 |
 |
|
|||||||||||||||||||||||||||||
 |
|||||||||||||||||||||||||||||
 |
 |
||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||
Common Diseases of Corn in Canada
Corn diseases, caused by bacteria, fungi and viruses, reduce yields and grain quality throughout Canada. Some of these diseases occur every year but do not cause much damage while others occur only sporadically and result in significant yield and quality losses under favourable weather conditions.
The purpose of this publication is to assist corn researchers and producers in identifying diseases likely to be encountered in Canadian corn fields. Some general recommendations for control are also given but all disease management decisions should only be made after consultation with agricultural extension professionals.
- BACTERIAL DISEASES
- FUNGAL DISEASES
- Gibberella Ear Rot (Pink Rot, Red Rot)
- Fusarium Ear and Kernel Rot
- Diplodia Ear Rot
- Common Smut (Boil Smut, Blister Smut)
- Head Smut
- Gibberella Stalk Rot
- Fusarium Stalk Rot
- Diplodia Stalk Rot
- Anthracnose Top-Die Back and Stalk Rot / Anthracnose Leaf Blight
- Pythium Stalk Rot
- Northern Leaf Blight (white blast, crown stalk rot or stripe)
- Eyespot (brown spot)
- Common Rust
- Crazy Top Downy Mildew
- VIRAL DISEASES
1.0 BACTERIAL DISEASES
Plant-pathogenic bacteria are single celled organisms. In suitable environments, bacteria populations can double within a few hours. Bacteria are spread by an animal or insect vector, rain splash, wind, and farm machinery. Identification of plant-pathogenic bacteria can be difficult. Symptoms of a suspected bacterial disease must be confirmed with laboratory identification of the bacteria; however, one of the more common signs is an ooze or bacterial stream from infected plant tissue. There are only two bacterial diseases that are commonly found in some parts of Canada: Stewart's bacterial wilt and holcus leaf spot.
1.1 Stewart's Bacterial Wilt (Stewart's wilt; bacterial wilt; bacteriosis)
Causal Organism: Erwinia stewartii (Smith) Dye (Pantoea stewartii Mergaert et al.).
Symptoms appear soon after tasselling and consist of long, pale green/yellow or tan coloured streaks with wavy margins that generally follow leaf veins (Fig. 1). Feeding scars of the flea beetle can sometimes be found as a small arc-like scratch on the leaf at the base of the streak.
The streaks soon dessicate and turn brown. If infection is severe, entire leaves may die and dry up. If the bacterium enters the stalk, the tassel becomes bleached and may die while the stalk decay causes a general wilting of the plant. Infected plants are also predisposed to fungal stalk rots. Infected stalks may ooze droplets of pale yellow fluid. Early infected plants, especially sweet corn, will be stunted, produce small ears or be barren (Fig. 2) or even die before tasselling.
Occurrence:
Most severe in southern Ontario, especially after a mild winter followed by hot summer (above 30C). Occurrence in the rest of Ontario and Quebec is sporadic. The corn flea beetle (Chaetocnema pulicaria) is the vector for this bacterium. Bacteria overwinter in adult beetle guts, so outbreaks are linked to the winter survival rate of the beetles. In the spring, the beetles feed on the plants and infect them. Stewart's wilt bacteria are also seedborne, but seed transmission is rare.
Control:
Use resistant hybrids, especially in areas where warm winters favour survival of the flea beetle. Dent corn is much less susceptible than sweet corn. Controlling weeds, especially grasses, may remove alternate hosts of the flea beetle. Insecticide applications can be used to reduce flea beetle populations but this may not be economical except in sweet corn crops. High levels of nitrogen and phosphorus increase disease incidence and severity. High levels of calcium and potassium decrease disease severity.
1.2 Holcus leaf spot
Causal Organism: Pseudomonas syringae pv. syringae van Hall (syn. P. holci Kendrick).
First symptoms appear as small (less than 10mm diameter) round-to-elliptical shaped dark green spots on the leaves. These spots first have the appearance of being water-soaked but eventually dry to a creamy-white or tan colour (Fig. 3).
Occurrence:
This disease is only occasionally found in Canada and is usually not severe. If plants are in the vegetative growth stage (before flowering), symptoms often appear a few days after a thunderstorm. Rainy, windy weather and warm temperatures (25-30C) favour disease development especially early in the season. The bacteria are splashed onto leaves and penetrate through openings on the leaf surface called stomata or through wounds caused by hail, blowing soil or wind.
