Corn, Sweet and Popcorn - Diseases, Pests and Problems
Problem: Corn ear worm Affected Area: Husk
Description: 2" long yellow, brown or green worms which feed inside the husk near the tip.
Control: Spray newly appearing silks with mineral oil or use eyedropper to put one drop in center of silks near top of ear.
* Sevin is also registered for this use, but spray silks, only. Apply early and at 3 to 5 day intervals. Sevin can be used to day of harvest
* Pesticide use and recommendations for various areas are constantly changing. Check with your County agent for current recommendations.
Problem: Cutworms Affected Area: Stalk
Description: Plants chewed off just above ground level.
Cutworms are caterpillars that are up to 1 1/2" long and mottled or striped green, brown or gray. When they are disturbed, they roll up in a coil.
They usually position themselves at the moisture line in the soil moving up and down according to the water content. If the soil surface is dry, they will be found a couple of inches below the surface where the moisture begins. When newly watered, they will be at the surface.
Control: Put cardboard collar around new transplants to extend 1" to 2"
Problem: Flea beetle Affected Area: Leaf and Ear
Description: Tiny, 1/16"-1/8" black beetles that jump when disturbed. They get into ear through open end of husk and damage
kernels. Leaves may have shot hole symptom (tiny holes in the leaves).
Control: Controlling corn earworm may protect the corn. Spray with *Diazinon, Rotenone or Sevin only if attack is severe. Sevin can be used to day of harvest. Allow 1 day between diazinon or rotenone sprays and harvest
* Pesticide use and recommendations for various areas are constantly changing. Check with your County agent for current recommendations.
Problem: Sap beetle Affected Area: Ear
Description: Black beetle that invades ripe ears. Lays eggs, which hatch into white maggots, found in ripe ear.
Control: Usually a secondary pest. Control earwigs, earworm and prevent damage to corn. Pick when ripe. Eliminate all dead and decaying material.
Problem: Smut Affected Area: Entire Plant
Description: Large galls on leaves, stems, ears or tassels. Deformation of the ears is most common. Smut a soil borne disease. The kernels are greatly swollen. They are white, and then turn brown, then black with spores inside.
Control: Use treated seed and let soil dry out between irrigations.
Problem: Damping-off Affected Area: Seedling
Description: Young seedlings develop stems, wilt and die.
Control: Use treated seed and let soil dry out between waterings.
Problem: Bacteria Soft Rot Affected Area: Leaf
Description: Leaves have streaky appearance, the stalk becomes brown and soft, leaves may turn gray, and the stalks may break and die.
Control: Keep water out of whorls and off plant.
Problem: Aflotoxin Affected Area: Grain
Description: Aspergillus flavus and A. parasiticus are ubiquitous in soil and on decaying plant material, cause heating and decay of stored grain, and, under certain conditions, commonly invade corn. Aflatoxicosis in domestic animals results in lesions in the liver and other tissues, reduced body weight gains and poor growth, and mortality. In poultry, besides fatty liver and kidney disorders, leg and bone problems as well as coccidiosis outbreaks can also result.
Control: Methods for aflatoxin analysis fall into two categories: rapid screening procedures determining the presence or absence of aflatoxin (fluoromotric iodine rapid screening and minicolumn tests), and laboratory procedures quantifying the actual amounts of toxin (thin-layer chromatography, gas-liquid chromatography, or high-pressure liquid chromatography tests).
Problem: Alternaria Affected Area: Leaf
Description: Chlorotic streaks, which later become necrotic, form on maize leaves of all ages. Colonies usually black or olivaceous black. Various forms of tissue damage, such as insect feeding scars and embedded sand grains, are apparently essential to infection that develops during periods of heavy dew.
Problem: Anthracnose Affected Area: Leaf
Description: Anthracnose has increased in importance since 1971 around the world. Symptoms vary greatly, depending on the genotype, age of the leaf, and environment. On PDA the mycelium is septate, sparingly branched, hyaline, and granular. The fungus overseasons as a saprophyte on maize crop residue and seed as spores and mycelia. The fungus affects a large number of grasses including the small grains, sorghums, bermudagrass, Dallis grass, Guineagrass, Bungalow grass, crabgrass, Guatemala grass, molasses grass, orchardgrass, Paragrass, redtop, red fescue, and Merker grass. Anthracnose is generally favored by high temperatures and extended periods of cloudy weather.
