Field Corn Diseases In Illinois

Field Corn Diseases in Illinois Many corn fields in every county in Illinois develop disease problems every year that affect yield and quality of the grain crop. As history has shown repeatedly, corn diseases can and do periodically cause significant yield losses in patterns that are difficult to predict in advance. Corn diseases typically cause minimal damage over the entire state, however, some acreage suffers significant disease damage each year. Fortunately, corn has effective genetic resistance to many of the important diseases, however, numerous challenges remain in management of corn diseases. This includes the seed and seedling diseases, leaf diseases, stalk diseases, and ear rots. The occurrence of any specific disease depends on environmental conditions, cultural practices, and the hybrid that is grown. Note: This site is under construction and materials will continue to be added, so please check back periodically for new information.

(http://cropdisease.cropsci.illinois.edu/corn/index.html) ( University of Illinois College of Agricultural, Consumer and Environmental Sciences)

1.www.sripmc.org/docs/SoutheasternVegetableGuide.pdf Similar(VEGETABLE CROP)

I. Leaf Diseases 1.Anthracnose Leaf Blight

Symptoms: • • • • •

Water-soaked, oval lesions usually appear first. Lesions enlarge up to 15 mm long and have tan centers with reddish-brown borders. Lesions on susceptible plants may join and blight the entire leaf and can cause the entire leaf to die late in the growing season. Symptoms develop on lower corn leaves early in the growing season, and develop on the upper corn leaves late in the growing season. Resistant corn lines may only have small, necrotic or chlorotic, lesions if infected.

Pathogen Involved: • •

Colletotrichum graminicola (a fungus) Overwinters on corn debris.

Time of Occurrence: •

Anthracnose is most likely to occur at the seedling phase and at full maturity of corn, but not in the middle of the summer.

Conditions Favoring Disease: •

High temperatures, prolonged wet weather

Disease Management: • • •

Use resistant hybrids Tillage may be beneficial where appropriate Crop rotation can reduce early season infection

2. Common Rust

Symptoms: • • • •

Pustules are oval to circular, and red to dark brown Pustules are common on both leaf surfaces Rust spores (urediniospores) are cinnamon brown Leaves as well as sheaths can be infected, and under severe conditions leaf chlorosis or death may occur

Pathogen Involved: • • •

Puccinia sorghi (rust fungus) Overwinters in the southern U.S. and Mexico (not in the corn belt) Urediniospores serve as the repeating stage of this fungus and are blown north to the corn belt in the early summer

Time of Occurrence: • •

Occurs in June or July when urediniospores are introduced into the U.S. corn belt via wind Pustules are more likely to form after corn silking

Conditions Favoring Disease: • • •

Cool temperatures (60 - 76°F), heavy dews, and high relative humidity (greater than 95%) Approximately, 6 hours of moisture is needed for germination and infection Young leaves are most susceptible to infection

Disease Management: • •

Use resistant corn hybrids Fungicides can be most beneficial if applied early and when are few pustules on the leaves

3. Common Smut

Symptoms: • • •

Galls develop on ears, leaves, stalk, or tassels Galls initially are covered with white to silvery tissue Interior of galls develop into a dark mass of dark spores

Pathogen Involved: • • •

Ustilago maydis (a fungus) Overwinters in crop debris or soil This pathogen is present where corn is grown around the world

Time of Occurrence: •

Can occur throughout the season

Conditions Favoring Disease: • • • •

Hail, wind, or mechanical wounding of young, actively growing tissue Rainy, wet weather may enhance this disease Poor pollination Excess nitrogen fertilization

Disease Management: • • •

Use resistant hybrids. Crop rotation Maintain proper soil fertility

4. Crazy Top

Symptoms:

• • • •

Extreme tillering (many tillers per plant), twisting, and rolling of top leaves The tassel may proliferate until it looks like a bundle of leafy structures Leaves of severely infected plants may be narrow and leathery as well as shortened with chlorotic stripes Symptoms will vary with time of infection

Pathogen Involved: • • •

Sclerophthora macrospore (an oomycete)\ This pathogen is widespread throughout the United States. The pathogen infects multiple grass species.

Time of Occurrence: • • •

This disease starts when the whorl emerges on young plants. Disease develop most often in plants from the seedling to the four to five leaf stage that have been flooded for 24-48 hours. Infection also can occur when soil and water collects within the whorl.