Control:
Little is known about the resistance levels in commercial hybrids. The bacterium survives in corn debris and has several alternate hosts including weeds, so crop rotation and effective weed control are important.
2.0 FUNGAL DISEASES
Most of the corn diseases in Canada are caused by fungi. The body of most plant-pathogenic fungi consists of a mass of numerous threadlike hyphae called a mycelium (often referred to as mold). Various other structures can be formed ranging from single cells such as spores to large fruiting bodies which produce spores. The asexual reproductive stage is usually associated with the mycelium while the sexual stage is usually associated with fruiting bodies. Spores can be produced from both stages and are the major means by which fungi reproduce. Spore size and shape is often used to identify fungi along with the characteristics of the mycelium and fruiting bodies. Fungi can attack all parts of the corn plant. Some fungi remain localized in the tissue that was first infected, e.g. the ear, while other fungi are systemic and can spread throughout the plant.
2.1 Gibberella Ear Rot (Pink Rot, Red Rot)
Causal Organism: Fusarium graminearum Schwabe [sexual state: Gibberella zeae (Schwein.) Petch].
The typical symptom of gibberella ear rot is a pink-to-reddish coloured mold, often starting at the ear tip or from an insect wound and growing down the ear (Fig. 4). Cobs can become quite spongy and husks become bleached and adhere tightly to the kernels. A powdery, cottony-pink mold may form later. Black coloured perithecia (fungal fruiting bodies that produce sexual spores) may be visible on husks.
Occurrence:
This is one of the most economically important ear rotting agents in Canada. The pathogen survives in soil and on crop residue; spores are produced in wet weather and dispersed by wind, rain splash and insects or birds. Infection takes place through the silks or through wounds created by birds or insects. Silks are highly susceptible 2-6 days after silking; kernels are susceptible until they reach physiological maturity. Infected ears are contaminated with mycotoxins such as deoxynivalenol (DON, vomitoxin) and zearalenone, which are highly toxic to livestock, especially swine, and humans. Gibberella ear rot is favoured by cool, wet weather shortly after silking.
Control:
Use resistant hybrids if available. Since this fungus also causes scab in wheat, corn should not be rotated with wheat or other cereals susceptible to scab if this disease is present. Hybrids with tight husk coverage and upright ears tend to get less infection. Crop rotation and fall tillage will reduce crop debris and thus may reduce disease levels in the next season. Insect control will reduce symptoms, especially with the use of Bt hybrids. Fields should be scouted as the corn begins to dent; if mold problems are identified, these fields should be harvested as soon as possible to prevent further mold growth. Adjust combines to eliminate light weight, moldy kernels and reduce damage to other kernels that may lead to continued mold growth in storage. Dry moldy corn immediately and rapidly. Test grain for mycotoxins before using as feed.
2.2 Fusarium Ear and Kernel Rot
Causal Organisms:
There are three main species responsible for fusarium ear and kernel rot: Fusarium verticilliodes [= Fusarium moniliforme J. Sheld. (sexual stage: Gibberella moniliformis Wineland)]; F. proliferatum (T. Matsushima) Nirenberg (sexual stage: G. fujikuroi var. intermedia Kuhlmann); and F. subglutinans (Wollenweb. & Reinking) Nelson, Toussoun and Marasas (sexual stage: G. subglutinans Nelson, Toussoun and Marasas).
The typical symptom of fusarium kernel or ear rot is a whitish to pink coloured mold, often starting at the ear tip or butt of the ear, commonly from an insect wound. Fusarium verticilliodes symptomatic kernels tend to be randomly scattered on the ear (Fig. 5) unlike other ear rots that spread from an initial point of entry. Infected kernels may also exhibit a 'starburst' symptom in which white streaks radiate from the point of silk attachment to the kernel. Husks may become bleached and adhere tightly to the kernels. Black coloured perithecia (fungal fruiting bodies that produce sexual spores) may be visible on husks.
Occurrence:
This disease is more important in the warmer, dry areas of Canada such as southern Ontario. The pathogens survive in soil and on crop residue; spores are produced in wet weather and dispersed by wind, rain splash and insects or birds. Infection takes place through the silks or through wounds created by birds and/or insects. There is also evidence that F. verticilliodes can invade the ear by systemic infection from the stalk. Infected ears may be contaminated with mycotoxins such as fumonisins, which are highly toxic to livestock, especially horses and humans.