Control: Resistant hybrids and varieties to leaf blight are not very highly correlated with resistance to anthracnose stalk rot. Crop rotation, clean plowdown of crop residues and balanced soil fertility are a few control measures.
Description: Bacterial leaf blight has been reported throughout the world. Water-soaked linear lesions occur on the leaves as they emerge from the whorl. The pathogen is a Gram-negative, polar-flagellated rod approximately 0.6 X 1.6 ?m. The bacterium attacks several other plants, including oats, foxtails, and Vasey grass.
Problem: Bacterial Stalk Rot Affected Area: Leaf and Stalk
Description: Primary symptoms generally appear in midseason when plants suddenly fall over. The bacterium is a short, Gram-negative rod, 0.6-0.9 X 0.8 - 1.7 ?m, actively motile with peritrichous flagella, usually in pairs and rarely in short chains, noncapsulated, and nonacid-fast. The bacterium lives saprophytically on crop residue in the soil and invades maize plants through stomata, hydathodes, or wounds in the leaves or stalks. Bacterial stalk rot is most prevalent and destructive in areas with high rainfall, where plants are watered by sprinkler irrigation, and on land subject to flooding.
Control: Resistant hybrids and varieties along with avoiding flooding are good control measures.
Problem: Bacterial Stripe Affected Area: Leaf
Description: Typical primary symptoms are amber-colored to olive-colored, oil- soaked, translucent lesions with parallel sides that tend to elongate and coalesce. The bacterium is a short rod with rounded ends, 0.5-0.8 X 1.5-2.5 ?m, Gram-negative, nonacid-fast, and motile with a polar flagellum. Like other bacterial diseases, bacterial stripe and leaf spot becomes more severe under extended periods of warm, wet weather.
Problem: Boron Deficiency Affected Area: Leaf and Growth
Description: Boron (B) is rarely deficient in maize. Boron deficiency is most likely to occur on sandy, low-organic-matter soils. Drought enhances boron deficiency, and some symptoms of drought and boron deficiency, such as delayed ear-shoot and tassel emergence and poor pollination, may occur simultaneously and are often confused. Deficiency appears first on newly formed leaves as elongated water-soaked or transparent stripes scattered between leaf veins. Leaves become brittle, and the growing point often dies. Upper internodes fail to elongate. Because boron plays an important role in pollination, deficiency is manifest in poorly filled ears with missing rows. Boron deficiency is best corrected by broadcast applications of soluble boron salts. Rates are generally 0.5 to 1.0 lbs boron/acre. Corn is very sensitive to excess boron and band applications are not recommended.
Problem: Charcoal Rot Affected Area: Seedlings
Description: Charcoal rot commonly attacks seedlings or plants approaching maturity. The minute sclerotia are black, numerous on diseased plant parts, and globular to irregular in shape. M. phaseolina overwinters and is disseminated as sclerotia. Soil temperatures near 37 ?C are favorable for disease development, while either low soil temperatures or high soil moisture decreases severity.
Control: In irrigated areas, charcoal rot can be minimized by maintaining moist soils during dry periods after tasseling. Planting resistant hybrids in lower plant populations, balancing soil fertility, and avoiding high levels of N and low levels of K can be effective.
Problem: Common Corn Rust Affected Area: Leaf
Description: Pustules (sori) may appear on any aboveground part, being most abundant on the leaves. The uredospores are cinnamon brown, mostly spherical to broadly ellipsoidal, and 21-30 X 24-33 ?m. Teliospores germinate in the spring in certain areas of the world to form basidia on which small, thin-walled, hyaline, haploid basidiospores are produced. Cool temperatures (16-23 ?C) and high relative humidity (100%) favor rust development and spread. Aecial infection occurs infrequently in temperate regions around the world on Oxalis spp.
Control: Planting resistant hybrids and varieties, applying fungicides early when the disease is first detected on the leaves may be feasible and a good form of control, especially in seed-production fields.