Conditions Favoring Disease: •

Flooding

Disease Management: • •

Soil drainage Avoid planting in wet, low areas

5. Eye Spot

Symptoms: •

Initial lesions are small, circular (1 to 4 mm diameter) and water soaked, with yellow halos

• • • •

Lesions develop into an "eyespot", which is a tan spot surrounded by a brown to purple ring and yellow halo These lesions most often will appear in patches on leaves, and can join together to form large necrotic areas Lesions will mainly be concentrated on the upper surface of corn leaves, but can sometimes be found on leaf sheaths and husks These symptoms may be confused with lesions of Curvularia leaf spot, a physiological leaf spot, or a genetic leaf spot

Pathogen Involved: • • •

Kabatiella zeae (a fungus) Overwinters in corn residue Corn is the only reported host

Time of Occurrence: •

The initial lesions can occur as early as silking or as late as corn maturity

Conditions Favoring Disease: • • •

Cool temperatures and humid conditions Continuous corn Conservation tillage

Disease Management: • • •

Use resistant corn hybrids Crop rotation Reduction of corn residue with tillage where appropriate

6. Goss's Bacterial Wilt and Blight

Symptoms: • •

Gray to yellow to reddish striping with wavy margins that follow veins on leaves Dark, water-soaked, irregular small spots inside the lesions

• • • • •

Lesion surfaces may contain patches of dried bacterial exudates Symptoms can be confused with Stewart's wilt These symptoms may vary on different corn inbreds or hybrids Orange vascular bundles may be seen if a corn stalk is cut into two The seedling blight phase of this disease is not common but this disease may cause wilting and death of seedlings

Pathogen Involved: • • • •

Clavibacter michiganensis subsp. nebraskensis (a bacterium) This pathogen has not been reported outside the United States Overwinters in infested corn debris near the soil surface Infects by way of wounds

Time of Occurrence: •

Symptoms are often most severe at or near silking

Conditions Favoring Disease: • •

Wounding from hail, sand-blasting, rain, wind, or machinery Warm temperatures and humidity

Disease Management: • •

Use disease resistant hybrids Crop rotation and tillage after harvest

7. Gray Leaf Spot

Symptoms: • • •

Lesions first appear as small, necrotic spots with halos The lesions become rectangular, 2-4 mm to1-6 mm in size and have a gray appearance Mature lesions have distinct parallel edges and appear opaque when put up to the light

Pathogen Involved: •

Cercospora zeae-maydis (a fungus)

• •

Corn is the only known host of this pathogen Overwinter in corn debris on the soil surface

Time of Occurrence: •

This disease will occurs from silking to maturity

Conditions Favoring Disease: • • • • •

Susceptible hybrids or inbreds Corn on corn rotation Moderate to warm temperatures Extended periods of high humidity or wet weather Minimum tillage

Disease Management: • • • •

Use resistant corn hybrids Conventional tillage where appropriate Crop rotation Foliar fungicides if economically warranted. Typically only used with susceptible inbreds; or a combination of susceptible hybrids, high yield potential, prolonged humid conditions, and evidence of disease development.

8. Holcus Spot

Symptoms: • •

• •

Initial lesions are water-soaked and dark green Lesions develop into round or elliptical, tan to white spots that are 2 to 10mm in diameter. Lesions become necrotic with red to brown margins and may develop yellow halos Lesions will often form on the tips of lower leaves Symptoms can be easily confused with damage from paraquat drift

Pathogen Involved: • • •

Pseudomonas syringae pv. syringae Overwinters in crop debris and will infect through stomata. The pathogen has a wide host range including many grasses and dicots

•

This pathogen can have ice nucleating activity and may enhance frost injury to corn leaves.

Time of Occurrence: •

Occurs early in the season.

Conditions Favoring Disease: • • •

Prolonged wet weather Warm temperatures (76 to 86°F) Wind and early season thunderstorms

Disease Management: • •

Crop rotation Tillage may be beneficial where appropriate

9. Maize Chlorotic Dwarf Virus Symptoms: • • • • •

Leaves may be reddish and yellow, but this will depend on environment, hybrid, or time of infection Veinbanding occurs when young leaves become chlorotic within the whorl Plants may be stunted and internodes may be shortened or stunted Leaves may also be torn, twisted or have swollen veins with a dull, rough surface Plants may also be infected with maize dwarf mosaic virus

Pathogen Involved: • • • •

Maize chlorotic dwarf (a virus) Transmitted by adult or nymph leafhoppers semi-persistently. Also transmitted mechanically by physical rubbing between plants Overwinters in Johnson grass, and infects sorghum, wheat, sudan grass

