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Corn, Zea mays, is an annual grass in the family Poaceae and is a staple food crop grown all over the world. The corn plant possesses a simple stem of nodes and internodes. A pair of large leaves extends off of each internode and the leaves total 8–21 per plant. The leaves are linear or lanceolate (lance-like) with an obvious midrib and can grow from 30 to 100 cm (11.8–39.4 in) in length. The male and female inflorescences (flower bearing region of the plant) are positioned separately on the plant. The male inflorescence is known as the 'tassel' while the female inflorescence is the 'ear'. The ear of the corn is a modified spike and there may be 1–3 per plant. The corn grains, or 'kernels', are encased in husks and total 30–1000 per ear. The kernels can be white, yellow, red, purple or black in color. Corn is an annual plant, surviving for only one growing season prior to harvest and can reach 2–3 m (7–10 ft) in height. Corn may also be referred to as maize or Indian corn and is believed to originate from Mexico and Central America.
Corn and cornmeal (dried, ground corn) are staple foods in countries all over the world. The ears can be cooked and eaten from the cob as a vegetable or the kernels can be removed and either eaten as is or used to produce a wide variety of foodstuffs including cereals and flour. Corn is also a major source of starch and the starch can be processed into oils and high fructose corn syrup. Corn is also commonly grown as a feed for livestock.
Corn is best grown in warm, tropical and sub-tropical regions as it requires warm soils to develop optimally. One of the most important requirements for growing corn is a high quality soil which is deep, fertile and well-draining with a pH between 6.0 and 6.8. Corn plants are very heavy feeders and even the most fertile of soils may need to supplemented with nutrients as the plants develop, particularly nitrogen. Corn also requires ample space as it grows large in size and is pollinated by wind. It should be planted where it will receive full sunlight for most of the day and provided with ample moisture.
Planting dates for corn depend on the variety being grown. Standard varieties should be planted when the soil has warmed to at least 12.7°C (55°F) and supersweet varieties when the soil reaches 18.3°C (65°F). Soil can be brought up to temperature faster by laying black plastic mulches approximately 1 week prior to planting. Seeds should be sown about 2.5 cm (1 in) deep and 10–15 cm (~3–4 in) apart allowing 76–91 cm (~30–36 in) between rows. Corn should be planted in blocks (numerous rows) rather than in a single long row as it is wind pollinated and pollen can transfer between plants much more efficiently. Seedlings should be thinned to a final spacing 20–30 cm (8–12 in) when they are approximately 7.5–10.0 cm (3–4 in) in height. It is common to stagger corn plantings to ensure a continuous harvest over the summer months.
General care and maintenance
Corn plants are heavy feeders, particularly of nitrogen (N) and care should be taken to provide them with adequate nutrients by applying side dressings of fertilizer. Corn undergoes a rapid growth period between 30 and 40 days after planting and should be fertilized just prior to this. All fertilizer applications should be made before the tasseling period to ensure the plant maximizes N use. Be vigilant regarding signs of nutrient deficiency, plants should be a deep green color. Purple tinged leaves indicate that the plants are suffering from a lack of phosphorous whereas light green leaves indicate a lack of nitrogen. Apply fertilizer. Plants also require adequate soil moisture throughout the growing period in order to tassel and form silks. Soaker hoses can be used to great effect in small to mid-size plantings. Pollination occurs when pollen is transferred from the male tassel to the female silk by the wind. Each silk produced a single kernel of corn and partially filled ears are usually a result of poor pollination.
Each corn stalk should produce 1 large ear of corn. Under ideal conditions, the stalk will often produce a second, slightly smaller ear which reaches maturity slightly later than the first. Corn ears should be harvested at the “milk stage” of development when when the kernels within the husk are well packed and produce a milky substance when the kernel is punctured. Check ears for ripeness by gently peeling back a small portion of the husk. Be sure to check the ears frequently for ripeness and harvest as required as ears can quickly become over-ripe and lose their sweetness. Remove the ears from the stalk by pulling quickly downward while twisting and then refrigerate until consumption.
The typical symptom of nitrogen deficiency is the plant turns pale green; a ‘V’ shaped yellow coloration on leaves. This pattern starts from leaf end to leaf collar. The symptom begin from lower to upper leaves.
The deficient plants are dark green and lower leaves show reddish-purple discoloration.
The leaf margins turn yellow and brown coloration which appears like firing or drying. The symptoms progress from lower leaves to upper leaves.
Symptom appears on younger leaves where we will see yellow color striping or interveinal chlorosis.
Upper leaves shows broad bands of yellow coloration and later turn pale brown or gray necrosis. The symptom first appears in the middle of leaves and progress outward.
