southeastern U.S.   In Canada, most reports of MDMV or SCMV are on late planted seed, sweet and grain corn if aphids build up on infected early plantings.  Oats and possibly barley may be symptomless carriers of the virus. Strain A is mechanically transmissible to corn and Johnson grass but not to wheat. In the USA the strains apparently overwinter in susceptible perennial grasses.

Management Strategies

In field corn, use resistant hybrids. Most seed and sweet corn cultivars are susceptible. Separate early and late plantings, destroy or till under crop debris following harvest, and control aphids on young plants to help delay infection of late seeded crops. Early planted corn may escape aphid infestation at the seedling stage.

Wheat Streak Mosaic Virus

Wheat streak mosaic is an important disease of wheat.  Corn is seldom seriously affected but may play a role in harboring both the virus and its vector, the wheat curl mite.

Symptoms

Infection of young, rapidly growing corn appears mainly in the newly expanding whorl leaves as small, oval to elliptical yellowish spots and streaks.   These streaks elongate and develop parallel to veins.  Later this mosaic fades, but purplish tints may develop as the infected leaves mature. Infected leaves of mature plants may show no symptoms. Severely affected plants may be stunted and yellowed and form small ears with poor seed set.  As the plants mature, the mites feed on the sides of the kernels, to which their saliva is toxic, causing brownish-red streaks (kernel red streak).The same mites can also transmit wheat spot mosaic to corn.

See Wheat Streak Mosaic Virus image

Disease Cycle

This virus is carried by the wheat curl mite (Aceria tulipae), which is about 0. 1-0. 2 mm long. After overwintering on winter wheat the mites leave this crop as it matures and are blown by the wind to spring wheat, barley, and corn. In the fall, mites leave these crops and are blown to newly emerged winter wheat. Some perennial grass weeds are alternate or overwintering hosts for the virus and mites.

Management Strategies

In southern Ontario, seed and commercial corn is not infected often enough for this disease to require a control strategy.  Use resistant hybrids. Avoid late planting of corn close to maturing wheat as it will carry the vector between crops

stalk rots

The stalk rot diseases are often more of a problem in commercial corn than in seed production fields because of differences in management practices.  For instance, seed corn fields in most cases are havested earlier than comparable commercial corn fields and are more likely to have a fungicide application for leaf diseases.  Both of these practices can reduce susceptibility to stalk rots and reduce stress that would promote these diseases. 

Fungi cause corn stalk rots, and the amount of damage they cause increases when the crop is under stress. Stresses that contribute to an increase in stalk rot infection include wet or dry conditions; cool evening and daytime temperatures; cloudy weather; leaf diseases (such as rust and Stewart’s wilt); leaf and ear damage from hail, birds and frost; incomplete pollination; unbalanced fertility; insect damage (e.g., from European corn borer), high plant populations; hybrid susceptibility and poor soil conditions. The distribution and prevalence of stalk and ear rot diseases vary from year to year, but the diseases are present in most years even though it may be at low levels. The majority of stalk rot damage in Ontario is caused by three fungi: Anthracnose, Gibberella and Fusarium, however, both Diplodia and Pythium have also been observed in Ontario. Refer to the sections below for more details on each of these stalk rot diseases.

See Stalk Rot provincial trend graph

Impact of Stalk Rot

Although these fungi cause different symptoms, their ultimate effect on the corn plant is the same; they reduce grain fill and stalk integrity and accelerate senescence. Stalk rot fungi affect the nutrient movement of the corn plant in three main ways.

1. Sugars (photosynthates) that would be produced through photosynthesis or carbohydrates in the root and stalk are diverted to the fungus and not to the ear. These nutrients allow stalk rot fungi to grow and flourish.



2. There is a reduction in stalk integrity. To meet the nutrient needs of both the developing ear and the stalk rot organisms, the corn plant will begin to cannibalize itself by moving soluble carbohydrates from the root and stalk. Problems arise when the plant is unable to meet the nutrient requirements of the developing ear and thus is forced to move more carbohydrates than usual. The result is a weaker stalk (prone to lodging) and less resistance to stalk rot fungi.



3. Finally, the infection and colonization process inhibits or blocks many of the pathways that the plant would ordinarily use to move nutrients. Yield losses (generally 10%–20%) arise from poorly filled ears and harvest losses from lodging.

Stalk Rot Management Strategies

Management begins by reducing crop stresses through:

• inbreds that have good resistance or tolerance to leaf diseases and stalk rots
• managing insects such as European corn borer
• good weed control
• appropriate plant populations
• a balanced N and K fertility program
• crop rotation
• tillage, etc.

Fields should be scouted to determine the amount of stalk rots present, and seed corn fields that have 5%–10% of the stalks rotted should be harvested as early as possible. Crop rotation reduces the potential for stalk rots.

