May 2006

Corn Production in Manitoba

Diseases and Disorders of Corn

Disease management can be a real challenge. Disease outbreaks are dependent on three key factors, the presence and type of pathogen, the status of the host, and the environment that influences the pathogen and the corn plant. When all the factors interact in a suitable combination, disease can occur.

To effectively manage corn for disease, it is preferable to prevent or manage a disease outbreak when the disease is at low levels, as opposed to attempting to deal with a disease in which significant damage has already occurred. Field scouting for disease throughout the growing season, planting disease-resistant varieties (if possible) and crop rotations can all reduce the likelihood of most disease outbreaks in the future.

Field scouting on a weekly basis can provide information on what diseases are present, the severity, and potential for crop loss if untreated. There is no better way of determining the status of disease on a corn crop, than actually being in the field to view these problems for yourself, and make informed decisions on what management tactics should be employed.  Reviewing the field history, identifying the diseases, and mapping the location of disease problems in the field, are all beneficial investments of time that will assist in the management of corn diseases.

The decision on what variety/varieties of corn to grow can be a difficult, and accounting for disease resistance can increase the difficulty. In some cases, higher yield performance and a high level of disease resistance, may not be possible, (as in the cases of stalk rots) or resistant varieties simply may not be available. Whenever possible, it is always a good idea to use varieties resistant to a disease, especially if a particular disease has been a problem in the past.

Growing corn in the same field for successive years may be desirable for a number of reasons. There are risks however, in not rotating other crops through the field, as the population of corn disease organisms can increase over time, increasing the likelihood of a large outbreak of disease with subsequent crop loss. In areas where disease is becoming a problem, the field should not be planted with corn (or any related crops) for several years, in order to reduce the pathogen level (and the risk of disease outbreaks) in the field. Generally diseases of corn are not of great concern in Manitoba, however there are increasing instances of crop loss due to root and stalk rots, in addition to the recurring problems of both common and head smut.

Brief descriptions of the more common diseases observed in Manitoba corn are provided here, along with a section on nematodes, which have not been reported as problems in corn in Manitoba, but could be a potential concern in the foreseeable future. General recommendations for disease management are also included.  Specific information on disease management can be found in the Guide to Crop Protection, published annually by Manitoba Agriculture, Food and Rural Initiatives.

Seedling Diseases/Seed Decay Diseases

Damping-off and Seedling Blight

When to look for: End of May to end of June

Damping-off and seedling blight are caused by a large number of soil and seed borne fungi. Germinating corn kernels may be attacked and severe infection may kill the embryo before germination (pre-emergence seedling blight) or destroy the seedling before or after emergence (post-emergence seedling blight). These diseases are prevalent in poorly drained, cold (less than 10-13°C), and wet soils. Planting depth, soil type, age and quality of seed, mechanical injury to the seed coat, and genetic resistance to infection all influence disease severity. 

The disease organisms responsible for seed rot and seedling blights, can be divided into two main groups:

1. Pathogens in or on the seed at planting. Fungi associated with seed at planting are usually involved in ear rots, and include species of Fusarium, Penicillium, Aspergillus, Trichoderma, Bipolaris and Alternaria. (see Ear and Kernel Rot Section). The degree of damage depends on how much rotting occurred prior to harvest and conditioning of the seed.

2. Pathogens in the soil at planting. Species of Pythium are the most common soil fungi that can cause seed rot and decay of seedling roots, though species of Fusarium can also act as soilborne seedling pathogens. Penicillium oxalicum is a fungus that can cause a seedling blight, which unlike other seedling blights, is favoured at high temperatures.

Symptoms
Poor stand establishment, varying emergence times, and gaps in rows are generally an indication of seed rot or seedling blight. Individual plant symptoms include stunting, yellowing, wilting, and death of leaves. Seed rots and blights may be confused with mechanical or chemical injury, or insect damage. Examination of plant parts under the ground is therefore necessary for accurate diagnosis.

In pre-emergence seedling blight, the coleoptile and developing root system appear brown, wet and slimy. In post-emergence seedling blight, the seedlings may have a constricted stem at the soil line, appear yellow, wilt, and die.