Anthracnose symptoms vary widely depending on numerous factors such as genotype, age of plant and environmental conditions; small oval or elongated water-soaked spots which enlarge up to 15 mm long appear on leaves; lesions develop a tan center and red-brown or orange border; lesions may coalesce to form large necrotic patches; severely infected leaves on susceptible hybrids may wither and die; fungal fruiting bodies develop on dead tissues and may produce pink or orange spore masses; the fungus may also cause top dieback and stalk rot.
Fungus overwinters on crop debris; emergence of disease is favored by high temperatures and extended periods of wet and cloudy weather; seedlings and mature plants are most susceptible to the disease.
Plant hybrids resistant to anthracnose; rotating crops and plowing crop debris into soil may help reduce incidence of early season infections.
Heavy infestations can result in curled leaves and stunted plants; honeydew secretions promote growth of sooty mold; corn leaf aphids are blue-green in color, peach aphids are green-yellow in color; aphids may transmit viruses when feeding.
Grassy weeds also serve as hosts for corn-leaf aphids; peach aphids have a wide host range.
It is rare for aphids to reach levels that are damaging to the plant and no control is generally warranted as insecticide sprays will not prevent transmission of viruses.
Singular, or closely grouped circular to irregularly shaped holes in foliage; heavy feeding by young larvae leads to skeletonized leaves; shallow, dry wounds on fruit; egg clusters of 50-150 eggs may be present on the leaves; egg clusters are covered in a whitish scale which gives the cluster a cottony or fuzzy appearance; young larvae are pale green to yellow in color while older larvae are generally darker green with a dark and light line running along the side of their body and a pink or yellow underside.
Insect can go through 3–5 generations a year.
Organic methods of controlling the armyworm include biological control by natural enemies which parasitize the larvae and the application of Bacillus thuringiensis; there are chemicals available for commercial control but many that are available for the home garden do not provide adequate control of the larvae.
Water-soaked linear lesions on leaves as they emerge; lesions turn brown and may subsequently turn gray or white; lesions may have a red border; after the leaves are mature, lesions do not tend to extend any further; no new lesions tend to appear after tasseling; if corn variety is susceptible, mature leaves may shred after maturity.
Bacteria can also cause disease in oats, barley, wheat, some millets and sorghum.
Resistant hybrids should be planted in areas where the disease is prevalent; plowing crop debris into soil and rotating crop may not be effective at controlling the disease due to its extensive host range.
The infected leaves initially shows narrow stripes between the veins. The initial symptoms are generally confused with gray leaf spot disease. But the lesions from bacteria appear brown, orange, and/or yellow when you infected leaves are back-lit. Also in Bacterial Leaf Streak disease the lesions show slightly wavy edges when compared to the smooth, linear lesion margins of gray leaf spot.
The bacteria causes gumming disease on sugarcane in several part of the world. First reported on corn in South Africa. Currently this disease is reported in Nebraska (Aug. 26, 2016), Colorado, Illinois, Iowa, and Kansas on corn.
Use healthy and disease free seeds. Remove the infected plant debris and burn them. Follow crop rotation.
Plants suddenly beginning to lodge (bend to lie along the ground) midway through season; one or more internodes above soil line turning brown, water-soaked, soft and slimy; tissue has foul odor and mushy appearance;
Disease is most commonly found in plantations which have overhead irrigation systems or in areas with high rainfall; disease emergence is favored by high temperatures and high humidity.
Plow all crop debris into soil in Fall; plant corn in well-draining soil to prevent waterlogged plants.
Small necrotic spots with chlorotic halos on leaves which expand to rectangular lesions 1-6 cm in length and 2-4 mm wide; as the lesions mature they turn tan in color and finally gray; lesions have sharp, parallel edges and are opaque; disease can develop quickly causing complete blighting of leaves and plant death.
Disease emergence is favored in areas where a corn crop is followed by more corn with no rotastion; severity and incidence of disease is likely die to continuous corn culture with minimum tillage and the use of susceptible hybrids in in the midwestern corn belt of the USA; prolonged periods of foggy or cloudy weather can cause severe Cercopora epidemics.
Plant corn hybrids with resistance to the disease; crop rotation and plowing debris into soil may reduce levels of inoculum in the soil but may not provide control in areas where the disease is prevalent; foliar fungicides may be economically viable for some high yeilding susceptible hybrids.
Symptoms are usually first apparent at the tasseling stage; plant stalks become shredded and pith is completely rotted with stringy strands of vascular tissue left intact; small, black fungal fruiting bodies are visible in the vascular strands and give the tissue a gray coloration; fungus grows into internodes of the stalk causing the plant to ripen early and causing the stalk to weaken; plant may break.
