Plant Health Problems
Diseases caused by Fungi:
Rust, Puccinia maydis.
The disease is caused by a fungus and appears as small rusty pustules on the leaves. The pustules can coalesce producing large necrotic areas on the leaves. In severe cases, it can cause leaves to drop off. The disease is less severe on tolerant varieties.
Control can also be achieved with the use of fungicide sprays. Fungicide sprays should be applied if there is at least one pustule on 80% of the leaves prior to the tasseling stage. After tasseling, fungicide sprays will not improve yield but may improve the cosmetic appearance of the ear. Among the compounds registered for use in Connecticut are chlorothalonil and mancozeb. Consult the label for dosage rates and safety precautions.
Smut, Ustilago maydis.
Corn smut is a fungus disease which appears as enlarged, malformed overgrowths on all parts of the corn plant. These overgrowths, called "boils," are most prominent on the corn ear and tassels. These boils, which enclose the fungus spores, are grayish-white when they are first formed. As the spores of the fungus ripen, the boils darken, until at last they burst releasing the spores. The ripened spore masses have a dark oily appearance. The disease occurs sporadically from season to season, but is most prevalent when temperatures are between 80 and 90 F and nitrogen fertility is high.
Sanitation measures and a three-year crop rotation will reduce chances of an outbreak of corn smut. The boils may be cut out and burned. Infected refuse should not be left in the field, but removed and destroyed. Fungicides have not been effective for control of this disease.
Helminthosporium leaf blight, Helminthosporium sp.
This fungus disease shows as a sudden leaf scorch late in the summer following a period of wet weather. The diseased leaves appear as if hit by an early frost. The fungus makes black threadlike growths on the injured tissue. Late planting and poor fertilization usually contribute to infection by this fungus.
To help avoid the disease, fertilize the plants properly and keep them in good growing condition. The disease is rarely observed on the sweet corn varieties.
Fusarium root and stalk rot, Fusarium spp.
This disease is caused by a fungus which lives in the soil. The roots are rotted off and the base of the stalk is diseased. Stalks which lose their supporting roots because of this disease may be blown over easily. The diseased stalk usually shows a reddish discoloration when it is split open.
There are no control measures except crop rotation, planting on well-drained soils, and proper nutrition.
Diseases caused by Bacteria:
Bacterial wilt or Stewart's wilt, Erwinia stewartii.
Corn infected with this disease is stunted and has wilted leaves on dry days. Brown streaks often appear along the leaf veins. Leaves dry up rapidly, remaining grayish-green in color even though withered. A sticky yellow ooze exudes from the cut ends of the "strings" when an infested stalk is cut. This ooze shows only after a rain or heavy dew. Plants infected as seedlings may be killed outright. Plants infected later may produce a satisfactory crop. The bacteria enter the field on seed or in the bodies of adult overwintering corn flea beetles.
Because of the importance of the flea beetle in carrying over the bacteria from one growing season to the next, the severity of the winter is one of the factors which controls the amount of Stewart's wilt. A series of severe winters kills most of the flea beetles, so that Stewart's wilt practically disappears. A series of mild winters allows the flea beetle population to build up and once again Stewart's wilt becomes important. Control can also be achieved with resistant cultivars.
Diseases caused by Viruses:
Maize dwarf mosaic virus.
This disease causes the plant to become stunted and the leaves to shrivel and develop a patchy mosaic pattern. The virus is seedborne but can also be spread by aphids. The virus can also reside in annual grass weeds, such as Johnston grass.
Control can be achieved by using resistant varieties.
Aphids, Rhopalosiphum sp. and Aphis sp. Corn leaf aphid, Rhopalosiphum maidis; corn root aphid, Aphis maidiradicis
These two species are often found on corn. They are not usually abundant and no special control measures are needed. See Aphid fact sheet.
Armyworm, Pseudaletia unipuncta.
In certain seasons, this caterpillar may be destructive to corn. Measures used to control European corn borer and fall armyworm will generally also control this insect.
Corn earworm, Helicoverpa zea.
This is one of our most destructive corn insects, feeding directly on the ear. The moths, with a wingspread of about 1.5 inches, are tan with darker markings. They arrive in Connecticut each season from more southern areas after which they lay eggs singly on the corn silk. After hatching, the caterpillars feed on the silk and then the kernels at the tip of the ear. They reach a length of up to 2" and vary greatly in color. They may be brown, tan, green, or pink with light and dark longitudinal stripes. The head is golden brown and the body has small bumps and spines, giving it a rough texture. There can be two or three generations in a year, depending on when the adults arrive on winds from the south. Varieties with long silk channels and the husk tightly covering the tip of the ear are better protected from the corn earworm damage.