Disease Cycle
Strands of fungus growth (mycelium) contact seed or seedling tissue and enter the seed through cracks in the seed coat or by direct penetration. The mycelium grows rapidly through and between the cells, killing the seed. Similar attacks may occur through rootlets and stems by direct penetration or through wounds. The mycelium proliferates in young cells causing rapid collapse and death of tissues.

Management
Sow injury-free seed of resistant varieties (consult seed suppliers about disease resistance of their varieties).

Follow good cultural practices, e.g., plant in warm, moist soil; prepare the seed bed properly and place fertilizer correctly relative to the seed to insure maximum seedling vigour.

Apply seed protectant fungicides.

Root Rots

When to look for:  Throughout season

Often root rots are caused by the activity of a number of different fungi and bacteria that attack plants approaching maturity. Identifying a single cause of a root rot can be problematic as the effects of environment and the growth stage of the plant will influence a specific pathogen’s ability to cause a root rot. Generally the disease cycles are similar, as many of these fungi also cause stalk rots, and root rot organisms are present in living and dead corn plants and can survive on corn refuse and in the soil. They attack dead or dying plant tissue and help rot down crop residues. Spores of some types of the fungi causing root rot are wind-borne.

Specific root rots in Manitoba include Fusarium and Pythium root rots.

Fusarium Root Rots (many different Fusarium species)

Symptoms
Root symptoms may range in appearance from a slightly abnormal browning of the roots, to complete destruction of the roots. A pink tinge of the roots could indicate Fusarium graminearum infection (cause of Fusarium head blight of cereals).

Management
Species of Fusarium are commonly found in soil. Evaluating varieties for Fusarium root rot, is very difficult, thus root rot resistant varieties are difficult to produce. Seed treatments may impact seedling blights, but protection is lost as the roots develop. Crop rotation appears to reduce the occurrence of Fusarium root rot.

Pythium Root Rot (various Pythium fungi)

Symptoms
Infected roots may appear brown to black in colour. Aboveground, plants may appear yellowed and stunted. The outer portion of the root may be discoloured, while inner portions remain white.

Management
Improving soil drainage, is perhaps the only effective way to deal with Pythium root rot. Crop rotations do not appear to have any effect.

Stalk Rots

When to look for: Mid August to Early October

The term “stalk rot” has been used to refer to any situation in which corn stalks are broken, lodge, or die prematurely. In the strictest definition, this section will refer to stalk rots as decay of the internal stalk tissues. Generally stalk rot fungi, do not significantly damage healthy plants. While the stalks contain a high level of carbohydrates, corn plants are able to tolerate most stalk rot organisms. As grain filling reduces the reserves in the stalks, susceptibility to infection increases. Stalk rots are generally observed later in the season, often some time after the infection has taken place, when the grain is drying down in the field.
 

The first indication of stalk rot is wilting of the plant. Over the next few days, leaves appear grey, the ear drops, and the outside of the lower stalk turns brown. When the outer stalk tissue is brown, the pith tissue in the lowest internode is rotted and separated from the rind. As the rotting pith tissue degrades, the once solid stem becomes a hollow and much weaker tube, that will lodge more readily. Plants with rotted stalks almost always have rotted roots. Diagnosis of specific causal agents of stalk rots can be difficult, as a number of fungi and bacteria can be involved in causing the stem to rot.

Scouting for Corn Stalk Rots

As most damage attributed to stalk rots, is difficult to detect until later in the season, corn fields should be scouted 40-60 days after pollination, before any lodging is observed. Two methods that can be used to scout for stalk rots, include the “Push Test” and the “Pinch or Squeeze Test”. 

1. The Push Test – Randomly select 20 plants from 5 different areas of the corn field (100 plants total). Push the top portion of the plant and note whether the plant lodged or not.

2. The Pinch or Squeeze Test – Randomly select 20 plants from 5 different areas of the corn field (100 plants total). Remove the lower leaves and pinch or squeeze the stalk above the brace roots. If the stalk is easily squeezed, with moderate pressure, it is rotting on the inside. Record the number of rotted stalks.

Regardless of which method is used, if 10-15% of the plants are lodged, then early harvesting should be considered. The extra drying costs that may be incurred, will be covered by the increased efficiency of harvest, as corn will have less time to rot and subsequently lodge in the field.