Emergence of the disease is favored by warm soils with a low moisture content; fungus overwinters in the soil and can also survive on other host plants which include sorghum and soybean.
There are currently no available fungicides to treat the disease; avoid stressing plants by practicing good water management; rotating crops with small grains may help reduce disease incidence.
Oval or elongated cinnamon brown pustules on upper and lower surfaces of leaves; pustules rupture and release powdery red spores; pustules turn dark brown-black as they mature and release dark brown powdery spores; if infection is severe, pustules may appear on tassels and ears and leaves may begin to yellow; in partially resistant corn hybrids, symptoms appear as chlorotic or necrotic flecks on the leaves which release little or no spore.
Disease is spread by wind-borne spores; some of the most popularly grown sweet corn varieties have little or no resistance to the disease.
The most effective method of controlling the disease is to plant resistant hybrids; application of appropriate fungicides may provide some degree on control and reduce disease severity; fungicides are most effective when the amount of secondary inoculum is still low, generally when plants only have a few rust pustules per leaf.
Tumor-like galls on plant tissues which are initially green-white or silvery white in color; interior of galls darken and turn into masses of powdery dark brown or black spores (with the exception of galls on leaves which remain greenish in color); galls may reach up to 15 cm in diameter and are common on ears, tassels, shoots or midrib of leaves; galls on leaves remain small and do not burst open.
Fungus overwinters on crop debris or in the soil and can survive for several years; fungus usually enters the plant through wounds; application of nitrogen fertilizer increases incidence of disease, while application of phosphorous fertilizer decreases infection.
Although many practices may be recommended for the control of common smut, the only method that is completely effective is to grow resistant corn hybrids.
Feeding damage to leaves, tassel and leaf whorls; preferred feeding site is the ear and insect produces extensive excrement at the tip of the ear; younger larvae feed on silks, severing them from the plant; young caterpillars are cream-white in color with a black head and black hairs; older larvae may be yellow-green to almost black in color with fine white lines along their body and black spots at the base of hairs; eggs are laid singly on both upper and lower leaf surfaces and are initially creamy white but develop a brown-red ring after 24 hours and darken prior to hatching.
Adult insect is a pale green to tan, medium sized moth; can be a very damaging pests ofcorn; insect overwinters as pupae in the soil.
Corn earworms are most problematic on sweet corn varieties and treatment should be applied at egg hatch; monitor plants for eggs and young larvae and also natural enemies that could be damaged by chemicals; Bacillus thuringiensis or Entrust SC may be applied to control insects on organically grown plants; appropriate chemical treatment may be required for control in commercial plantations.
Stems of young transplants or seedlings may be severed at soil line; if infection occurs later, irregular holes are eaten into the surface of fruits; larvae causing the damage are usually active at night and hide during the day in the soil at the base of the plants or in plant debris of toppled plant; larvae are 2.5–5.0 cm (1–2 in) in length; larvae may exhibit a variety of patterns and coloration but will usually curl up into a C-shape when disturbed.
Cutworms have a wide host range and attack vegetables including asparagus, bean, cabbage and other crucifers, carrot, celery, corn, lettuce, pea, pepper, potato and tomato.
Remove all plant residue from soil after harvest or at least two weeks before planting, this is especially important if the previous crop was another host such as alfalfa, beans or a leguminous cover crop; plastic or foil collars fitted around plant stems to cover the bottom 3 inches above the soil line and extending a couple of inches into the soil can prevent larvae severing plants; hand-pick larvae after dark; spread diatomaceous earth around the base of the plants (this creates a sharp barrier that will cut the insects if they try and crawl over it); apply appropriate insecticides to infested areas of garden or field if not growing organically.
Symptoms of all maize downy mildew pathogens are similar although may vary depends on cultivar, age and climate. The disease appear as early from two weeks after sowing resulting in chlorosis and stunting. In older plants the leaves shows mottling, chlorotic streaking and lesions and white striped leaves. Usually the leaves are narrower and more erect when compare to healthy plants and are covered with a white, downy growth on both surfaces.
The disease is both air and seed born. The pathogen have several alternative hosts.
Grow available resistant varieties and hybrids. Follow crop rotation with non host crops. Use suitable systemic fungicide for both seed treatment and foliar spray. Keep the fields free from weeds. Drying seeds before sowing reduces the disease incidence.
Small holes or pits in leaves that give the foliage a characteristic “shothole” appearance; young plants and seedlings are particularly susceptible; plant growth may be reduced; if damage is severe the plant may be killed; the pest responsible for the damage is a small (1.5–3.0 mm) dark colored beetle which jumps when disturbed; the beetles are often shiny in appearance.