Problem: Common Smut Affected Area: Entire Plant
Description: Common smut or boil smut occurs worldwide wherever maize is grown. Losses from this disease vary greatly from one area to another and may range from a trace to (rarely) 10% or more. All aboveground parts of the plant are susceptible, particularly young, actively growing, or meristematic tissues. The chlamydospores overwinter and germinate under favorable conditions to produce sporidia carried by air currents or splashed by water to young, developing tissues of maize. Development of common smut is favored by dry conditions and temperatures between 26 and 34 ?C. Somewhat resistant hybrids and varieties. Avoid mechanical injuries to plants during cultivation and spraying. Maintain well-balanced soil fertility. In home gardens, remove and burn galls from infected plants before they rupture. Ears, tassels, leaves with gray gnarled growths that become powdery.
Control: Removing and destroying growths as soon as they are noticed, keeping the black powder in the galls from getting into the soil and planting resistant varieties early, can help control the problem.
Problem: Copper deficiency Affected Area: Leaf and Stalk
Description: Copper (Cu) is rarely deficient in corn. Deficiency usually occurs in organic, or peat and muck, soils and very sandy soils of low organic matter content. Copper deficiency is exhibited as yellowing of young leaves. Tips of leaves may wilt and die. Leaf streaking similar to iron chlorosis may be observed, and some necrosis of leaf margins similar to potassium deficiency may occur. Stalks may become soft and limp.
Control: Copper deficiency is prevented by application of soluble copper fertilizers, such as copper sulfate or copper EDTA. Fertilizer rates are generally about one lb Cu/acre. Rates of 2-3 lbs copper/acre may be effective for several years, but these rates should be broadcast not banded. Foliar sprays of copper sulfate or copper EDTA are also effective, but repeated applications may be necessary.
Problem: Corn Earworm Affected Area: Kernels and Tassel
Description: Range in color from green to almost black with lengthwise stripes of various colors. Worms up to 13/4-inches long eating down through kernels of corn. Prior to tasseling, worms found in whorl of plant, feeding on developing tassel.
Control: With a medicine dropper, apply mineral oil to the silk just inside the tip of each ear, 3 to 7 days after silks first appear. Use 20 drops per ear or Break off and discard wormy end of ear, Insecticides will not control worms that are inside the ear. Preventive treatments to silks are intended to kill worms before they enter ears.
Problem: Crazy Top Affected Area: Roots, Stalk, Leaf, and Floral Structures
Description: Crazy top occurs in most areas with temperate or warm-temperate climates and rarely in tropical areas. Symptoms vary greatly with time of infection and degree of host colonization by the fungus. Sporangia are hyaline, lemon-shaped, operculate, 60-100 X 30-65 ?m, and attached to short, simple, hyphoid sporangiophores emerging from stomata. The mycelium of S. macrospora can be detected in brace roots, stalk tissue, leaves, and floral structures of affected plants by staining with zinc chloriodide. The oospores germinate in saturated soil to produce sporangia from which zoospores escape that penetrate host tissues. Unlike other sclerophthoras, the crazy top fungus develops systemically where soils have been flooded shortly after planting or before plants are in the four-leaf to five-leaf stage. S. macrospora has been reported on more than 140 species of the Gramineae.
Control: Provide adequate soil drainage or avoid planting in low, wet spots to help control this disease
Problem: Diplodia Ear Rot Affected Area: Husks, Leaf, and Stalk
Description: The husks of early infected ears appear bleached or straw-colored. The causal fungus overseasons as conidia in acervuli on maize leaf and stalk debris and as dormant resting mycelial cells in stalk debris. Dry weather early in the season followed by wet conditions just before and after silking favors ear infection.
Control: A couple control measures that can be used in the field are using resistant hybrids and varieties and harvest the maize early. Proper storage (below 18% moisture initially for ears, 15% for shelled grain) will prevent further growth of the pathogen, but is not a control measure in the field.