Anthracnose Stalk Rot

Anthracnose stalk rot is the easiest to identify. It appears as large, dark brown- to-black shiny areas or streaks on the outer stalk rind. These shiny or discoloured areas are often found at the base of the stalk. Cutting the stalk lengthwise will reveal a discoloured and rotted pith. Another symptom that is associated with this disease is “top dieback.” Typically, top dieback symptoms begin in late August or early September as corn plants begin to wilt and die from the top down. Premature death occurs above the ear with the plant tissue below the ear remaining green. Examination of the stalk in these dead areas will show the same shiny black areas that are found at the stalk base. Plants with top dieback symptoms correspond to areas of the field that had late-season stresses.

Disease Cycle:

The fungus that causes Anthracnose stalk rot survives in the previous corn crop residues and therefore is most often a problem in second-year corn. Warm, wet and humid weather favours anthracnose development.

See Anthracnose Stalk Rot images

Gibberella, Fusarium and Diplodia Stalk Rots

These fungi all cause general stalk rot symptoms, include wilting and death. Affected leaves turn a grey-green colour, which resembles frost damage. All three rots cause a dark external lesion or spots at the lower nodes. Diplodia stalk rot produces small black spots (pycnidia) that are embedded in the stalk rind. These spots are hard to remove. This is in contrast to Gibberella stalk rot, which also produces small, round, black spots at the lower node, except these spots can be easily scraped from the stalk surface. The pith is shredded and has a pink to red colour. Fusarium stalk rot symptoms appear as light brown-to-black lesions near the nodes. The internal stalk symptom of Fusarium stalk rot is a salmon-pink fungal growth in the pith.

Disease Cycle:

See the section Ear Rots or Moulds, for each stalk rot disease.

See Gibberella, Fusarium and Diplodia Stalk Rot images

Pythium Stalk Rot

Symptoms and Disease Cycle

Pythium stalk rot gives the same general above-ground symptoms that are associated with the other stalk rot organisms. Pythium is in a unique group of fungi (that also includes Phytophthora) that are called “oomycetes” or “water moulds” because of their preference for wet conditions. The unique characteristic feature of this group of fungi is their production of mobile spores that can migrate or move through the water film in saturated soils. These spores (infection stage) are able to physically move to the corn plants roots and, once inside, cause disease. Corn plants infected with Pythium will not have any visible signs of fungal growth at the base of the plant that is different from the other stalk rots that produce overwintering structures (black dots) or mould. When the plant is cut lengthwise through the stalk base and roots, Pythium-infected tissue will appear wet and soggy and will disintegrate (“a wet rot”) at the root base.

See Pythium Stalk Rot images

ear rots

For detailed information on the incidence and disease cycles for each ear rot disease, refer to the sections below.

See Ear Rot provincial trend graph

Fusarium Ear Rot

Fusarium ear rot is another common ear rot that occurs in Ontario. Unlike Gibberella, Fusarium-infected kernels will be scattered around the cob among healthy-looking kernels or on kernels that have been damaged, for example, by corn borer or bird feeding. Silks are susceptible to infection during the first 5 days after initiation.

Symptoms

Fusarium infection produces a white-to-pink or salmon-coloured mould. A “white streaking” or “star-bursting” can be seen on the infected kernel surface. Although many Fusarium species may be responsible for these symptoms, the primary species we are concerned about in Ontario is Fusarium verticillioides (formerly Fusarium moniliforme).

See Fusarium Ear Rot images

Disease Cycle

Fusarium survives in corn debris. The significance of this fungus is that it produces a toxin called fumonisin that has been shown to cause cancer (carcinogen) in humans. The environmental conditions that favour disease development are warm, wet weather 2-3 weeks after silking.

Gibberella Ear Rot

The most common and important ear mould in Ontario is Gibberella zeae, which is the sexual reproductive stage of Fusarium graminearium. This fungus not only infects corn (seed and hybrid) but also small grains such as wheat. Many plant pathologists believe that in years with a high occurrence of Fusarium head blight in wheat, the potential exists for increased Gibberella ear rot in corn.

Symptom

Although, the fungus can produce a white mould that makes it difficult to tell apart from Fusarium ear rot, the two can be distinguished easily when Gibberella produces its characteristic red or dark pink (purple) mould.

See Gibberella Ear Rot images

Disease Cycle

Infection begins through the silk channel and thus, in most cases starts at the ear tip. In severe cases, most of the ear may be covered with mould growth. Corn silks are most susceptible 2-10 days after initiation, and cool and wet weather during this period is ideal for infection.