Flea beetles may overwinter on nearby weed species, in plant debris or in the soil; insects may go through a second or third generation in one year.
In areas where flea beetles are a problem, floating row covers may have to be used prior to the emergence of the beetles to provide a physical barrier to protect young plants; plant seeds early to allow establishment before the beetles become a problem - mature plants are less susceptible to damage; trap crops may provide a measure of control - cruciferous plants are best; application of a thick layer of mulch may help prevent beetles reaching surface; application on diamotecoeus earth or oils such as neem oil are effective control methods for organic growers; application of insecticides containing carbaryl, spinosad, bifenthrin and permethrin can provide adequate control of beetles for up to a week but will need reapplied.
Plants wilting and leaves changing color from light to dull green; lower stalk turns straw yellow; internal stalk tissue breaks down; interior of stalk has a red discoloration; black fungal fruiting bodies may be visible on the stalk, often at internodes, and can be easily scraped off; if fungal infection affects the ears, it produces a red mold at the tips of the ear which spreads down; early infection may result in the ear being covered in pink mycelium which causes the corn husk to adhere to the ear.
Fungus can enter through wounds to stalk or ear; ear rot is caused by the fungus infecting silks and moving down through the ear; fungus survives on corn debris in soil and on debris of other host plants such as wheat.
Stressed plants are more susceptible to Gibberella - providing adequate fertilization and irrigation can help reduce incidence of disease; control insects, especially stem and ear borers; hybrids differ in their susceptibility to the disease and further information is required in order to develop specific control measures.
Gray or yellow stripes with irregular margins on leaf surfaces; stripes follow leaf veins and contain characteristic dark green to black water-soaked spots; if infection occurs early then plant may become wilted or withered; it is common to find a crystalline residue on leaves caused by dried bacterial exudate.
Disease overwinters in diseased crop debris on, or close to, the soil surface; temperatures below 12°C (53.6°F) and above 40°C (104°F) bacterium grows more slowly and may even be killed off.
Plant resistant sweetcorn hybrids; rotate crop; plow crop debris into soil immediately after harvest.
Circular or elliptical spots 2-10 mm across near the tips of lower leaves which are dark green and water soaked initially but become cream to tan before turning dry and brown; lesions may have red-brown margins; large lesions may have a yellow halo.
Disease can occur anywhere where corn is grown but is not usually very damaging.
Disease is usually not severe but if it does become a problem crops should be rotated and any debris should be plowed into the soil after harvest.
Chlorotic spots and streaks on leaves which develop into a mottled or mosaic pattern; susceptible plants may be stunted; ear formation and development cease; mosaic and mottling with no red discoloration are characteristic symptoms of the disease.
Virus is transmitted by more than 15 different species of aphid and is passed to the plant from the insect in seconds to minutes of feeding; sorghum is also a major host of the virus.
Many commercial corn hybrids are highly tolerant of the disease and no control is needed; control aphid populations on plants and remove any Johnson grass growing in the vicinity as it can act as a reservoir for the virus.
The disease occurs at all stages of crop. The main symptoms includes appearance of chlorotic mottling on leaves which starts from base and extends upwards. Also the leaves shows necrosis at margins which later extends to mid rib and results in drying of entire leaf. The necrosis of young leaves in the whorl causes dead heart symptom. Other symptoms are premature plant death, shortened male inflorescences with few spikes, and/or shortened, malformed, partially filled ears.
The virus mainly spread by vectors (maize thrips, aphids, rootworms and leaf beetles) and infected seeds. The first report of this disease in Africa continent was in Kenya (2011). And later the disease spreads to other countries like Tanzania, Uganda and South Sudan.
Use healthy, disease free certified seeds. Keep the fields free from weeds. Remove the infected plants and burn them. Control vectors by treating seed and/ foliar spray with suitable insecticide. Follow crop rotation with non cereals at least for two seasons. Plant maize only in main rainy season instead of short rainy season. Grow available resistant varieties.
In the beginning we will notice elliptical gray-green lesions on leaves. As the disease process this lesions become pale gray to tan color. Later stage the lesions looks dirty due to dark gray spores particularly under lower leaf surface. The disease can be easily identified in the field due to its long, narrow lesions which are unrestricted by veins.
The disease mainly spread through rain splash and wind.
Follow proper tillage to reduce fungus inoculum from crop debris. Follow crop rotation with non host crop. Grow available resistant varieties. In severe case of disease incidence apply suitable fungicide.