Direct application of vegetable oil can be used for control, but precise timing is critical. The best time to apply the oil is 4 days after growth of corn silk starts or 2 days after the silk is fully-grown. At this time, the tips of the silks are just beginning to turn brown and dry up. If applied earlier, the oil can interfere with pollination, and later application gives poorer control. Only 3-5 drops with an eyedropper or oil can are needed. Bacillus thuringiensis var. kurstaki (Bt) is registered for control of this pest in Connecticut. Mixing one part Bt to 20 parts oil will improve control. Consult the label for dosage rates, safety precautions, and preharvest intervals.
Corn root webworm, Neodactria caliginosella.
The larvae of this insect and that of a closely related species occasionally injure corn, especially on sod land. They eat into the side of the stalk just above the surface of the ground. The larva is about 1/2" long and pale white or gray with dark tubercles. Control is rarely necessary.
Corn rootworms, Diabrotica sp.
Several species of corn rootworms are found in Connecticut, including the western corn rootworm, Diabrotica virgifera virgifera, the northern corn rootworm, Diabrotica barberi, and the southern corn rootworm, Diabrotica undecimpunctata howardi. The life cycles of the northern and western corn rootworm are similar. The adults feed on corn silk and other flowers (especially squash and other cucurbits, where they may be mistaken for striped cucumber beetles) in August and into the fall. They lay their eggs at the base of corn plants and other grassy weeds. Corn is at risk of injury if replanted in the same location, or in a field with large numbers of grassy weeds the previous year. In these situations larvae hatching in the spring feed on the roots. Southern corn rootworm (which is the same species as the spotted cucumber beetle) feeds and lays eggs on a much wider range of hosts than the other corn rootworm species.
Although the western corn rootworm is a major pest of corn in the Midwest, it has not been a serious problem so far in Connecticut. Crop rotation combined with good weed management is an effective control.
Several species, which vary greatly in numbers from year to year, may injure corn. See Cutworm fact sheet.
European corn borer, Ostrinia nubilalis.
This insect feeds not only on corn, but on other crop plants (including potatoes, peppers, and beans), many weeds, and on a variety of other herbaceous plants. The larvae tunnel in the stalk and ears in all directions, weakening the stalks so that they break over. The larva is pale white or gray with black tubercles and is not more than an inch in length when fully grown. Adults have a wingspread of an inch or so and are buff to brown in color. There are usually two generations annually. Eggs are laid on the underside of leaves, and the larvae tunnel in the stalks and pupate in the burrows. Second generation larvae and those of the single generation corn borer overwinter in the stalks and pupate in the spring.
Control begins with tillage of the corn stubble, either in the fall or early in the spring. This reduces the survival of borers from the previous year. Pheromone traps may be used to monitor flight of the adults. Then, from the time the tip of the tassel first becomes visible, looking down into the whorl of leaves, until it emerges, watch carefully for larvae or signs of larval damage. Among the compounds registered for control of this pest in Connecticut are Bt formulations. Bacillus thuringiensis var. kurstaki (Bt), in either a granular or liquid formulation, may be applied to the top of the plant so that it flows down into the center of the whorl. Additional applications may be needed when caterpillars or signs of feeding damage are visible, or when pheromone traps indicate the flight of adults. As the ear develops, applications should be directed to the ear zone of the plant. Consult the label for dosage rates, safety precautions, and preharvest intervals.
The parasitic wasp Trichogramma has been used as alternative method of control. This tiny wasp attacks the egg masses of the corn borer, and the eggs of other caterpillars, too. Be sure to purchase the insects from a reputable supplier and make sure the strain you purchase is known to be well adapted to attacking corn borer on corn. Bt will not harm the Trichogramma wasps, but other insecticides will.
Fall armyworm, Spodoptera frugiperda.
This caterpillar may be tan, brown or greenish with thin light colored stripes down the back and a dark brown head capsule with a "Y" on top. The adult moth is tan with gray mottled patches on the wings and has a wingspread of 1 1/2". This caterpillar feeds voraciously on corn in the whorl stage, before the tassel develops. Ragged holes and brownish droppings indicate the presence of a caterpillar inside the rolled-up leaves. Bt var. kurstaki is among the compounds registered for control of this pest in Connecticut. Bt should be applied so that it gets into the center of the whorl. Fall armyworms may also feed in the ear. Control methods used for European corn borer and corn earworm should also control ear feeding by this insect.