Problem: Diplodia Stalk Rot Affected Area: Stalk
Description: The disease commonly appears several weeks after silking. The fungus produces globose or flask-shaped pycnidia containing olive-colored to brown, elliptical, two-celled, straight to slightly curved spores, 5-6 X 25-30 ?m in size. D. maydis overseasons as spores in pycnidia on debris and as spores or mycelium on the seed. Dry conditions early in the season and warm (28-30 ?C), wet weather two to three weeks after silking favor development of Diplodia stalk rot.
Control: Planting resistant hybrids and varieties at lower populations, balancing soil fertility, and avoiding high levels of N and low levels of K can help control this disease.
Problem: Ear Rot Affected Area: Ear and Kernel
Description: Maize is susceptible to a number of ear and kernel rots, some of which are widely distributed.
Problem: Earwigs Affected Area: Silk and Kernels
Description: Feed on silk and prevent pollination and thus lolling of kernels. Ears only partly filled, shortened silks, presence of earwigs on silks.
Control: Traps; check daily for earwigs and destroy.
Problem: Frost Affected Area: Entire Plant
Description: Young maize plants are sometimes killed by a hard frost or freeze.
Problem: Fusarium Ear Rot Affected Area: Kernels
Description: A salmon-pink to reddish-brown discoloration first appears on the caps of individual kernels or groups of kernels scattered over the ear. The fungi develop on crop residue in or on the soil surface. Disease development and spread are favored by dry, warm weather.
Control: Planting resistant hybrids and varieties at lower populations, balancing soil fertility, avoid high levels of N and low levels of K can all be used to control this disease.
Problem: Fusarium Stalk Rot Affected Area: Stalk and Sheath
Description: This rot is difficult to distinguish from Gibberella stalk rot. Both fungi produce asexual spores on mycelium, which appears as a cottony-pink growth on the leaf sheaths and at the nodes. The perithecia are globose, smooth, and blue-black. The macroconidia of F. moniliforme var. subglutinans are slightly less curved than those of F. moniliforme and usually are three-septate. The fungi develop on crop residue in or on the soil surface. Dry conditions early in the season and warm (28-30 ?C), wet weather two to three weeks after silking favor development of Fusarium stalk rot.
Control: Planting resistant hybrids and varieties at lower populations, balanced soil fertility, and avoiding high levels of N and low levels of K can help control this disease.
Problem: Gibberella Ear Rot Affected Area: Ear
Description: A reddish mold, often beginning at the ear tip, is the characteristic sign of Gibberella ear rot. The perithecia are bluish black, spherical, and borne superficially on diseased stalks. The macroconidia of F. roseum 'Graminearum' are hyaline, curved, and tapering toward the tips, three- to five-septate, and 4-6 x 30-60 ?m. Perithecia on infected maize stalks mature under warm, wet conditions. Cool, wet weather within three weeks of silking favors development of this disease.
Control: Planting with the proper resistant hybrids and varieties and harvesting early are effective controls in the field. Proper storage (below 18% moisture initially for ears, 15% for shelled grain) will prevent further growth of the pathogen away from the field.
Problem: Gibberella Stalk Rot Affected Area: Leaf and Stalk
Description: Leaves on early infected plants suddenly turn a dull grayish-green while the lower internodes soften and turn tan to dark brown. The perithecia are bluish black, spherical, and borne superficially on diseased stalks. The macroconidia of F. roseum 'Graminearum' are hyaline, curved, and tapering toward the tips, three- to five-septate, and 4-6 x 30-60 ?m. Perithecia on infected maize stalks mature under warm, wet conditions. Dry conditions early in the season and warm (28-30 ?C), wet weather two to three weeks after silking favor development of Gibberella stalk rot.
Control: Planting resistant hybrids and varieties at lower populations, balancing soil fertility and avoiding high levels of N and low levels of K are effective controls for this disease.
Problem: Hail Affected Area: Entire Plant
Description: Hail damage is easily recognizable by shredded leaves hanging in tatters.
Problem: Head Smut Affected Area: Ear and Tassels
Description: Head smut occurs periodically in the deltas and intermountain valleys around the world. Head smut first appears when ears and tassels are formed. Sphacelotheca reiliana (Kuhn) Clint. (Syn. Sorosporium reilianum (Kuhn) McAlp., Ustilago reiliana Kuhn) S. reiliana is primarily soilborne. The infection level is related to the concentration of teliospores in the soil.