Above-ground, plants may be yellow and stunted; roots have obvious lesions and roots are discolored; root cortex will come away when pulled gently, exposing the white stele; can also cause damping-off of seedlings.
Fungus overwinters in soil and crop debris; disease emergence is favored by high soil moisture and low temperatures leading to low soil oxygen levels.
Control of the disease relies on improving soil drainage or planting corn in areas where the soil is well-draining; systemic fungicides can be used to treat seed prior to planting to protect seedlings from disease.
Below ground we can see galls on the roots due to female nematode feeding. Above ground the plants are stunted, yellow and patchy in growth. Severely infested plant may die before harvest.
The galls are formed by female nematode feeding resulting in formation of giant cells.
Deep summer ploughing helps in reducing nematode population. Follow crop rotation with nematode antagonistic plants. Grow resistant varieties. In severely infected field follow soil fumigation with suitable nematicide.
Irregularly shaped holes in leaves and stems; leaves may be shredded; slime trails present on rocks, walkways, soil and plant foliage; several slug species are common garden and field pests; slugs are dark gray to black in color and can range in size from 2.5 to 10 cm (1-4 in).
Slugs prefer moist, shaded habitats and will shelter in weeds or organic trash; adults may deposit eggs in the soil throughout the season; damage to plants can be extensive.
Practice good garden sanitation by removing garden trash, weeds and plant debris to promote good air circulation and reduce moist habitat for slugs and snails; handpick slugs at night to decrease population; spread wood ashes or eggshells around plants; attract molluscs by leaving out organic matter such as lettuce or grapefruit skins, destroy any found feeding on lure; sink shallow dishes filled with beer into the soil to attract and drown the molluscs; chemical controls include ferrous phosphate for organic gardens and metaldehyde (e.g. Buggeta) and carbaryl (e.g Sevin bait) for non-organic growers.
Foliar symptoms vary with hybrid and different fungal isolate; lesions on leaves may be tan and elongated and run between leaf veins; lesions may have a buff or brown colored margin; another race of the fungus causes tan, spindle shaped or elliptical lesions with a water-soaked margin that turns into a yellow halo.
Fungus overwinters in corn debris in soil; disease occurs worldwide but is emergence favors areas with a warm, damp climate.
The most effective method of controlling the disease is to plant resistant hybrids; cultural control methods include plowing crop debris into soil after harvest and rotating crops.
Leaves stippled with yellow; leaves may appear bronzed; webbing on underside of leaves; small kernel size; mites may be visible as tiny moving dots on the webs or underside of leaves, best viewed using a hand lens; usually not spotted until there are visible symptoms on the plant; leaves turn yellow and may drop from plant;
Spider mites thrive in dusty conditions; water-stressed plants are more susceptible to attack.
In the home garden, spraying plants with a strong jet of water can help reduce buildup of spider mite populations; if mites become problematic apply insecticidal soap to plants; certain chemical insecticides may actually increase mite populations by killing off natural enemies and promoting mite reproduction.
The main symptoms are appearance of water soaked lesions initially. As the disease progress the lesions become long and turn pale yellow with irregular margins running in the length. The pathogen may infect the stem and causes stunting, wilting and death of plant.
The pathogen is mainly transmitted by maize flea beetles and to lesser extent by infected seeds.
Grow available resistant varieties. Use certified healthy seeds. Remove the crop debris and burn them. Use suitable insecticide to control flea beetle.
If population is high leaves and may be distorted and curl upwards; edges of leaves may dry up and are speckled with black feces; insects are small (1.5 mm) and slender and best viewed using a hand lens; adult thrips are pale yellow to light brown and the nymphs are smaller and lighter in color.
May be found on corn at any time during the growing season.
Avoid planting next to onions, garlic where very large numbers of thrips can build up; use reflective mulches early in growing season to deter thrips; apply appropriate insecticide if thrips become problematic; young plants will recover from damage and treatment is not often necessary as the thrips are beneficial for controlling mites.
CABI Crop Protection Compendium. (2012). Zea mays datasheet. Available at: http://www.cabi.org/cpc/datasheet/57417. [Accessed 14 November 14]. Paid subscription required.
Espinoza, L. & Ross, J. (Eds.) Corn Production Handbook. University of Arkansas Division of Agriculture Cooperative Extension Service.Available at: http://www.uaex.edu/publications/pdf/.... [Accessed 14 November 14]. Free to access.
White, D. G. (1999). Compendium of Corn Diseases. American Phytopathological Society Press. Available at: http://www.apsnet.org/apsstore/shopap... Available for purchase from APS Press.
ELDAD How many bags of maize should a farmer expect to harvest per acre after planting PAN variety of maize seed?