Control:
Use resistant hybrids if available. Hybrids with tight husk coverage and upright ears tend to get less infection. Crop rotation and fall tillage are important to reduce crop debris and thus disease levels in the next season. Insect control or use of Bt hybrids will reduce symptoms. Fields should be scouted as the corn begins to dent; if mold problems are identified, these fields should be harvested as soon as possible to prevent further disease development. Adjust combines to eliminate light weight, moldy kernels and reduce damage to other kernels which may lead to continued mold growth in storage. Dry moldy corn immediately and rapidly. Test grain for mycotoxins before using for feed.
2.3 Diplodia Ear Rot
Causal Organism: Diplodia maydis (Berk.) Sacc. [= Stenocarpella maydis (Berk.) Sutton].
Symptoms:
With Diplodia ear rot, infected husks become dry and bleached but the rest of the plant remains green. Eventually, the husks and kernels become covered with a white to grayish brown coloured mold. Kernels appear glued to the husks and cob by white mycelium. Infected ears are light and shrunken. Black pycnidia (fungal fruiting bodies that release asexual spores; look like small raised black bumps) will form on husks and kernels late in the season, this is a characteristic of Diplodia ear rot. If infection occurs several weeks after flowering, symptoms are much less apparent but close inspection of the ear will reveal white mycelial growth between the kernels. Symptoms are most severe if infection occurs just after silking.
Occurrence:
This disease is not as common as gibberella or fusarium ear rot. Diplodia maydis overwinters on corn debris so the incidence is increasing with the increased use of conservation tillage. The disease is most severe when corn follows corn in rotation, especially if conditions are wet after silking. The pycnidia on the debris produce spores that are splashed onto the silks and then grow down the silks into the ear. This fungus can also penetrate the husks especially at the base of the ear. Bird and insects may also vector the disease and create wounds on the ear for easy fungal entrance. Infection is favoured by cool, wet weather during grain fill. This fungus is not known to produce mycotoxins.
Control:
Use resistant hybrids. Hybrids with tight husk coverage and upright ears tend to get less infection. Crop rotation and fall tillage are important in reducing crop debris and thus disease levels in the next season. Fields should be scouted as the corn begins to dent; if mold problems are identified, these fields should be harvested as soon as possible to prevent further disease development. Adjust combines to eliminate light weight, moldy kernels and reduce damage to other kernels which may lead to continued mold growth in storage. Dry moldy corn immediately and rapidly.
2.4 Common Smut (Boil Smut, Blister Smut)
Causal Organism: Ustilago zeae (Beckm.) Unger [ = U. maydis (DC.) Corda].
Symptoms
|
|
can occur on all plants parts above the ground, especially young and actively growing plant tissues. Large 2-10 cm galls (swollen, distended growths) are formed on stalks (Fig. 6), tassels and ears (Fig. 7). The galls are first covered by a silvery-white membrane, which changes to a grey mass containing black powdery spores.
When mature these galls will erupt and release black spores. Galls may also form on leaves but are generally small, brown and hard (Fig. 8). Early infection may kill young plants. Plants with large galls on the lower stalks may be stunted, barren or produce small ears. Infection of ears is usually from spores germinating on the silks and mycelia growing down the silks to infect the kernels
Occurrence:
This disease is found in most corn growing regions of Canada. Black teliospores of the pathogen overwinter in soil or crop debris and can survive for several years. Initial infections occur from wind-borne or water-borne spores in the spring. Spores from galls on the plants can infect other plants. Infection can occur through unwounded tissue, but wounds caused by insects, detasseling, cultivation, hail or blowing soil are important sources of fungal entry. The fungus favours high temperatures of 26-34C. There is no consensus on whether smut favours humid or dry conditions. During combine harvesting, spores are spread locally by the wind. Spores can also be spread from field to field by contaminated farm equipment. In certain parts of Mexico, galls that form on sweet corn ears are considered a delicacy known as huitlacoche or maize mushroom.
Control:
Use resistant hybrids and avoid mechanical injury to plants. Destruction of galled plants is useful where feasible. Crop rotation will reduce disease severity in the following season. Maintain balanced soil fertility since high nitrogen and manure applications will promote vigorous plant growth making the plant more susceptible; phosphate fertilization tends to decrease incidence. Herbicide injury will also promote infection.
2.5 Head Smut
Causal Organism: Sporisorium holci-sorghi [= Sphacelotheca reiliana (Kuhn) G.P. Clinton].