Time of Occurrence: •

After infected leafhoppers feed on corn plants

Conditions Favoring Disease: • •

Conditions that favor leafhopper development and feeding Nearby Johnson grass infected with this virus

Disease Management:

• •

Use tolerant or resistant commercial corn hybrids. Control of Johnson grass with herbicides may be beneficial

10. Maize Dwarf Mosaic Virus

Symptoms: • • • • •

Symptoms often begin as chlorotic spots and streaks on green, young leaves, which later develop into a mottle or a mosaic pattern Viral strain, corn genotype, and stage of corn development at the time of infection will affect the type of symptoms Upper internodes of corn may be shortened, and excessive tillering may occur Ear formation and development may slow, which may cause grain yield loss Hybrids infected early in their growth stage may be stunted

Pathogen Involved: • • • • •

Maize dwarf mosaic (a potyvirus) This pathogen consists of 4 strains: C, D, E, and F Aphids (= 15 species) can transmit MDMV non-persistently Can be seed transmitted at a low frequency or mechanically transmitted by leaf rubbing, etc. Infects Johnson grass and sorghum

Time of Occurrence: •

Symptoms appear six weeks after aphids feed and transmit this virus

Conditions Favoring Disease: • •

Average to warm temperatures Nearby Johnson grass infected with MDMV may increase disease

Disease Management: • • •

Use tolerant commercial corn hybrids Avoidance of aphid vectors Control of Johnson grass with herbicides may be beneficial

11. Northern Corn Leaf Blight

Symptoms: • • •

Lesions are gray to green in color, cigar shaped, and (3 - 15 cm) long Lesions may contain prominent, dark areas of sporulation as they mature The length or size of lesions may vary with their different reactions to resistant genes within corn hybrids

Pathogen Involved: • • • • •

Setosphaeria turcica (a fungus) Overwinters on corn leaf debris. Races of this pathogen are grouped according to their virulence to resistant Ht genes Race 1 is not considered important, because of use of the Ht1 gene for resistance in most commercial corn hybrids Race 2 may infect commercial corn hybrids that only have the Ht1 gene for resistance .

Time of Occurrence: • • •

Occur at silking or after silking of corn Most prevalent during warm and prolonged periods of moisture during the growing season Significant grain losses can result if this disease begins before silking

Conditions Favoring Disease: • •

Moderate temperatures of 66 - 80 F Long periods of dew

Disease Management: • •

Use resistant hybrids Fungicides may be warranted during the early stages of this disease for seed corn

12. Stewart's Wilt

Symptoms •

•

•

This disease has two phases and primarily is a problem on sweet corn and susceptible inbreds, although resistant hybrids may be affected if flea beetle populations are very high in the spring. The first phase affects corn seedlings. Seedling leaves have light green to yellow linear streaks with wavy margins that can follow the length of parallel veins. Plants can become necrotic and die. The plants may produce bleached tassels if it survives. The second phase is leaf blight after tasseling. Leaves develop light green or yellow streaks along veins. Infected areas will die. The disease can be severe enough to kill the entire leaf. With the absence of resistance, the leaf blight phase can be a threat to field corn.

Pathogen Involved • • • •

Erwinia (=Pantoea) stewartii (a bacterium) Overwinters in and is transmitted by the corn flea beetle Many grass hosts are susceptible Can be seed transmitted at a low rate

Time of Occurrence •

Seedling stage and the leaf blight phase occur after tasseling

Conditions Favoring Disease • • • •

Warm winters and high corn flea beetle populations in spring Susceptible inbreds and sweet corn High levels of soil phosphorus and nitrogen High temperatures and soil moisture

Disease Management • • •

Use resistant corn hybrids Seed-applied insecticides can be beneficial Disease forecasting is based on the sum of mean temperatures for December, January, and February. In mild winters, the corn flea beetle will have a high

survival rate and disease severity may be high. In cold winters, corn flea beetles will have low survival and disease would likely be low severity or absent. 13. Northern Corn Leaf Spot

Symptoms: • • • • •

• •

Lesions are generally oval to circular tan spots The five different races of this pathogen can cause different symptoms Race 1 causes oval to circular, tan lesions that are 1.2 x 2.5 cm in size on leaves. Rings of spores may be present within lesions Race 2 causes oblong, brown spots on leaves, which are 0.5 x 2.5 cm in size Races 3 and 4 will produce narrow, long, gray or tan lesions with a border on leaves, sheaths, and husks, which are (.5 -2 x 15-20 mm) in size. However, the lesions may vary with different corn genotypes. Race 4 may also have concentric rings of spores within lesions Race 0 causes only small flecks or spots on leaves Northern corn leaf spot lesions can be confused with lesions produced by northern corn leaf blight, southern corn leaf blight, and anthracnose

Pathogen Involved: • • • • •

Cochiobolus carbonum (= Bipolaris zeicola) (a fungus) This pathogen overwinters on residue of corn Generally, race 2, 3, and 4 cause important diseases of seed corn production Race 1, 2, 3, and 4 may be found in the corn belt Race 1 is not considered to be a threat.

