Common Problems of Sweet Corn

By Sherri Sanders

Corn earworms are a problem in sweet corn every year. Early plantings are not badly infested, but later harvests will usually have severe earworm damage unless timely control measures are followed. Corn 

earworm moths deposit eggs on the developing silks or on the leaves near the ear. The tiny caterpillars follow the silk down into the ear, where they feed on the tip. Once the worm is inside the protective husk covering, there is no effective control. A suggested insecticide must be applied before the worms enter the silk channel. For good control in heavy infestations, make several applications two to three days apart from the time silks appear until they turn brown. To restrict worm infestation, tighten the tip of the husk with a rubber band or clothespin after the silk appears or insert mineral oil (one-half medicine dropper full) in the silk tube to decrease damage.

European corn borers damage stalks, tassels and ears. As their name suggests, corn borers bore into the plant. The stalks break off when damage becomes severe. Corn borers may also bore into the cob and be found after cooking. Apply a suggested insecticide at five-day intervals, beginning with eggs hatching in mid-May.

Spraying applications for corn earworms will adequately control the corn borers. Flea beetles often attack early in the spring as the corn plants emerge through the soil. They can be quite damaging in large numbers and may also carry Stewart’s bacterial wilt disease. A suggested insecticide must be applied early to control flea beetles.

Stewart’s wilt is a bacterial disease spread by the flea beetle. This disease causes yellow streaks in the leaves, stunted growth and death of young plants of susceptible varieties. The disease generally is not severe after cold weather or when resistant varieties are planted.

Smut is caused by a fungus that invades the kernels. It develops as a swollen black pustule (gall) in the ear and sometimes infects the tassel. Some sweet corn varieties are more tolerant of smut than others. Smut occurs most frequently on white varieties and is often severe when kernels are extremely dry and during tasseling. Remove and destroy smut galls while they are moist and firm. Do not discard these galls in or near the garden. Place in the garbage or burn them. The smut is not poisonous, and in certain areas, it is cultivated and eaten as one would use a mushroom. Break off the infected part of the ear. The remainder is suitable for eating.

diseases – Stewart’s wilt, a bacterial disease spread by flea beetles; smut (especially on white varieties); stunt (transmitted by leafhoppers). insects – corn earworm, European corn borer, flea beetles, Japanese beetles (eat silks), corn sap beetles (damage kernels after husk is loosened).

cultural – Poor kernel development (failure to fill out to the tip) caused by dry weather during silking stages, planting too close, poor fertility (especially potassium deficiency), too few rows in a block, resulting in poor pollination. Lodging (falling over) from too much nitrogen.

Why didn’t my sweet corn ears fill out to the tips?

Several conditions can cause poor kernel development at the tip of the ear: dry or cool, wet weather during silking and pollination; planting too close; poor fertilization, especially lack of potassium; and poor natural pollination. These conditions may be overcome by watering in dry weather, planting at recommended spacing (9 to 12 inches apart in the row), proper fertilization and planting short rows in blocks of two or more for self-pollination.

Why are ears of corn underdeveloped at the tip end?

This condition may be caused by nutrient deficiency, cool temperatures during ear maturity and low moisture. Corn is pollinated by windborne pollen from the male flowers or tassels at the top of the plant to the female flowers or silks about midway up the stalks. Each kernel develops from an individually pollinated silk. Kernels which develop near the middle and base when conditions occur, such as those mentioned above, are pollinated first and will take precedence over those pollinated last. This often results in failure of the kernels near the tip to develop properly or in irregular kernel development.

My sweet corn grew for a while and then had a mosaic appearance. The corn did not develop properly. The ears that formed were poorly filled.

This is maize dwarf mosaic virus. It overwinters in johnsongrass around a garden. To control the problem, remove the johnsongrass, and follow a good insect control program. Some varieties are more resistant to this disease than others.

I planted corn in my garden this fall and it turned out beautifully, but the corn ear worms ate more corn than I did. What can I do to prevent this?

Spray or dust the ear silks with Sevin (carbaryl) to prevent adult insects from entering and laying eggs. Begin dusting and spraying at an early stage and repeat every two days. A drop of mineral oil on the silks is recommended to prevent earworm damage.

The centers of my corn plants are full of little green insects. What do I do about them?

Corn leaf aphids infest the sworl of young corn plants. The plants will tolerate large numbers of these aphids. If plants begin to wilt or die, spray with malathion. Use as directed on the label.