Symptoms first appear when ears and tassels are formed. Infected ears are smaller, rounded, lack silks and may have galls that are first covered with a thin layer of tissue that breaks open to expose the black spore masses and threadlike remains of the plant (Fig. 9). Leaf-like proliferations may develop on the tassels and ears. Tassels can become completely changed into a mass of sooty looking branches resembling a 'wire brush' (Fig. 10). Ears may be aborted and replaced with a leaf-like proliferation of tissues. Plants can become severely dwarfed. Galls may form on leaves in long narrow strips but the leaves and stalks are rarely symptomatic.
This is a systemic disease (Fig. 11) found on some farms in Canada. Black teliospores of the fungus overwinter in soil and then infect the growing point of young seedlings as the plants grow to the surface. The fungus then grows internally (systemically) into the ear and tassel. Infection is more common in dry years with temperatures of 21-28C. Spores can survive in the soil up to 10 years. Spores may also be seedborne and spread through contaminated animal feed and manure because the spores pass through the animals unaffected. Spores can be spread locally by the wind during combine harvesting. Spores can also be spread from field to field by contaminated farm equipment.
Control:
Use resistant hybrids. Hybrids with more rapid seedling development may escape infection. Rotate crops; do not plant corn for several years in contaminated fields. Care must be taken to prevent dissemination of spores in seed, feed, manure and on farm equipment. Fungicide may be useful in sweet corn fields. Maintain balanced nitrogen levels because this disease is more common in soils with nitrogen deficiencies.
2.6 Gibberella Stalk Rot
Causal Organism: Fusarium graminearum Schwabe [sexual state: Gibberella zeae (Schwein.) Petch].
Symptoms:
|
Fig. 12 - Full View |
Gibberella stalk rot infected plants may wilt and the leaves may change from a light to dull green colour while the lower stalk becomes dry and the pith tissue disintegrates to a shredded appearance (Fig. 12). Distinctive symptoms of gibberella ear rot are a tan to dark brown discolouration of the lower internodes and a pink to reddish discolouration of the pith tissue. Bluish-black coloured perithecia (fungal fruiting structures that releases sexual spores) or reddish-white asexual spores form on the stalk surface. Plants may lodge if the infection is severe.
Occurrence:
This fungus is one of the more potentially damaging stalk rotting agents in Canada. The pathogen survives in soil and on crop residue; spores are produced in wet weather and dispersed by wind, rain splash and insects or birds. Infection takes place through roots or through wounds in the stalk often shortly after pollination. Like most stalk rots, occurrence is strongly linked to stress during grain filling by any conditions that reduce photosynthesis and production of sugars (leaf diseases, drought or soil saturation, insect damage, hail damage, lack of sunlight, cool weather, etc.). Many stalk rot infections can be traced back to wounding of the stalk by boring insects such as the European corn borer.
Control:
Planting resistant hybrids, clean plowing, crop rotation, avoidance of high plant populations and balanced soil fertility are some of the methods to control this disease. Since this fungus also causes scab in wheat, corn should not be rotated with wheat or other cereals susceptible to scab if this disease is present. Control of stalk boring insects is helpful. Fields should be scouted 40-60 days after pollination by looking for symptoms and pinching stalks. When pinched, infected stalks are easily compressed or crushed. Fields should be harvested as early as possible if more than 10-15 % of the stalks have rot because rotted stalks will lodge, making harvest difficult.
2.7 Fusarium Stalk Rot
Causal Organism: There are three main species responsible for fusarium ear and kernel rot: Fusarium verticilliodes [= Fusarium moniliforme J. Sheld. (sexual stage: Gibberella moniliformis Wineland]; F. proliferatum (T. Matsushima) Nirenberg (sexual stage: G. fujikuroi var. intermedia Kuhlmann); and F. subglutinans (Wollenweb. & Reinking) Nelson, Toussoun and Marasas (sexual stage: G. subglutinans Nelson, Toussoun and Marasas).
Symptoms:
Symptoms of fusarium stalk rot are not much different from gibberella stalk rot. Plants may wilt, the leaves change from a light to dull green, the lower stalks dry and the pith tissue disintegrates to a shredded appearance. Brown streaks appear on the lower internodes and the rotted pith tissue may be whitish pink to salmon in colour as opposed to the distinct red-pink colour of gibberella stalk rot. Infected plants may lodge. Symptoms usually appear late in the season.