Anthracnose Stalk Rot (Colletotrichum graminicola)

Symptoms
Unlike the other stalk rots, the fungus Colletotrichum graminicola may rot several internodes of the plant. Lodging before maturity is the most obvious symptom, and in some cases, portions of the plant above the ear die while the lower portions of the plant remain green. The most obvious symptom of anthracnose stalk rot is the shiny black discolouration at the base of the stalk initially and later in the top killed portions of the plant. This discolouration is attributed to masses of fungal strands (mycelium), just under the rind surface.  Symptom appearance can be quite variable, as the blackened areas may be uniform or in patches. They may occur anywhere on the stalk or sometimes only near the nodes. The internal stalk tissues, are often blackened or discoloured, appearing shredded. The stalk can be easily squeezed between the thumb and forefinger. The external and internal symptoms do not always reflect each other, as plants with a discoloured outer rind may have internal tissues that appear healthy, or stalks that have discoloured pith tissues may have rinds that appear green and symptomless.

Disease Cycle
The fungus can survive in some weed species, infected seed, and corn residues. In addition to a stalk rot, this pathogen also causes a leaf blight. Spores produced in leaf lesions may splash behind the leaf sheaths where the stalks are then penetrated. Insect feeding sites and other wounds, may also serve as entry points. When infested corn residues are left in the field, anthracnose leaf blight and stalk rot occurrence is generally higher with decreasing levels as distance increases from crop residue. In fields with no residues, the fungus can enter new fields by wind dissemination of spores associated with dry leaf pieces or by penetration of the roots by buried inoculum.

Management
Many commercial hybrids are usually not susceptible until after flowering when senescence of pith tissue occurs. If high levels of the fungus are present, rotation to a crop other than corn is advisable. Any reduction in stress, such as balanced fertility, and minimizing European corn borer damage (which creates entry wounds for the fungus), are beneficial for management of anthracnose stalk rot.

Fusarium Stalk Rot (Fusarium moniliforme, F. subglutinans, F. profileratum)

Symptoms
With symptoms that are not easily distinguishable from Gibberella stalk rot, and no easily recognizable signs of the fungus, Fusarium stalk rot can be quite difficult to diagnose. Often the only means of implicating Fusarium stalk rot is the absence of signs of Gibberella stalk rot, Anthracnose stalk rot, and other stalk rots.

As with other stalk rots, a shredding of the internal tissues is observed. Brown streaks will appear on the lower internodes. The rotting stem tissues may appear a variety of colours, ranging from beige to whitish pink to orange pink in colour, which tend to be less intense than the pink-red stain of Gibberella stalk rot.

Decay tends to begin after pollination, increasing in severity as the plants mature, becoming more prevalent under warm, dry conditions.

Disease Cycle
The various fungi survive in soil and on crop residue. Under favorable conditions, it may infect roots or wounds on stalks or leaves. F. moniliforme may be present for the duration of the life of the plant, from infected seeds to new ear rots. Spore dispersal can occur by wind, rain, insects, or birds.

Management
Use of hybrids resistant to other stalk rot diseases usually display resistance to Fusarium stalk rot. Any practice that can reduce stress or injury to the plant, especially the roots, can reduce the likelihood of damage attributed to Fusarium stalk rot.

Diplodia/Stenocarpella Stalk Rot (Stenocarpella maydis)

Symptoms
Diplodia stalk rot, may initially be seen as a brown to tan discolouration of the lower stem, the internal tissues of the lower plant portions appear shredded, and easily crushed. White strands or mats of fungal growth may be observed, especially during extended moist periods.  As with Gibberella, tiny dark brown to black spots will appear on the lower portions of the stem. (See Gibberella stalk rot section for further differences).

Disease Cycle
The fungus can survive on stalk debris either in or on the soil surface. With warm moist conditions and rain, the spores can be released and spread by wind or even insects. Wet weather two to three weeks after silking is suitable for the development of Diplodia stalk rot. Infection is usually via the crown, roots, or lower nodes. Insect wounds can also provide an entry point for the fungus.

Management
Varieties that have resistance to Gibberella stalk rot will usually display comparable levels of resistance to Diplodia stalk rot. Reduction of any stress on the plant is the best method of management. Keeping weed populations down, proper plant density, balanced fertility, and prevention of wounds, will all help to reduce the risk of Diplodia stalk rot. Rotations of at least a year, or reduction of debris in the field may reduce future outbreaks of the disease.