Time of Occurrence: • •

Lesions usually occur on lower leaves at the time of silking or at full maturity Some races may cause small lesions early in the growing season

Conditions Favoring Disease: •

Most severe in warm and humid weather

Disease Management: • •

Generally, this is not an important disease of hybrid corn Resistance to all races of northern corn leaf spot is available

• • •

Crop rotation Tillage of corn debris at the end of the growing season where appropriate This disease should often be controlled on inbred lines and foliar fungicides may be warranted for hybrid seed corn production

14. Physoderma Brown Spot

Symptoms: • • •

Initial lesions are tiny, round, and yellow spots Later, lesions turn to a brown or reddish color and merge together to form irregular blotches Lesions often occur in bands on leaves as well as on sheaths and stalks, but rarely occur on ear husks and tassels

Pathogen Involved: • • • •

Physoderma maydis (a fungus) Overwinters in infested tissue or soil Primarily occurs in regions with high temperatures and much rainfall This disease is not considered to be important in the United States, however, when conditions are favorable, severe damage can occur

Time of Occurrence: • •

Corn plants are most susceptible 50 to 60 days after germination This disease will occur most often after late whorl

Conditions Favoring Disease: • •

Infection usually occurs on leaf tissue within the whorl, where water has gathered High temperatures (72 - 86°F) and heavy rainfall

Disease Management: • •

Infection may be reduced by tillage or destroying crop residue Resistant corn hybrids may be available

15. Sorghum Downy Mildew

Symptoms: • • • • •

Leaves have yellow to white stripes extending from the base of the leaf toward the leaf tip Infected leaves often will be more narrow than usual, erect, and shredded A white, downy mycelium may develop on both surfaces of leaves Plants may be stunted, chlorotic, and have an irregular seed set Symptoms vary according to plant tolerance

Pathogen Involved: • • •

Peronosclerospora sorghi (a fungal-like pathogen) Pathogen is located worldwide and causes a systemic infection Overwinters in the soil

Time of Occurrence: •

Occurs before mid-whorl

Conditions Favoring Disease: • •

High humidity or free water Warm temperatures of 70 - 80°F

Disease Management: • • • • •

Use resistant hybrids Plant in cool soils Avoid rotation of corn with sorghum and adjacent planting of corn and sorghum Destroy shattercane, because it is an alternate host for the pathogen Systemic fungicide seed treatments may be helpful

16. Southern Rust

Symptoms: • • •

Pustules are circular to oval, and light cinnamon brown to orange The pustules will be mainly scattered on the upper surface of the leaf, however a few pustules may be seen on the lower leaf surface Pustules may also appear on husks, ear shanks, stalks, and leaf sheaths

Pathogen Involved: • • • •

Puccinia polysora (a fungus) Overwinters in the southern U.S. and Mexico (not in the corn belt) Urediniospores serve as the repeating stage of this fungus and are blown north to the corn belt in the early summer This pathogen most often is present in the southwestern United States, but spreads to the U.S. corn belt in some years

Time of Occurrence: • •

This disease occurs most often after the silking of corn june to August

Conditions Favoring Disease: •

Warm temperatures and high humidity

Disease Management: • •

Primarily is controlled by planting resistant corn hybrids. A protectant fungicide or a locally systemic fungicide may also be used against southern rust

17. Southern Corn Leaf Blight

Symptoms: • • • •

Common lesions are elongated, tan lesions between veins on leaves Different isolates of this pathogen will cause lesions of different sizes Race O causes long, tan, lesions that have brown borders on leaves. Lesions will develop differently on various inbreds and hybrids

Pathogen Involved:

• • • • •

Cochliobolus heterostrophus (=Bipolaris maydis) (a fungus) Overwinters in corn residue Especially prevalent in areas with a warm, humid climate Three races known of this pathogen: race O, race T, and race C Race T and race C are known to be specifically virulent to corn with cytoplasm male-sterile T and cytoplasm male-sterile C, respectively. Since the switch from cytoplasm male sterile T to normal cytoplasm corn, race T is not considered to be a threat.