Occurrence:
This disease is more important in the warmer, dry areas of Canada such as southern Ontario. The pathogens survive in soil and on crop residue. Spores are dispersed by wind, rain, insects and birds. This fungus may also be present in the seeds and/or the plants but not create disease symptoms. Infection can take place through the roots or through wounds on the leaves or stalk. Like most stalk rots, occurrence is strongly linked to stress during grain filling by any conditions that reduce photosynthesis and production of sugars (leaf diseases, drought or soil saturation, insect damage, hail damage, lack of sunlight, cool weather etc.). Many stalk rot infections can be traced back to wounding of the stalk by boring insects such as the European corn borer.
Control:
Planting resistant hybrids, clean plowing, crop rotation, avoidance of high plant populations and balanced soil fertility are some of the methods to control this disease. Control of stalk boring insects is helpful. Fields should be scouted 40-60 days after pollination by looking for symptoms and pinching stalks. When pinched, infected stalks are easily compressed or crushed. Fields should be harvested as early as possible if more than 10-15 % of the stalks have rot because rotted stalks will lodge, making harvest difficult.
2.8 Diplodia Stalk Rot
Causal Organism: Diplodia maydis (Berk.) Sacc. [= Stenocarpella maydis (Berk.) Sutton].
Symptoms:
Like most stalk rots, Diplodia rotting symptoms include a disintegration of the pith tissue causing the pith to have a shredded appearance and stalks that are easily crushed or compressed when pinched. Diplodia stalk rot is characterised by numerous black pycnidia (flask shaped fungal fruiting structures that produce asexual spores) that appear as small dots about the size of a pinhead on the lower internodes of the stalk. If conditions are wet, a white mold may appear on the surface of the stalk.
Occurrence:
This fungus overwinters as spores on corn debris and as pycnidia or mycelium in corn seed. Incidence of this stalk rot are increasing in minimum tillage areas especially if corn follows corn in rotation. During wet weather, pycnidia produce spores that are dispersed by splashing water and wind. These spores infect the plant through the crowns, roots, and lower nodes. Insects may also carry the spores to plants and into feeding wounds. Dry conditions early in the season, followed by wetter conditions after silking, favour disease development. Like most stalk rots, occurrence is strongly linked to stress during grain filling by any conditions that reduce photosynthesis and production of sugars (leaf diseases, drought or soil saturation, insect damage, hail damage, lack of sunlight, cool weather etc.).
Control:
Planting resistant hybrids, clean plowing, crop rotation, and balanced soil fertility are some of the methods to control this disease. Fields should be scouted 40-60 days after pollination by looking for symptoms and pinching stalks. When pinched, stalks are easily compressed or crushed. Fields should be harvested as early as possible if more than 10-15 % of the stalks have rot since rotted stalks will lodge, making harvest difficult.
2.9 Anthracnose Top-Die Back and Stalk Rot / Anthracnose Leaf Blight
Causal Organism: Colletotrichum graminicola (Ces.) G.W. Wils. (sexual state: Glomerella graminicola Politis).
|
|
Stalk rot with this disease is characterized by a distinctive blackening of the stalks as a result of black streaks that appear on the lower stalk late in the season. The pith turns dark brown and has a shredded appearance (Fig. 13). Numerous, black, spiny asexual fruiting bodies (acervuli) form on the surface of the dead tissue. Like most stalk rots, the most obvious symptom is a sudden death of plants before grain maturity. Since the leaves wilt and die after infection, the plant has the appearance of being 'frosted'. Another common symptom is lodging. If seedlings are infected, the plants may die before flowering. Top-die back refers to symptoms after the dough stage in which the top nodes, leaves and the tassel dry out, but the bottom leaves remain green and normal (Fig. 14). This same fungus can cause Antracnose Leaf Blight, but this is rarely found in Canada. Symptoms of the leaf blight include oval or spindle-shaped lesions that are tan to brown in colour with a brown or purple margin; acervuli develop on the dead leaf tissue.
Occurrence:
This disease is becoming more severe in Canada. The fungus survives in corn debris, infected seed, and some weed species. For stalk rot, the fungus infects plants through the roots or by rain splash onto stalks. Spores can also infect through wounds on the stalk or leaves. Insects may also carry spores into wounds. Like most stalk rots, occurrence is strongly linked to stress during grain filling by any conditions that reduce photosynthesis and production of sugars (leaf diseases, drought or soil saturation, insect damage, hail damage, lack of sunlight, cool weather etc.). Many stalk rot infections can be traced back to wounding of the stalk by boring insects such as the European corn borer.