Control: Planting resistant hybrids and varieties that are treated, chemical treatments of soil in rows at or before planting time, along with sanitation and crop rotation after harvest are effective controls for this disease.
Description: Lesions of Race 1 are tan, oval to circular, usually with concentric zones and measure 1.2 x 2.5 cm. The conidia are golden yellow to dark olive-brown, curved, long- elliptical to spindle-shaped, with rounded ends, 2 to 12 septate, 7- 18 X 25-100 ?m. Similar to northern and southern leaf blights. Moderate temperatures and high relative humidity favor the disease. There are at least three races of H. carbonum.
Problem: Iron Affected Area: Leaf
Description: Iron chlorosis, the name given to iron deficiency symptoms, may occur in corn grown on high pH soils containing free calcium carbonate. Iron availability decreases rapidly as soil pH increases. Water-logging of the soil worsens the symptoms, but the effect may be temporary and disappear as the soil dries. Iron chlorosis typically occurs on the new leaves as a distinctive yellow striping of the tissue between veins while the veins remain green. Severely chlorotic leaves may be almost white and have necrotic areas.
Control: The best method of prevention is variety selection. So-called iron-efficient cultivars are less susceptible to chlorosis and should be planted if chlorosis has been observed previously and soil conditions are conducive to development of deficiency. Soil applications of iron fertilizers are generally ineffective in correcting iron deficiency, but banding of high rates (75-100 lbs/acre) of ferrous sulfate in the seed row have been shown to be effective. The most effective corrective treatment is foliar sprays of soluble iron materials, such as iron chelates or ferrous sulfate. A 0.5 to 1.0% solution of ferrous sulfate with a surfactant sprayed in enough water to wet the leaves (about 20 gals/acre) is effective. Multiple applications at 7-14 day intervals may be necessary.
Problem: Magnesium Deficiency Affected Area: Leaf
Description: Magnesium deficiency normally occurs on acid soils of low magnesium limed with calcitic limestone. High soil K or high rates of fertilizer K can induce magnesium deficiency, especially in low magnesium soils. Symptoms normally occur as interveinal chlorosis on the lower leaves. Small necrotic spots may also develop giving the appearance of spotted striping. Symptoms are similar to manganese deficiency except, because magnesium is mobile in the plant, the symptoms appear first on older leaves. As severity increases leaf tips, edges and underside may become reddish-purple and eventually die.
Control: Magnesium deficiency is prevented by liming with dolomitic limestone. On high pH soils, or where dolomite is not available, soluble salts of magnesium are effective. Magnesium sulfate or potassium-magnesium sulfate is commonly used.
Description: The earliest symptom caused by maize chlorotic dwarf virus (MCDV) is a chlorosis of the young leaves in the whorl. Double infections of MCDV and MDMV do occur in some areas. MCDV is transmitted by the leafhopper Graminella nigrifrons (Forbes) and G. (Deltocephalus) sonora (Ball) in a semipersistent manner. The isometric (31 nm in diameter) maize chlorotic dwarf virion contains RNA.
Problem: Maize Dwarf Mosaic Affected Area: Leaf and Growth
Description: Symptoms are highly variable and the following description depicts frequently seen effects. MDMV is commonly confused with maize stunt and maize bushy stunt. Maize dwarf mosaic virus (MDMV) is a long, flexuous, rod-shaped virion 12-15 x 750 nm that is transmitted mechanically.
Problem: Maize Streak Affected Area: Leaf
Description: Initial symptoms consist of minute, round, cream-to-white scattered spots on the youngest leaves. Maize streak virus (MSV) is transmitted by five leafhopper species of the genus Cicadulina of which C. mbila (Naude) is the most important.