Time of Occurrence: •

When corn has reached mid-whorl up to maturity

Conditions Favoring Disease: • • •

Warm, temperatures (70 - 90° F) and wet conditions Monoculture Reduced tillage

Disease Management: • • •

Use of resistant hybrids Crop rotation Tillage may be helpful

II. Stalk Rot Diseases 1. Anthracnose Stalk Rot Symptoms • • • • •

Symptoms usually most apparent just before crop maturity Uniform or blotchy, shiny, black color on the outer stalk Inner stalk may also be black and systemically infected, or may not have any discoloration Several internodes of the plant may be rotted and easily crushed with finger pressure The plant above or below the ear may die and lodging may result.

Pathogen Involved

• • • •

Colletotrichum graminicola (a fungus) It is the same that causes the Anthracnose leaf blight disease The only common stalk rot pathogen that can cause a leaf blight disease Overwinters in corn residue

Time of Occurrence •

Usually occurs after corn pollination

Conditions Favoring Disease • • •

Corn residue left on the soil surface Insect injury Environmental stress

Disease Management • • • •

Use disease resistance hybrids Maintain balanced fertility Reduce plant stress Control the corn borer

2. Bacterial Stalk rot Symptoms • • •

Symptoms are similar to those of Pythium stalk rot One or more of lowest internodes will turn tan to brown and become water soaked, soft, or slimy and there may be a foul odor Upper leaves wilt and a soft rot can occur at the base of the whorl, and this may spread rapidly down the plant and cause lodging

Pathogen Involved • • • • •

Erwinia chrysanthemi pv. zeae (bacterium) Wide host range, including monocots as well as dicots Infection sites are stomata, hydathodes, or the wounds of leaves or stalks Overwinters in stalk residue on the soil surface line May be transmitted by larvae of the maize borer

Time of Occurrence • •

Occurs in midseason after pollination of corn Top rot can occur in plants that are sprinkler irrigated during periods of rapid growth

Conditions Favoring Disease • • • •

High Temperatures (88-95°F) High amounts of rainfall and flooding Overhead irrigation Insect injury

Disease Management • •

Incorporate soil debris with fall plowing Avoid flooding and excessive overhead irrigation

3. Charcoal Rot Symptoms • • •

Symptoms may first appear similar to Fusarium, Diplodia, & Gibberella stalk rot A key sign of charcoal rot is gray or black discoloration in the inside the rind due to the presence of many tiny, black sclerotia (can be seen with hand lens) Plant may die and lodge

Pathogen Involved • • • •

Macrophomina phaseolina (a fungus) Has a very wide host range that include sorghum and soybean. Infects soybeans more often than corn in the Midwestern U.S. The pathogen overwinters on host crop residue

Time of Occurrence • • • • •

Occurs after pollination Conditions Favoring Disease Hot temperatures (over 85°F) and low soil moisture Drought conditions when corn is senescing Insect injury may be associated

Disease Management • • •

Use hybrids resistant to Diplodia and Gibberella stalk rot. Minimize early senescence of stalk tissue Reduce moisture stress

4. Diplodia Stalk rot

Symptoms • • •

Symptoms may be similar to those of Gibberella stalk rot, but Diplodia doesn't develop pink color. White mycelium can develop in lower stalks. Tiny black specks (pycnidia) embedded in rind of lower stalk Lower, inner stalk may have shredded tissue and small, dark specks (pycnidia)

Pathogen Involved • • •

Stenocarpella (Diplodia) maydis (a fungus) Corn is the only host of this pathogen This pathogen overwinters on corn debris

Time of Occurrence • •

Usually occurs after pollination The crown, mesocotyl, or roots are often the point of infection, however, the nodes between the crown and the ear can also be infected.

Conditions Favoring Disease • • • •

Warm and wet weather Corn on corn rotation Stress from other diseases or insect injury Conservation tillage

Disease Management • • • • •

Use commercial corn hybrids that are resistant to Gibberella stalk rot Maintain balanced fertility Use the proper plant population Reduce plant stress Control stalk-boring insects

5. Fusarium Stalk rot Symptoms • • • •

Similar to Diplodia and Gibberella stalk rots Inner stalk will have a light pink discoloration, but no black specks (fungal fruiting bodies) will be present in or on the stalk Ears may be small and lower nodes may be shredded or break Eventually, the stalks may die and lodge

Pathogens Involved

• • •

Fusarium moniliforme, F. proliferatum, and F. subglutinans (a fungus) These fungi are found wherever corn is grown Overwinters in crop residue