Control:
Use resistant hybrids; resistance to both the stalk and leaf blight forms are not necessarily found in the same hybrid. Clean plowing may reduce inoculum levels. Crop rotation is essential especially in reduced tillage areas. Balanced soil fertility is important to control this disease. Control of the corn borer will reduce entry of the pathogen through feeding sites. Fields should be scouted 40-60 days after pollination by looking for symptoms and pinching stalks. When pinched, infected stalks are easily compressed or crushed. Fields should be harvested as early as possible if more than 10-15 % of the stalks have rot since rotted stalks will lodge, making harvest difficult.
2.10 Pythium Stalk Rot
Causal Organism: Pythium aphanidermatum (Edson) Fitzp.(= P. butleri L. Subramanian) and other Pythium species.
Symptoms:
A typical symptom before flowering is a decay of the first internode above the soil. Infected stalks may have a strong odour. Damaged internodes may twist and fall over but the plant can remain green for several weeks. After the milk stage of plant development, the roots and several lower internodes become water-soaked and decay and can result in early plant death.
Occurrence:
This disease can occur any time during the season, especially when conditions are warm and wet. The fungus overwinters in the soil and plant debris as oospores that germinate and produce mycelium or zoospores, both of which can infect corn plants.
Control:
Use resistant hybrids. Stress factors such as unbalanced fertilization, high plant populations, use of unadapted hybrids, insects, weeds, poor drainage and infection by leaf diseases may increase plant susceptiblity. Fields should be scouted 40-60 days after pollination by looking for symptoms and pinching stalks. Infected stalks are easily compressed or crushed when pinched. Fields should be harvested as early as possible if more than 10-15 % of the stalks have rot since rotted stalks will lodge, making harvest difficult.
2.11 Northern Leaf Blight (white blast, crown stalk rot or stripe)
Causal Organism: Setosphaeria turcica (Luttrell) K.J. Leonard & E.G. Suggs [asexual state: Exserohilum turcicum (Pass.) K.J. Leonard & E.G. Suggs = Helminthosporium turcicum Pass.].
Symptoms consist of long, elliptical, grayish-green or tan coloured lesions appearing first on the lower leaves (Fig. 15). Lesions may be as large as 4 x 15 cm. When infection is severe, entire leaves die and it is difficult to see individual lesions; this is sometimes referred to as a 'burning' of leaves.
Lesions may also occur on husks but the kernels are not infected. A severely infected plant may turn grayish-green and die pre-maturally. Hybrids with Ht resistance genes develop smaller lesions (Figs. 16 and 17).
Occurrence:
This is one of the most damaging leaf diseases of corn in Canada and northern regions of the world. The fungus overwinters on corn debris. Spores are wind-borne up to 2 km under moderate temperatures (18-25C) and can also be dispersed by rain splash. Heavy dews favour disease development. Secondary infection can rapidly spread from field to field. In Canada, the initial source of infection may be from spores blowing in from the U.S. Like most leaf diseases, plants infected with northern leaf blight may be predisposed to stalk rot.
Control:
This fungus has four races. Hybrids resistant to a single race or to all races are available. Crop rotation and tillage help to reduce surface residues and pathogen populations. Fungicide applications are useful in sweetcorn but must be applied early and often several times.
2.12 Eyespot (brown spot)
Causal Organism: Aureobasidium zeae (Narita & Hiratsuka) J.M. Dingley (= Kabatiella zeae Narita & Hiratsuka).
Symptoms on leaves consist of round-to-oval lesions (2-5 mm in diameter) with a tan-to-cream coloured centre and a brown (Fig. 18) or purple (Fig. 19) margin surrounded by a yellowish halo, giving the characteristic 'eyespot' appearance. These lesions are easy to recognize if the leaf is held up to a light. The lesions may fuse to form large necrotic areas. The upper leaves may wither and die prematurely. Symptoms can be confused with non-infectious physiological leaf spots or insect damage found on some corn varieties.
Occurrence:
The incidence of this disease is increasing in Canada. The disease occurs during cool, wet weather in late August and September. The fungus overwinters in corn debris and spores are spread by rain splash and wind. Higher incidence in conservation tillage areas and when corn follows corn has been reported. Like most leaf diseases, plants infected with eyespot may be predisposed to stalk rot.