The adults of several species of flea beetles feed upon the leaves of corn and, besides eating holes in the leaves, may cause damage by their ability to disseminate Stewart's wilt disease. It is helpful to plant varieties resistant to wilt. See fact sheet on Flea beetles.
Grass thrips, Anaphothrips obscurus.
The leaves of corn are sometimes injured by thrips, which make narrow chain-like markings where they have eaten on the surface. Control is not usually required.
Japanese beetle, Popillia japonica.
The adults of this colorful insect may cluster at the tips of ears of sweet corn where they feed on the fresh silk during July and August if the cornfield adjoins a large area of turf that is infested with Japanese beetles. Controlling the larvae in nearby turf will assist in reducing their numbers.
Leafminers, Cerodontha sp. and Agromyza sp.
The grass sheathminer, Cerodontha dorsalis, makes a linear mine, usually near the margin or midvein. The corn blotch leafminer, Agromyza parvicornis, makes a broad linear or blotch mine near the base of the leaf. These are of minor importance, and control measures are not ordinarily needed.
Lesser cornstalk borer, Elasmopalpus lignosellus.
In some seasons, particularly in dry weather and on light soil, young corn may be attacked by bluish-green, brown-striped caterpillars which feed first upon the lower leaves and later infest the roots and stems. They characteristically form dirt covered silken tubes which lead away from the stem tunnels in the soil. Control has not been necessary.
Both the larvae and adults of this small active beetle feed on the ears of sweet corn. The adults make small round holes through the husk to enter the ear, or enter the ear where the husk has been pulled back to view the ear. Control measures have not been needed in the past.
Seedcorn maggot, Delia platura.
The maggots of this small fly may infest seed corn in the ground and injure or destroy them so that they do not produce desirable plants. Damage is much greater in cold, wet weather in soils high in organic matter, especially if a cover crop or fresh animal manure has just been turned in. These small grayish-brown flies lay eggs in newly plowed fields, and the dirty yellowish-white maggots that develop attack the seeds of bean and other vegetables, injuring or destroying them in the ground so that they do not produce good plants. These maggots can be controlled by delaying planting until the weather is warm enough and new organic matter added to the soil has adequately decomposed. Once the damage has been recognized, it is too late for effective control.
Stalk borer, Papaipema nebris.
Occasionally, the larvae of this insect will bore into the stalks of corn, particularly at the edge of a planting. Control is not commonly needed. The eggs of this caterpillar are laid in the fall by the moths on surrounding grasses and weeds. After hatching in the spring the borers feed on these plants and later may migrate into adjacent fields where they cause injury. The caterpillars are very active. Their restless habit of frequently changing from one plant stem to another increases the damage. Keeping the borders of the vegetable garden well-mowed and controlling weeds may help deny these insects their breeding grounds.
The larvae of several species of white grubs, which are the immature forms of scarab beetles, may cause injury by feeding on the roots of corn. The grub is white, "C" shaped, has six legs, and a brown head. The eggs are generally laid in grasslands, and damage to crops is, subsequently, greatest when they are planted on land that has recently been taken out of sod. Native species have a 2 to 3 year life cycle but are rarely observed. Exotic species (Japanese beetle, oriental beetle, Asiatic garden beetle and European chafer) have annual life cycles and can be damaging any year in which there is adequate moisture for egg hatch. By not planting corn and other susceptible crops on newly plowed sod land, most of the injury can be avoided.
These shiny, hard-skinned, wire-like larvae are the immatures of click beetles (Elateridae). Wireworms feed on seeds, roots, and stems, producing weak plants. Several species of wireworms are involved and, like the white grubs, are more injurious to crops planted on soil which is the first or second year out of sod. The corn wireworm, Melanotus communis, or the wheat wireworm, Agriotes mancus, usually occur in heavier organic soils and tend to disappear with continued cultivation. Light sandy soils favor the eastern field wireworm, Limonius agonus. Continued cultivation encourages this species. The presence of wireworms in soil may be detected with baits, such as a cut potato, a mixture of untreated wheat and corn seeds, or rolled oats, buried a few inches deep in the soil, marked with a flag, then dug up 2-3 weeks later. If present in large numbers, they may also be visible when the soil is turned over.
Wireworms are difficult to control with available insecticides. Some tactics which may reduce damage are: 1) delay planting for several weeks after plowing so that little food is available, 2) use plastic mulch between rows to heat the soil above the wireworms' preferred temperature, and 3) rotate wireworm-susceptible crops with plants in the cabbage family (including canola or mustard used as a cover crop) and then incorporate plant residues.