This plant disease, also called Stewart''s wilt, is caused by a bacterium (Erwinia stewartii). The bacteria spend the winter in the bodies of corn flea beetles and infect healthy plants as the flea beetles feed on the corn leaves. The bacteria then move through and infect the entire plant. The severity of the wilt disease depends on the number of flea beetles that survive the winter. Bacterial wilt is most severe after mild winters when numerous beetles survive. Plants growing in rich soil are the most susceptible to attack, as are early yellow varieties. Spotted cucumber beetles also spread the bacteria during the growing season.
Remove and destroy all infected plants. Plant varieties tolerant of this disease. Control flea beetles and cucumber beetles an insecticide labeled for the pest, following directions on the label.
Helicoverpa zea, (formerly Heliothis zea) This is the most serious pest of corn. Also known as the tomato fruitworm and the cotton bollworm, it attacks other garden vegetables and flowers as well. The worm is the larva of a light gray-brown moth with dark lines on its wings. In the spring, the moth lays single yellow eggs on corn silks and the undersides of leaves. Worms hatch and feed on new leaves in the whorls. This feeding doesn''t reduce the corn yield, but ragged leaves develop and plant growth may be stunted. Worms cause more serious damage when they feed on the silks, causing poor pollination, and on the developing kernels. They enter the ear at the silk end, or bore through the husk. In the South, where these pests survive the winter, early and late plantings suffer the most damage. Adult moths migrate into northern areas, where late plantings are severely damaged. Several generations occur per year. Uneaten parts of infested ears are still edible.
Insecticides are ineffective once worms are in the ears. For future plantings, apply an insecticide labeled for corn earworm, following directions on the label. Select varieties with long, tight husks, or pinch the silk end of the husk closed with a clothespin.
This plant disease is caused by a fungus (Ustilago maydis) that attacks any aboveground part of corn. Germinating seedlings are not affected. Galls are full of black powdery spores that survive the winter in soil and corn debris. Spores spread from plant to plant by wind and water, and a gall forms only where one lands, without spreading throughout the plant. Younger corn plants are more susceptible; most are infected when they are 1 to 3 feet tall but are less susceptible after the ears have formed. Corn smut is most prevalent in warm temperatures (80° to 95°F) and when dry weather early in the season is followed by moderate rainfall as the corn matures. Smut doesn''t reduce the corn yield directly but tends to sap the plant''s energy, reducing ear development.
Cut off smuts before they break open and release the black powdery spores. Grow varieties tolerant of corn smut. Clean all plant debris from the garden after harvest.
Ostrinia nubilalis The European corn borer is one of the most destructive pests of corn, but also feeds on tomatoes, potatoes, and peppers. Early plantings are most affected, but late plantings can be severely damaged in areas where more than one generation occurs in a season. Overwintering in corn plants, the borers pupate in spring, and emerge as adult moths in early summer. The moths, tan with dark wavy lines on the wings, lay clusters of about twenty cream-colored eggs on the undersides of lower corn leaves. Hatching borers feed first in the leaf whorls, riddling them with shot holes. Later they bore into stalks and the bases of ears, resulting in broken stalks and tassels, poor ear development, and dropped ears. The borers feed for a month, pupate, and emerge as moths to repeat the cycle. Cool, rainy weather in early summer inhibits egg laying and washes hatching larvae from the plants, reducing populations. Very dry summers and cold winters also reduce borers.
At the first sign of borers, apply an insectide that''s labeled for these pests, following the directions on the label. Destroy the plants at the end of the season. Avoid early planting.
These beetles jump like fleas but are not related to fleas. Both adult and immature flea beetles feed on a wide variety of garden vegetables. Some flea beetles are responsible for spreading the bacterial wilts that kill corn plants. The immature beetle, a legless gray grub, injures plants by feeding on the roots and the lower surfaces of leaves. Adults chew holes in leaves, feeding for up to 2 months. Flea beetles primarily damage seedlings and young plants. Leaves of seedlings riddled with holes dry out quickly and die. Adult beetles survive the winter in soil and garden debris. They emerge in early spring to feed on weeds until vegetables sprout or plants are set in the garden. Grubs hatch from eggs laid in the soil and feed for 2 to 3 weeks. After pupating in the soil, they emerge as adults to repeat the cycle. One to four generations occur per year.
Remove all plant debris from the garden after harvest to eliminate spots for adult beetles to overwinter. Treat corn with an insecticide labeled for flea beetles, following the directions on the label.
Hylemya platura Seedcorn maggots feed on seeds and seedlings. They are attracted to large-seeded vegetables such as peas, beans, cucumbers, watermelons, and corn. The maggots are most numerous in cool periods in the spring and fall and in cold soil that is high in organic matter. The black, hairy adult flies are attracted to the organic matter and lay eggs in the soil. The maggots that hatch from these eggs burrow into the seeds and eat the inner tissue, leaving a hollow shell. Rot fungi may enter a damaged seed and further destroy it. After 1 to 2 weeks, the maggots burrow deep into the soil and pupate. The adult flies that emerge feed on nectar and plant juices before laying more eggs. Several generations occur per year. In warm-winter areas, these pests are active year-round. In cold-winter areas, they survive the winter as pupae and emerge as adults in the early spring.
Since adult flies are attracted to organic matter, don''t add manure to the soil in the spring when planting beans, peas, cucumbers, watermelons, or corn.