Gibberella Stalk Rot (Giberella zeae)

Symptoms
Gibberella stalk rot has the typical symptoms of a stalk rot, wilting, leaves that resemble frost damage, appearing a light to a dull grey-green colour. The lower portion of the stalk softens and turns a light brown colour. Near and at the nodes, tiny superficial round black specks (perithecia) are produced. These can be easily scraped off from the stalk surface. The internal pith tissue breaks down, leaving only the thread-like vascular bundles. A diagnostic sign of Gibberella stalk rot is the reddish-pink discolouration observed inside the stalk.

Disease Cycle
The fungus can survive on overwintering crop residues infested with the fungus. With the onset of warm, wet conditions in the spring, wind disseminated spores are produced which may infect corn plants. Inoculum may also be produced as splash dispersed conidia in summer. Stalk infections usually occur shortly after pollination, developing at the base of the leaf sheaths or near brace roots. The fungus also may enter through the roots and grow up into the lower stem. G. zeae also causes scab and seedling blights of wheat, barley, oat, and rye.  The fungus is widespread and levels of the fungus do not get low enough in areas of corn production to completely eliminate the risk of Gibberella stalk rot outbreaks.

Management
The fungus that causes Gibberella stalk rot is the same fungus that causes Fusarium head blight in cereals. Therefore crop rotations of a minimum of one year of crops other than cereals or grasses are critical, especially if reduced tillage is practiced. Plowing under of residue and controlling weeds and volunteers will reduce inoculum levels. Any source of stress, can increase the likelihood of stalk rots, including high plant densities, high levels of leaf disease, damage from stalk boring insects, high levels of nitrogen and low levels of potassium. Maintaining a balanced soil fertility based on soil tests, will also reduce the impact of Gibberella stalk rot.

Pythium Stalk Rot (Pythium aphanidermatum)

Symptoms
Generally restricted to the internode closest to the soil line, symptoms of Pythium stalk rot, include soft, collapsed, and dark green tissues. Even after lodging, the plants usually remain green as the vascular system remains intact.  As opposed to the other stalk rots which have visible overwintering structures of the fungus (such as the black specks on Gibberella stalk rot), there are no readily apparent signs of fungal growth associated with Pythium stalk rot.

If infection occurs before flowering, the lowest internode decays, and infected stalks may have a strong odor. Internodes may twist, causing the plant to lodge, but the greenish-brown stem colour remains for weeks. Should infection occur during the milk stage (R3), the roots and several lower internodes will become water soaked, leading to early plant death.

Disease Cycle
Species of Pythium are considered water moulds, and act differently from the other stalk rot fungi. Spores that can move on their own (zoospores), swim through moist soil, towards the roots of a corn plant, gain entry inside the plant and initiate disease.

The fungus overwinters as tiny hardened structures known as oospores, which can survive the dry, cold conditions of winter in soil and crop residues.  In the spring, the overwintering oospores germinate, releasing the swimming zoospores or threads of mycelium, each capable of infecting plants.  The oospores may remain viable for years, and the fungus may be able to survive on weeds.

Management
There is very little information on management that is known to be effective. Avoiding poorly drained areas or improving soil drainage, could be useful in Pythium stalk rot management.

Summary of Stalk Rot Management: 

• Plant resistant varieties, if available;

• Adjust plant populations according to variety, fertility, and area. Excessively high populations cause stress on individual plants that increases the incidence of stalk rot and lodging;

• Maintaining good soil fertility balance helps manage fungal stalk rots. These rots are generally more severe when nitrogen is in excess in relation to potassium. Excess nitrogen can lead to increased lodging.

  Plant sound disease-free seed treated with a fungicide.

Diseases of Aboveground Portions of the Plant

Anthracnose Leaf Blight (Colletotrichum graminicola)

What to look for: Late June to Late September

The anthracnose fungus can cause both a stalk rot and a separate leaf blight disease (see the stalk rot section for more information on anthracnose stalk rot).

Symptoms
Anthracnose leaf blight produces large oval spots, up to 15 mm in diameter, with tan centers and reddish-brown or yellowish-brown margins. Infection occurs first on the lower leaves, but can spread to the upper leaves under wet conditions. Infected leaves wither and die prematurely late in the season or during prolonged wet weather.