Diplodia Ear Rot

Of the three primary ear rots that occur in Ontario, Diplodia ear rot is the least common. Diplodia ear rot is caused by Diplodia maydis and favours cool, wet conditions through grain fill.

Symptoms

The characteristic ear symptom is a white mould that begins at the base of the ear and will eventually cover and rot the entire ear. Mould growth can also occur on the outer husk, which has small black bumps (pycnidia) embedded in the mould. These reproductive structures are where new spores are produced. Unlike Gibberella and Fusarium, Diplodia does not produce any known toxins.

See Diplodia Ear Rot images

Disease Cycle:

Overwinters in corn debris left on the soil surface from the previous crop. Spores (conidia) that are produced during wet weather can infect silks and husks or enter through tissue damaged by birds or insects. Ears are most susceptible 3 weeks after.

Other Problems Affecting Corn Production

Cold
Early-Season Cold

Frost damage in May or June will generally have little impact on seed corn, providing the growing point of the corn plant is still below the soil surface, which is the case until the young plant reaches roughly the sixth-leaf stage. On more advanced plants and/or where damage is more severe, split stalks to see if the growing point has been damaged. This procedure will require some time to make the correct recommendation as it usually takes 3-5 days following a frost to accurately determine the degree of damage, to verify the presence of healthy growing points (yellowish-white and firm) or to see new leaf growth.

In some cases, frozen leaf tissue, which bleaches to a straw colour several days after freezing, also develops a “knot,” which may restrict expansion of the undamaged tissue lower in the whorl. Often it appears that clipping these knots by mowing the field aids in the plant's recovery. This is, however, mostly cosmetic. Tests conducted on frosted corn fields over the years have arrived at the conclusion that clipping appeared to help the fields “green up” but that unclipped sections of the same fields often recovered as quickly and yielded as much or more than the clipped sections.

There is very little that can be done to minimize the potential problem but here are some management factors that increase the risk of frost damage to seed corn should temperatures fall:

• inter-row cultivation
• side-dressing nitrogen (where soil is disturbed)
• herbicide applications
• presence of weeds
• high levels of previous crop residue

If the forecast calls for a risk of frost, growers may elect to delay inter-row cultivation, nitrogen side-dressing or herbicide applications until warmer temperatures return. Soil disturbance at the surface introduces more air into the soil and insulates the corn plants from the heat of the soil mass, thus increasing the risk of frost damage. Similarly, crop residues and weeds act as a barrier for heat transfer from the soil to the corn plant. Also, dry soils are more prone to frost damage because they have a lower capacity to store heat during the day and thus less heat to transfer or release to protect the corn plant overnight.

See Cold images

Heat

Heat stress is different than drought stress. Corn can usually tolerate temperatures as high as 38°C (100°F) before injury occurs, as long as drought conditions are not present as well. Temperature and drought sensitivity varies by inbred. Drought-tolerant hybrids may display a yield drag; they are likely not good choices for a normal growing season. Historically, seed corn in Ontario has performed better under hot and dry soil conditions than under cool, wet growing conditions.

See Heat images

Flooding

Flooding stresses the plant by cutting off the supply of oxygen to the root system. Younger corn plants die if submerged in water for more than 5 days, especially in warmer weather conditions. If air temperatures are high, death may occur in only a few days. Under warm conditions plant processes are faster and the need for a supply of oxygen to the roots is high. In cooler weather, submerged plants may live for up to a week. After the 8-leaf stage of corn, plants can tolerate being submerged in water for more than 8 days but may be more susceptible to disease and may experience limited root development while under water. Yield loss due to flooding is most substantial for plants submerged immediately before and during tasselling and silking. Plants in the vegetative growth stages of 10-16 leaves and/or during the grain filling period display little yield response to flooding.

See Flood image

Drought

A corn crop requires approrximately 50 cm of water to produce high yields. This can be supplied to the corn crop over the growing season from a combination of stored water in the soil, rainfall or irrigation.

Lack of water causes the leaves to wilt and the plant to turn a greyish colour. Corn is most susceptible to dry conditions during the tasselling-to-silking stage and may demonstrate yield loss if under stress at this time . During the later vegetative stages of growth (V8-V14), the plant may benefit from dry conditions, as it forces the more rapid downward growth of the roots. Drought conditions during silking can reduce pollination and a lack of silk emergence, while drought after silking may cause a reduction in grain fill and subsequently seed quality.

See Drought images

Hail

Corn fields damaged by hail may experience a reduction in leaf surface area, bruising of the stalk and ear and, in serious incidences, stalk breakage Yield loss due to hail is dependent on the stage of the crop at the time of the hail event and the level of defoliation. Yield loss is greatest when the corn is defoliated during tasselling. Younger plants may experience a delay in growth and development due to hail, but yield loss is usually minimal. Defoliation of plants near maturity tends to cause little yield loss. Hail damage may also provide an entry point for diseases such as smut.