Control:
Resistant hybrids must be grown in areas with a history of eyespot. Crop rotation and clean plowing are recommended to control this disease. Fungicides can be applied in sweet corn fields.
2.13 Common Rust
Causal Organism: Puccinia sorghi Schwein.
|
|
Symptoms begin as small discoloured flecks on the leaves that soon turn into small, round to elongate, reddish brown pustules full of red coloured urideospores (Fig. 20). Pustules can be observed on both leaf surfaces as well as husks, leaf sheaths and stalks. The red spores can be rubbed off on hands or clothing. The pustules turn black as the plant matures (Fig. 21).
Pustules are frequently clustered in bands around the leaves as a result of infection that took place when the leaf tissue was in the whorl. Younger leaves are more susceptible than mature leaves. When the disease is severe, leaf tissue around the pustules turns yellow, withers and dies. Some hybrids have resistance genes; these plants may develop a hypersensitive response consisting of several small (pin-prick) pale coloured lesions (Fig. 22).
Occurrence:
This fungus does not overwinter in Canada; spores survive the winter on corn in the southern U.S. and then are carried by wind to northern corn regions of the U.S. and Canada. Rust prefers wet seasons with high humidity (above 95 %) and warm temperatures (16-25C). Like most leaf diseases, plants infected with common rust may be predisposed to stalk rot.
Control:
Use resistant hybrids. Early planting may allow the crop to develop to a less susceptible stage before spores are blown in. Cultural practices such as crop rotation and clean plowing have no effect on disease incidence since the rust spores blow in from southern regions. Fungicide applications may be economical in sweet corn fields.
2.14 Crazy Top Downy Mildew
Causal Organism: Sclerophthora macrospora [Sclerophthora macrospora (Sacc.) Thirumalachar et al. = Sclerospora macrospora Sacc.].
Symptoms:
Typical symptoms are a distortion and/or stunting of the plant and tassel which may be partially or completely replaced by a 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.
Occurrence:
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. This disease is not a major problem in Canada.
Control:
Providing adequate soil drainage is an effective method to control this disease. Wet spots should be avoided. Plants will not be affected if the growing point is not submerged.
3.0 VIRAL DISEASES
Despite the fact that viruses cause the most important diseases of corn elsewhere in the world, viral diseases of corn in Canada are very rare, often only occurring on very late planted corn and in the warmer corn growing regions of southern Ontario. Two viruses more likely to occur are maize dwarf mosaic and wheat streak mosaic.3.1 Maize Dwarf Mosaic
Causal Organism: Maize Dwarf Mosaic Virus (MDMV-A) and Sugar Cane Mosaic Virus (SCMV, formerly MDMV-B).
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 (Fig. 23). These spots can develop into narrow, light-green or yellowish streaks along the veins (Fig. 24).
Later in the season, plants may have streaks of red on the leaves (Fig. 25), 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.
Occurrence:
In Canada, most reports of MDMV or SCMV are on late planted sweet and grain corn. Both viruses are transmitted by the feeding of over 20 species of aphids. These insect vectors are wind blown from the U.S. Both viruses can infect over 200 species of wild and cultivated grasses but Johnsongrass is the only important overwintering host in North America.
Control:
Use resistant hybrids. Most sweet corn cultivars are susceptible. The control of aphids on young plants may delay infection of late seeded crops. Early planted corn may escape aphid infestation at the seedling stage.
3.2 Wheat Streak Mosaic
Causal Organism: Wheat Streak Mosaic Virus (WSMV).
Symptoms:
This virus is vectored by the wheat curl mite (Aceria tulipae). Small, oval to elliptical yellowish spots and streaks form on young leaves. These streaks elongate and develop parallel to veins. Severely affected plants may be stunted and yellowed and form small ears with poor seed set. As the plant matures, the mites feed on the sides of the kernels; it is believed that this is the cause of kernel red streak which is characterised by brownish-red streaks appearing on the kernels.
Occurrence:
This disease is only occasionally found in corn in Canada; however, it can be a serious disease in wheat. Grassy weeds may act as alternative hosts for the mites. It is believed that the mites overwinter on winter wheat and in the spring are blown from the developing winter wheat to corn, spring wheat and barley crops.
Control:
Use resistant hybrids. Late planting of corn close to maturing wheat should be avoided since it will carry the vector between crops.
Date Modified: 2006-01-10 |