Time of Occurrence •

Occurs after corn pollination

Conditions Favoring Disease • • •

Warm, dry weather Insect injury Environmental stress and other diseases

Disease Management • • • •

Use hybrids that are resistant to other stalk rot diseases. Minimize all crop stresses Control insects Use proper plant population

6. Gibberella Stalk rot Symptoms • • •

Pith of the inner stalk may deteriorate leaving only the vascular bundles intact The vascular bundles will appear reddish in color Small black specks develop on the stalk surface near the nodes and can be scraped off with fingernail

Pathogen Involved • • •

Gibberella zeae (=Fusarium graminaerum) (a fungus) A common stalk-rotting pathogen in the Midwestern U.S. Can also infect wheat, barley, oat, or rye and causes scab and seedling blights

Time of Occurrence • •

Infection occurs after pollination Infection may occur on the leaf sheath, brace roots, or roots and continue up into the lower stem.

Conditions Favoring Disease • •

Warm and wet weather Insect injury

•

Environmental stress and other diseases

Disease Management • • • • •

Use resistant commercial corn hybrids. Maintain balanced fertility Plant the proper plant population. Reduce plant stress. Control stalk-boring insects

7. Pythium Stalk rot Symptoms • • • • •

Usually limited to individual plants in small areas. Infection usually limited to first internode above the ground. Infection site will appear brown, water soaked, soft, and collapsed. Pith tissue may completely rot, stalks may be twisted or distorted, but will not completely break off. Vascular bundles remain intact, and the plant may remain green for several weeks.

Pathogen Involved • •

Pythium aphanidermatum (an oomycete fungal-like pathogen) Overwinters as oospores in the soil.

Time of Occurrence •

Can occur prior to flowering, but mainly infects after pollination.

Conditions Favoring Disease • • •

Extended periods of hot (90°F or more) and wet weather. Poor soil drainage High humidity

Disease Management •

Improve soil drainage

8. Red Root Rot Symptoms

• • • • • •

Red or pink discoloration of the root system and lower stalk tissue turns to a deep red to purple color along with death and shriveling Symptoms usually appear just before corn maturity The red color is deeper and darker than that of Gibberella stalk and root rot Symptoms vary among different corn genotypes Root mass can be reduced The rotting below ground can lead to wilting and grayish green foliage, premature plant death, and lodging in 4 to 5 days

Pathogen Involved • • • •

Phoma terrestris (fungus) in association with Pythium and Fusarium species Survives in soils with a wide range of temperature and pH conditions The pathogen overwinters as microsclerotia in soil This disease has had a high incidence in the Atlantic states and Delmarva region of the U.S.

Time of Occurrence •

Occurs as early as midsilking stage, but mainly late in the growing season

Conditions Favoring Disease • • •

As the plant senesces, the rate of disease development increases Moderate temperatures: (~75-80°F) High yield environment (=high populations, high fertility, and irrigation)

Disease Management •

Rotation with soybeans may provide some benefit

III. Seedling And Root Diseases 1. Seed and Seedling Diseases

Symptoms:

• • •

•

Disease damage may appear similar to some environmental stress, but general and specific symptoms can help with diagnosis. General effects: reduced emergence, slow growth and stunting in a random or circular pattern, wilting, chlorosis/yellowing, post-emergence damping-off. Specific symptoms of seed and root infections include: rotted seed and seedlings before or after emergence; red/yellow discoloration of leaves; complete or partially rotted roots with firm or soft, brown-reddish- to gray lesions or decay; discolored and soft coeleoptile; death of leaf tips; wilting; and sunken,discolored lesions on mesocotyl. Leaf spots and streaks can also occur, resulting from anthracnose and Stewarts wilt infections.

Pathogen Involved: •

• • •

·Some common genera of `fungal' pathogens that cause one or more of the symptoms noted above are Stenocarpella (Diplodia), Pythium, Rhizoctonia, Fusarium, Colletotrichum, and Penicillium. Two bacterial pathogens that can affect corn seedlings are Erwinia [=Pantoea] (Stewart's Wilt) and Pseudomonas (holcus spot). The source of these pathogens is soil, infested residue remaining from previous years crops, or infested seed. Nematodes can also damage corn seedlings, especially in sandy soils.

Conditions Favoring Disease: •

The frequency of infection and importance of these pathogens will vary and depend in part on location, seed quality (cracked or infected seed), soil temperatures <55°F, soil water content, soil compaction, rate of emergence and growth, hybrid/inbred, fertilizer burn, herbicide injury, crusted soil, high temperatures (Penicillium infection), and population of flea beetles (Stewart's wilt). · Favorable conditions are cool, wet, and compacted soil, and poor seed quality. For example, Pythium is a common soil fungal-like pathogen that causes seed and seedling rots, and is favored by damaged seed and wet and cool soil conditions.