See Hail image

Birds

Birds can damage emerging seedlings, however, the more serious damage occurs to the ear in August and September. Birds eat the kernels off the cob, causing direct yield loss, and kernel damage may result in mould growth and impact seed corn quality. Bird damage can be easily confused with seedling damage caused by black cutworms or ear damage caused by grasshoppers.

Noisemakers, such as Av-alarms, propane cannons, exploding shotgun shells, the Phoenix Wailer and recordings of bird distress calls may be successful deterrents if more than one technique is used and their pattern is changed frequently. If crop damage due to birds or wildlife is substantial, contact your local Ministry of Natural Resources office for control options.

See Bird images

Herbicide Injury

All herbicides have the potential to cause crop injury. Some soil applied (preplant or preemergent) herbicides can be splashed up on the leaves to cause injury, or the roots can take up the herbicide. With post-emergent herbicide products, most injury results at or shortly after application. Spray from the target crop onto a neighbouring non-target crop can also cause injury when the non-target crop is susceptible to the herbicide being applied. Spray overlaps are also common places where crop injury from herbicides can occur.

Stage of growth, crop stress, weather conditions, variety, tank mix partners and adjuvants can all affect the amount and severity of crop injury. When the target crop is under stress, it is sometimes not able to metabolize the herbicide fast enough to avoid injury. The type of herbicide (mode of action) is a major factor in how herbicides affect crops. In general, while contact herbicide injury may look worse, systemic herbicides will have longer-lasting and more detrimental injury, which may be more severe to the crop. Always read the label for information on how to reduce the risk for herbicide injury.

Dinitroaniline (e.g., trifluralin, pendimethalin)

Dinitroaniline injury on corn caused by high rates or slow breakdown. Short roots with thickened root tips or stunted top growth with purplish leaves are common symptoms.

See Dinitroaniline image

Chloroacetamide (e.g., dimethenamid, flufenacet, s-metolachlor)

Leaves are shortened, puckered and the coleoptile is twisted, with the leaves unable to unfurl properly. Plant stress can reduce the plant metabolism and ability to breakdown herbicide and causes more injury.

See Chloroacetamide image

Bleaching Herbicides (e.g., isoxaflutole, clomazone)

Plants may emerge normally and then show symptoms at the 2-4-leaf stage. Symptoms may also occur after a rain. Leaves will appear yellow to white and are usually more pronounced near the midrib. Purpling may appear at the leaf margin or at the base of the whorl.

See Bleaching image

Auxinic Herbicides and Other Products Containing These Herbicides (e.g., 2,4-D, MCPA, dicamba, clopyralid)

Dicamba injury causes twisted brace roots on corn. Brace roots are gnarled and tend to grow together and do not grow into the soil to support the plant, leading to potential lodging late in the season. High rates, sensitive hybrids or late applications can all cause these symptoms.

See Auxinic image

Bromoxynil

Symptoms include a speckling of the leaf and browning on leaf tips and edges. Contact injury will show on leaves that are emerged at the time of treatment but not on new leaves emerging after treatment.

See Bromoxynil image

Acetolactase Synthase (ALS) and Other Products Containing These or Similar Herbicides (e.g., nicosulfuron, imazethapyr, chlorimuron-ethyl)

Sulfonylurea (ALS) injury on corn often appears as a “flash” on the leaf Typically, corn outgrows this level of injury. Symptoms can include a yellow discolouration of the leaves, yellow flash on leaf (as shown), stunting of the plant, a delay in plant development and crinkled leaves or bleaching near the whorl. Onion-leafing or a purpling of stem or leaves may also occur. Some ALS herbicides may also reduce the root mass by shortening lateral roots. These herbicides should not be applied to corn that has been treated with an organophosphorus insecticide due to the increased risk of injury.

See ALS image

ACCase Inhibitors (e.g., sethoxdim, fluazifop-p-butyl, quizalofop-p-ethyl)

Group 1 graminicide (ACCase) injury on corn appears as intervenal chlorosis on newer leaves Older leaves may show red or purple. The growing point turns brown and dies. The top of the plant can be easily pulled out of the whorl. New growth dies first, such that the plant appears to take considerable time to die after application.

See ACCase image

Glufosinate Ammonium

Susceptible plants turn yellow and die. Injury occurs within the first few days after application.

See Glufosinate image

Glyphosate Injury

Glyphosate drift on corn causes yellowing in whorl (some reddening on leaf edges) near the growing point. Leaves can become twisted, and plants will become stunted or die. Injury may take 1-2 weeks to develop and is is not reversible.

See Glyphosate image