Disease Cycle
The fungus survives on corn residues during the winter. It can survive in buried residues, though buried residues are more of a concern for the stalk rot phase, but not for the leaf blight phase.  Spores on crop debris can be dispersed to the leaf surfaces, with rainfall, and the fungus penetrates the leaf surface. Anthracnose leaf blight is most severe after prolonged periods of cloudy wet weather.

Management
Management of anthracnose involves crop rotation and the use of hybrids that are specifically resistant to anthracnose leaf blight. Leaf resistance to anthracnose and anthracnose stalk rot resistance are not highly correlated. Be aware of this when selecting “anthracnose resistant” hybrids, as leaf blight resistant forms may not necessarily be stalk rot resistant. This disease may be more prevalent in reduced tillage situations. Crop rotation and tillage of residues, could minimize early season infections, but has little impact on late season or stalk rot infections.

Common Rust (Puccinia sorghi)

When to look for: Late July to Late September

Symptoms
The rust organism produces many small golden-brown blisters on corn leaves. As the disease progresses, these spots split open releasing the characteristic powdery, rust-coloured spores. Because this disease is favoured by cool, damp weather, it usually does not develop until late in the growing season and is relatively minor.  In addition, most modern hybrids have adequate resistance to rust.

Disease Cycle
Windborne spores are blown north from southern states, where the rust overwinters. In the early spring, spores produced on the alternate host of wood sorrel (Oxalis spp.), are blown northward, where spore numbers increase as new corn plants are infected. The spores continue into Canada, arriving in Manitoba, as early as late June. Moderate temperatures and high relative humidity at the time of rust spore arrival will favour infection. The plants become increasingly resistant to infection as they mature, with younger leaves more susceptible to infection than older leaves.

Management
Growing resistant hybrids, and timely use of registered foliar fungicides, can effectively manage common rust, though fungicide use will tend to be more economical in high value seed or sweet corn, under high rust pressure. Planting earlier in the season can ensure the corn crop reaches a more resistant growth stage, prior to spore arrival. Crop rotation and burial of crop debris, will not effectively manage corn rust, as rust spores do not overwinter in Manitoba.

 

Disease Cycle
Head smut spores overwinter in the soil. The disease cycle is comparable to that of common smut, though this fungus attacks seedlings.  In the spring, the fungus grows systemically through the infected seedling, eventually invading the developing flower tissue.

Corn under stress during emergence appears to be more susceptible to infection.

Management
Susceptible cultivars can have significant yield losses due to head smut. Corn hybrids with good resistance to head smut are available. Systemic fungicide seed treatments can be an effective control measure. Maintaining an adequate level of nitrogen can also be beneficial. The highest incidence of head smut occurs in continuous corn when the levels of fungus have built up over time, though crop rotations and cultivation may have little impact on disease, as the smut spores can survive for long periods in the soil.

Crazy Top/Downy Mildew (Sclerophthora macrospora)

When to look for: Early July to Mid-August

This disease is of little economic importance but arouses considerable attention when it occurs because of the peculiar appearance of infected plants. Crazy top is caused by a systemic infection of downy mildew. Infection occurs when the soil is flooded for 24 to 48 hours between planting and the 4 to 5 leaf stage. Infected plants are stunted, crooked and produce abundant tillers. The tassel is often replaced by a mass of twisted, leafy tillers. Provision of adequate drainage offers the best means of management.

Ear and Kernel Rots 

When to look for: Mid August to October (and during storage)

Reduction in yield and grain quality due to infection by fungi of ears and kernels both during the growing season, and while in storage can be very severe. In addition, a major concern with many ear and kernel rots is the production of mycotoxins. These toxic compounds, produced by fungi, can adversely affect the feed value and marketability of the grain. Most mycotoxin contamination does occur in the field, but they may also be produced under storage conditions.

Generally, the likelihood of corn ear or kernel rots, is increased when insects or birds damage ears or kernels. Any factor that promotes the lodging of stalks and enables the ears to contact the ground, increases the risk of kernel or ear rots. Ears well covered by husks and maturing in a downward position tend to be less prone to rots than ears with open husks or that mature upright.