Disease Management: • • •

Plant into soil greater than 50-55°F. Use seed treated with fungicides. However, these chemicals are most effective for about 2 weeks after planting depending on soil water content and temperature. The primary fungicidal seed treatments used are of two main groups. The first group (ApronXL, Allegiance, and Apron) is most effective against Pythium. The second group of fungicides (Maxim, Captan, and Dynasty are common examples) protect against the other fungi.

2. Stewart's Wilt

Symptoms •

•

•

This disease has two phases and primarily is a problem on sweet corn and susceptible inbreds, although resistant hybrids may be affected if flea beetle populations are very high in the spring. The first phase affects corn seedlings. Seedling leaves have light green to yellow linear streaks with wavy margins that can follow the length of parallel veins. Plants can become necrotic and die. The plants may produce bleached tassels if it survives. The second phase is leaf blight after tasseling. Leaves develop light green or yellow streaks along veins. Infected areas will die. The disease can be severe enough to kill the entire leaf. With the absence of resistance, the leaf blight phase can be a threat to field corn.

Pathogen Involved • • • •

Erwinia (=Pantoea) stewartii (a bacterium) Overwinters in and is transmitted by the corn flea beetle Many grass hosts are susceptible Can be seed transmitted at a low rate

Time of Occurrence •

Seedling stage and the leaf blight phase occur after tasseling

Conditions Favoring Disease • • • •

Warm winters and high corn flea beetle populations in spring Susceptible inbreds and sweet corn High levels of soil phosphorus and nitrogen High temperatures and soil moisture

Disease Management • • •

Use resistant corn hybrids Seed-applied insecticides can be beneficial Disease forecasting is based on the sum of mean temperatures for December, January, and February. In mild winters, the corn flea beetle will have a high

survival rate and disease severity may be high. In cold winters, corn flea beetles will have low survival and disease would likely be low severity or absent.

3. Corn Nematodes

Symptoms: • • • • • •

Symptoms can look similar to nutrient deficiency, or insect or herbicide damage. Usually in parts of fields, not over the whole field. Symptoms and injury vary with type and numbers of nematodes involved. Wilting, stunting, small poorly-filled ears, nutrient deficiencies Short root systems with reduced number of fine feeder roots (dagger, stunt, and sting nematodes). Similar to grape colaspis injury. Short, stubby roots similar to dinitroaniline herbicide damage (needle, stubby root, and sting nematodes). Few fine roots and dark lesions on roots (lesion, dagger, lance, and spiral nematodes).

Nematodes Involved: • • • • • • • • •

Needle nematode (Longidorus sp.) Stubby root nematode (Trichodorus sp.) Sting nematode (Belonolaimus sp.) Dagger nematode (Xiphinema sp.) Stunt nematode (Tylenchorhynchus sp.) Sting nematode (Belonalaimus sp.) Lesion nemadotode (Pratylenchus sp.) Lance nematode (Hoplolaimus sp.) Spiral nematode (Helicotylenchus sp.)

Time of Occurence: • •

Damage can occur in May and June. Damage will often not be visible until crop is under moisture or other stress after early summer.

Conditions Favoring Disease: • • •

Sandy soils Continuous corn on corn cropping Corn stressed by other factors.

Disease Management:

• •

Scout and sample moist soil and plants to determine if nematodes are a problem Rotate out of corn

IV. Ear Rot Diseases

1. Aspergillis Ear Rot Symptoms • • • •

Patches of green to yellow spores on or between kernels Most common at tip of ear and scattered on a few kernels on ear The fungus can become dark green to brown as it ages Aspergillus flavus and other Aspergillus can also cause storage rot. In favorable conditions, Aspergillus can invade kernels with moisture levels as low as 14%. Kernels are more likely to be invaded by fungi if they have come from rotted ears or if they have been damaged.