Problem: Manganese Deficiency Affected Area: Leaf and Growth
Description: Corn has a relatively low manganese requirement and deficiencies are rare. Manganese deficiency is favored by high soil pH, low soil manganese levels, and sandy soils high in organic matter, such as peat or muck soils. Symptoms are vague but are generally characterized by interveinal chlorosis of the young leaves similar to iron deficiency. Manganese deficiency is best prevented by band applications of manganese sulfate. Banding manganese with an acid fertilizer such as ammonium sulfate improves manganese availability. Broadcast applications of manganese on high pH soils are not recommended. Manganese EDTA applications on high pH soils are not consistently effective and in some cases have actually worsened the deficiency. Insoluble manganese fertilizers such as manganese oxide are not effective.
Control: Manganese deficiency detected early in the season may be corrected with foliar sprays of manganese sulfate or manganese EDTA. Multiple applications may be necessary.
Description: Molybdenum is rarely, if ever, deficient in maize. If deficiency occurs, it is likely to be seen only in extremely acid soils. Leaves may die at the tip, along the margins, and between veins. In some cases symptoms may resemble nitrogen deficiency. Molybdenum deficiency is best prevented by liming acid soils.
Control: Because of the extremely small amounts of molybdenum needed by plants, seed treatments of molybdenum fertilizer are the best method of assuring adequate distribution.
Problem: Nematode Disease Affected Area: Root
Description: Nematodes, also called nemas or eelworms, are unsegmented roundworms that inhabit fresh and salt water, decaying organic matter, soil, plants, and animals throughout the world. Plant-parasitic nematodes are typically microscopic, transparent, mobile, and vermiform. Passive movement of nematodes occurs in water, soil, and infected plant parts. Nematodes are extracted from soil by various flotation, sieving, and centrifugation procedures and from roots by incubation techniques. Most plant-parasitic nematodes become inactive at temperatures below 10 ?C and over 35 ?C. More than 100 species of plant-parasitic nematodes have been reported to feed on or are associated with the roots of maize. Virtually all fields will contain more than one species of plant-parasitic nematode. The nematode genera Longidorus, Xiphinema, Trichodorus, and Paratrichodorus transmit viruses to various crop plants.
Problem: Nitrogen Deficiency Affected Area: Leaf
Description: Nitrogen deficiency in young plants is characterized by overall stunted, spindly appearance and pale, yellow-green foliage. In older plants lower leaves show a characteristic V-shaped yellowing from the tip down the leaf to the midrib. Yellowing progresses to firing of the leaves and premature death. The entire plant becomes progressively more yellow as severity increases. Ears are small and poorly filled at the tip. Low grain protein may also indicate nitrogen deficiency. Nitrogen deficiency may occur on all soils. Heavy rain or irrigation can leach available soil nitrogen and induce deficiency. Sandy soils of low organic matter are most prone to nitrogen deficiency.
Control: Applying nitrogen fertilizer at appropriate rates depending on soil, environment, and yield level prevents nitrogen deficiency.
Problem: Northern Leaf Blight Affected Area: Leaf and Husk
Description: Long, elliptical, grayish-green or tan lesions ranging from 2.5 to 15 cm in length develop first on the lower leaves. T. turcica, the sexual stage, occurs rarely, if ever, in nature but produces black, globose pseudothecia in the laboratory. Two biotypes of H. turcicum from maize isolates have been described. H. turcicum overwinters as mycelia and conidia in infected leaves, husks, and other plant parts, except in the northernmost areas where maize can be grown. Northern corn leaf blight occurs sporadically in most humid areas of the world where maize is grown. H. turcicum attacks sorghum, Sudangrass, Johnsongrass, gamagrass, and teosinte. At least two types of resistance are known: small lesion size and few lesions (polygenic), and chlorotic lesions with little or no sporulation and yellowish halos (monogenic).
Control: Planting resistant hybrids and varieties and/or using fungicides where practical, for in seed-production fields or on sweet corn for fresh market are the best controls.
Description: Phosphorus deficiency is most commonly observed on soils with insufficient phosphorus. Very acid and alkaline soils are most conducive to deficiency. Phosphorus deficiency may also be induced by cold soil temperatures and soils which are too wet or too dry. Root growth restrictions caused by compaction, insect damage, root pruning by cultivation, herbicide injury, and positional unavailability may cause phosphorus deficiency. Mild deficiencies are characterized by stunted growth and dark green leaves. As severity increases, some varieties may show purpling or reddening of the stalk and leaves beginning early in the growing season. Phosphorus deficiency causes reduced root growth, small ears with undeveloped kernels, and delayed maturity.