Disease Cycle
As with Gibberella stalk rot, the fungus survives in soil and on cereal (including small grains) crop debris. Fusarium head blight outbreaks in other cereals (such as wheat), during the season may also be potential sources of inoculum (spores), that can initiate new infections either directly through the silks, or by wounds attributed to insects or birds. Wet weather is necessary for spore dispersal, and cool temperatures within a week after silking is especially favourable for Gibberella ear rot. The kernels remain susceptible to infection until they reach physiological maturity.

Management
Varieties differ in resistance to Gibberella ear rot. In general, tighter husked hybrids tend to be more severely infected than those that have loose husks.

Practices that reduce crop residues are beneficial. Rotations that avoid wheat or other cereals susceptible to Fusarium Head Blight and/or fall tillage may reduce the impact of Gibberella ear rot in future growing seasons.

Fields should be scouted after silking to determine the levels of Gibberella ear rot in the field. If rotted ears are found, it is advisable to harvest the field as soon as possible.

After harvest, Gibberella zeae is still capable of growing and can continue to produce the DON mycotoxin, in the kernels.

Fusarium Ear and Kernel Rot (Fusarium moniliforme, F. proliferatum, F. subglutinans)

Symptoms
Fungal growth on the kernels and silks, may range in colour from almost white with a pinkish tinge to light purple in colour.  This growth often can be found at the tip or near damaged parts on the ear. A “starburst” (i.e. light streaks, radiating from a white center on the kernel surface) may be observed on infected kernels. In severe infections, whitish fungal growth may be observed on and between kernels, the entire ear has a whitish, weathered appearance.

Disease Cycle
The fungi overwinter in soil and on crop debris.  Moist weather is favourable for spore production. Spores in the air, come into contact with the silks and grow through them, infecting the kernels. Fusarium moniliforme, may also invade the ear via a systemic stalk infection.  Warm, wet weather, two to three weeks subsequent to silking, are ideal for disease development.

Management
Balanced moisture, fertility and any practice to reduce wounds (such as insect management) will reduce the likelihood of Fusarium ear rot.  Storage of kernels under dry conditions is necessary to prevent production of fumonisin mycotoxins in storage.

Ear and Kernel Rots that Develop in Storage

Storage rots are caused principally by species of Aspergillus (often dark brown to green colour), Penicillium (bluish-green in colour) and a number of Fusarium (often light pink to purple to red in colour) species. These fungi are the most important mycotoxin producers, and species of these fungi are well adapted to surviving at low moisture content levels. Invasion of grain in storage often results in discolouration, heating, caking, and the occurrence of a musty odor.

Storage decay is favoured when kernel moisture content is above 20% and storage temperature is between 21 and 32°C. At moisture contents below 15% and temperatures below 10°C, the risk of storage decay is reduced.

Mycotoxins and Mycotoxin Management

Some storage fungi that can cause ear and kernel rots in field and storage can produce poisonous by-products called mycotoxins, which can be toxic to animals (including humans) when consumed at high concentrations. Presence of mycotoxins in animal feeds may result in increased mortality, feed refusal, reduced productivity and depressed growth. Mycotoxins often do not produce specific clinical signs in animals that allow for a positive diagnosis by symptoms alone.

Deoxynivalenol (DON) is a mycotoxin responsible for a number of problems in livestock, most notably swine. Fusarium graminearum (the causal agent of both Fusarium Head Blight of wheat and Gibberella Ear Rot of corn) is the major producer of DON in corn. Fusarium proliferatum and some forms of Fusarium moniliforme fungi can produce fumonisins (a group of mycotoxins which have been linked to disorders in horses and swine) in corn.

After harvest, it is best to dry down the corn as soon as possible to reduce the growth of fungi, which can continue to produce mycotoxins, until the moisture content falls below 15% (depending upon the species of fungus involved). However, even though fungal growth has been stopped, the mycotoxin level in the corn will not be reduced. Mycotoxin contamination in storage is usually the result of improper drying or storage conditions.

To minimize mycotoxin production:

• Follow harvesting recommendations to minimize grain damage. Damaged kernels are more susceptible to storage decay than sound kernels

• Dry corn harvested above 18% moisture, unless it is frozen. Corn should be dried to 14-15% moisture as quickly as possible

• Thoroughly clean grain bins and grain, if feasible, before storage to remove soil, dust, crop debris, and cracked or broken kernels

• Prevent insect, rodent and water damage to corn in storage

Analysis of a feed sample is necessary to detect the presence of mycotoxins. Even if only a few kernels are highly contaminated, these kernels can still have concentrations at levels high enough to cause a problem if consumed. If sampling in a storage bin, attempt to collect from a number of areas within the bin, if possible. Storage bins can have “hotspots” within them that are very suitable for mycotoxin production, due to differences in moisture levels or insect activity. 