Pathogen Involved • • • •

Aspergillis flavus This fungus produces aflatoxin, a dangerous mycotoxin Overwinters in plant debris and soil Spread by insects or wind

Time of Occurrence • • • • •

Mid to late in the season Conditions Favoring Disease Hot and dry weather Insect damage to ears Stressed corn plants

Disease Management • • • • •

Reduce plant stress Irrigation or tillage may be of some benefit Some hybrids may develop less susceptible than others Insect control may be helpful Planting may timed to reduce stress after pollination

2. Diplodia Ear rot Symptoms

• • • • • •

Husks appear either bleached or straw colored Infection most often begins at the base of the ear The ear may become bleached and brown, but may show no external symptoms Coarse, white or grayish brown fungal mycelium develops on the husk or kernels The ear may turn brown and become shrunken, rotten, and light Late in the season, black specks (fungal fruiting bodies) may be produced on ears

Pathogen Involved • •

Stenocarpella (Diplodia) maydis (a fungus) This pathogen overwinters on infested corn debris from the previous year

Time of Occurrence •

This disease occurs after flowering

Conditions Favoring Disease • • • • •

Corn ears are most vulnerable for three weeks after silking Warm and wet weather conditions after silking Corn ears that have been damaged by insects or birds Conservation tillage Corn on corn rotation

Disease Management • • •

Corn hybrids differ in susceptibility, but resistant corn hybrids not reported Crop rotation Fall tillage of corn residue

3. Fusarium Kernel or Ear Rot Symptoms • • • • •

Infected groups or individual kernels may be scattered on the ear and be covered with a whitish pink to lavender mycelium Fungal growth is most often found at the ear tip Infected kernels may also have the "starburst" symptom, which is white lines radiating out from a point on the kernel Kernels can be infected at the embryo end and symptoms may not be visible Symptoms may vary according to the genotype of the corn hybrid, environment, or disease severity

Pathogen Involved •

Fusarium moniliforme, F. proliferatum, and F. subglutinans (fungi)

• • •

These pathogen produce fumonisin (a mycotoxin) in grain Overwinters in corn residue May infect systemically through stalk

Time of Occurrence •

Occurs during or after flowering

Conditions Favoring Disease • • •

Hot and dry weather after flowering Minimum tillage Damaged kernels and ears

Disease Management • • •

Corn hybrids differ in susceptibility, but resistant hybrids not reported Attempt to maintain appropriate fertility and moisture Insect control may be helpful

4. Gibberella or Red Ear Rot Symptoms • • • •

Reddish mold appears first at the ear tip and spreads downward The ear most often be only partially covered with the reddish mycelium Early infection may cause the entire ear to be colonized with mycelium and partially rotted, which may cause the ear to be glued to the husk Tiny black fruiting structures may be found on the husks and ear shanks.

Pathogen Involved • • • •

Gibberella zeae (=Fusarium graminearum) (a fungus) This pathogen produces multiple mycotoxins that are harmful to humans as well as animals. Overwinters in corn residue The same pathogen causes head scab (blight) of wheat

Time of Occurrence • •

Infection takes place within the silks or at the base of the ear when heavy rainfall occurs late in the season This disease occurs after silking

Conditions Favoring Disease

• • • •

Cool and wet conditions soon after silking Corn on corn rotation, or a wheat and corn rotation Wheat or corn residue on soil surface Damaged kernels

Disease Management • • • •

Corn hybrids differ in susceptibility, but resistant hybrids not reported Corn hybrid with tight husks may be more susceptible. Crop rotation Fall tillage may be helpful where appropriate

5. Nigrospora Ear Rot or Cob Rot Symptoms • • • •

Symptoms most apparent after harvest The cob often shreds at the butt end during mechanical shelling Kernels may be partly covered with gray mycelium with many tiny, black spores Kernels may be bleached

Pathogen Involved • • •

Nigrospora oryzae (a fungus) This pathogen is a weak parasite that attacks plants that have been stressed or killed before maturity Overwinters on corn residue

Time of Occurrence •

Occurs late in the season

Conditions Favoring Disease • • •

Occurs when corn is killed or weakened by a factor such as frost, drought, root injury, leaf blight, or stalk rot Corn grown on infertile soils Damaged kernels and ears

Disease Management •

Cultural practices that reduce ear damage may be beneficial

6. Penicillium Ear Rot

Symptoms • • • •

Infection is most prevalent on ears that have been injured by mechanical means or insects Powdery green or blue fungal growth between and on kernels, most often at tip of the corn ear Infected corn kernels may become streaked or bleached A discoloration of the embryo or "blue eye" may result if the pathogen invades corn kernels stored at high moisture conditions

Pathogen Involved • •

Penicillium oxalicum or other Pencillium sp. This pathogen is a common and damaging storage fungus and can grow in grain at 18% moisture

Time of Occurrence •

Late in season

Conditions Favoring Disease • •

Mechanically or insect injured corn ears Kernel moisture greater than 18%

Disease Management •

Prevent injury to ears