Control: Where low soil test indicates phosphorus deficiency, band application of phosphorus fertilizer is more effective. Soil test phosphorus levels should be maintained at sufficient levels for optimum yield levels.
Description: Potassium deficiency is most common in soils of low potassium content but may be induced by very wet or dry soils, compaction, and nutrient stratification by tillage. Soils most commonly low in potassium are sandy soils, organic soils, and highly weathered soils. Removal of forage crops containing large amounts of potassium may produce deficiencies in the following crop. Early deficiency symptoms are yellowing or firing of tips and margins of older leaves. Symptoms progress up the plant as severity increases. Potassium deficient plants have weak spindly stalks, have fewer brace roots, are prone to lodging, and are more susceptible to disease. High nitrogen rates accentuate the deficiency.
Control: Deficiencies are best prevented by application of potassium chloride or potassium sulfate according to the soil test.
Problem: Rust Affected Area: Leaf
Description: A disease caused by a fungus. Brown spots on leaves with powdery rust-colored spores. Leaves may discolor. Favored by cool temperatures and high humidity or overhead sprinklers.
Control: Planting resistant varieties and using proper fungicides are effective control methods.
Problem: Seed Rot Affected Area: Kernels and Seedlings
Description: Germinating maize kernels may be attacked by a number of soilborne or seedborne fungi that cause seed rots and seedling blights.
Description: Stewart's bacterial wilt, sometimes called Stewart's leaf blight or maize bacteriosis, is common around the world. Infected sweet maize hybrids wilt rapidly, resembling plants suffering from drought, nutritional deficiency, or insect injury. Although most field or dent maize is not as susceptible as sweet maize, there are some very susceptible inbred lines and hybrids. The corn flea beetle (Chaetocnema pulicaria Melsh.), both adult and larvae of the twelve-spotted cucumber beetle (Diabrotica undecimpunctata howarti Barb.), toothed flea beetle (C. denticulata Ill.), as well as larvae of the seed corn maggot (Hylemya cilicrura Rond.), wheat wireworm (Agriotes mancus Say), and May beetle (Phyllophaga sp.) are known vectors of E. stewartii. Mineral nutrition influences the susceptibility of maize hybrids to infection by E. stewartii. Disease forecasting is based on the sum of the mean temperatures (C) for December, January, and February.
Control: Planting resistant hybrids and varieties with early applications of insecticides (e.g., carbaryl) to kill corn flea beetles are effective methods of control for this disease.
Problem: Wheat Streak Mosaic Affected Area: Leaf
Description: Early symptoms appear as small chlorotic spots or broken streaks at the tips of young leaves.
Description: Corn is very sensitive to zinc deficiency. Symptoms are most often observed in corn grown in high pH soils of low zinc content. Low soil organic matter, loss of topsoil by erosion or leveling, cold soil temperatures, high phosphorus levels, poor root growth, and compaction can all contribute to deficiencies. Symptoms usually occur early in the season and include shortening of the internodes, lack of vigor, and light streaking of leaves followed by a broad stripe of bleached tissue on each side of the midrib. Mild deficiency may cause yield losses and delayed maturity without visible symptoms.
Control: Zinc deficiency is best prevented by soil application of water soluble zinc fertilizers such as zinc sulfate. Zinc EDTA is also effective, but costs significantly more. Insoluble zinc sources such as zinc oxide in granular form are ineffective. Band applications of 1-5 lbs Zn/acre, such as in a starter application, are most effective. Banding of nitrogen and zinc together improves zinc uptake. Broadcast applications can also be used, but higher rates of 5-10 lbs Zn/acre are normally required for high pH soils. These rates may have residual effects for several years. Foliar application of soluble zinc fertilizers is also effective. A 0.5 to 1% zinc sulfate solution applied with a surfactant in enough water to wet foliage is an effective treatment. Repeated applications at 7-14 day intervals may be necessary.