Nematodes

When to look for: numbers will vary throughout the year, anytime from the end of June to mid-September

Nematodes are very small worm-like organisms, commonly found in a wide range of environments, some of which can feed on plants, including corn. While nematodes have yet to be confirmed as a significant problem in corn grown in Manitoba, different kinds of nematodes that could become potential problems to corn production in the future have been found. Virtually all plant parasitic nematodes live in the soil, feeding on roots and underground stem portions.

Soil temperature, moisture, and aeration affect survival and movement of nematodes in the top 15-30 cm of soil. Distribution in a cultivated field can be quite irregular and they are usually concentrated in close proximity to the plant roots. Nematodes move through the soil on their own power, and unassisted, wouldn’t move more than a few metres in a season.  However, any practice that moves soil (movement of farm equipment, irrigation or runoff water, birds and dust storms) can potentially move nematodes, over greater distances, into new production areas.

Nematodes that are known to cause problems in corn that have been found in Manitoba soils, (though not necessarily in corn fields), include the following:

Root Knot Nematodes (species of Meloidogyne) 

Symptoms
Damage from root-knot nematodes, appears very similar to symptoms of nutrient deficiencies and soil compaction, and low pH soils. Aboveground, plants may appear stunted or water stressed. In the root zone, “knotted” or galled, stunted roots, may be observed. Some corn plants are capable of  dealing with a large root-knot nematode population, may have numerous galls, and show very little indication of a stunted root system or necrosis due to secondary decay organisms. On a field scale, plants under root-knot attack generally appear in scattered clumps of plants.

Root-Lesion Nematodes (species of Pratylenchus)

Symptoms
Fibrous and coarse corn roots have dark brown dead patches that may cover the entire root system. Roots invaded by lesion nematodes, may develop a significant number of lateral roots. Plants can be severely stunted, and may be distributed in patches within a field, or suppress the growth of the entire field.  Yield losses can be as high as 100%. Plants may appear chlorotic (yellow) in severe field infestations.

Spiral Nematodes (species of Helicotylenchus)

Symptoms
Generally spiral nematodes are considered moderate to weak pathogens, though high numbers have led to a reduction in yield. Plants subjected to spiral nematodes, have numerous small, light or dark brown lesions.

Stunt Nematodes (species of Tylenchorhynchus)

Symptoms
Forms of this nematode feed on the surface of the roots, though may be found partially embedded in the root. At high population densities (greater than 200 nematodes per 100cm3 of soil), damage to corn may occur. Stunting and chlorosis of the shoots may occur at these high levels and result in a corresponding reduction in yield. Defoliation and wilting may occur. The roots may appear brown, reduced in size and number, with an absence of root hairs on the most recent growth. Feeder roots may be brown and wither, reduced in size, and appear swollen near the tip.

Not yet found in Manitoba:

Stubby Root Nematodes (species of Paratrichadorus)

Symptoms
As the name suggests, a stunted, stubby root system, is the most apparent symptom of root infection by this nematode. As a seedling, the root tips are fed on by the nematodes, reducing root elongation. The root systems show little in terms of dead areas or discolouration initially. However, as the season progresses, secondary invaders (fungi, bacteria, other nematodes, insects etc.) enter the plant, creating dead patches on the root. The above ground symptoms are much like those of plants with few functional roots. Stunted, chlorotic, wilting, and death after dry periods, may be observed.  While there are no reports of this nematode ever being found in Manitoba, this nematode has caused significant problems in the United States, and could be a concern if found in Manitoba.

Note: Diagnosis on the basis of symptoms alone, is not sufficient evidence to conclude a nematode problem is responsible for a reduction in yield. A nematode test is required to identify the levels of nematodes present in the soil. A number of laboratories can perform this service.

Control of nematodes can be quite difficult, and is not always cost effective. As nematodes have very broad host ranges, crop rotations may only reduce the numbers of some nematodes, not eliminate them. 

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