Sweet Corn

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Insect Damage

Holes in seeds, seedlings or cotyledons


Cause

The seed corn maggot (Delia platura), the yellowish-white larva of a small fly accidentally imported from Europe. Seed corn maggot are roughly ¼" long and legless. There are several generations per year in Minnesota with the first, which emerges in early spring, being the most destructive.

Treatment

Chemical controls are usually not effective. Once damage is noticed, treatment is usually too late. Adult flies are attracted to rotting plant debris and larva overwinter in crop residue. For this reason cover crops and green manures should be plowed under as early in the spring as possible. The most effective management technique is to time planting with declining fly populations. Growers can achieve this by placing yellow pails filled with soapy water in fields to be planted. Adult flies are attracted to the water and become stuck. By checking the buckets daily growers will quickly notice when fly populations are declining and thus when to begin planting.

Brown, chewed roots or holes in the base of the stem


Cause

Wireworms, the larval for of the click beetle (Melanotus cribulosus). Wireworms vary from ½ to 1 ½ inches in length and are typically brown with hard bodies. Wireworms are long-lived in the soil, often taking several years to reach maturity and emerge as adult beetles. As adults they do not cause damage. Wireworm populations are higher in fields that were previously covered with sod or left fallow.

Treatment

Treatment is not possible once damage has been observed. If damage is noticed over several seasons, treated seed should be used. Approved seed treatments include formulations of lindane and diazinon. It is important to note that seed treatments only protect the seed and will not protect the germinated seedlings.

Wilting plants with ragged holes around hte base of stem in June or early July


Cause

The common stalk borer (Papaipema nebris), the larva of a moth. Larva are up to 2 inches in length, brown or purple in color with white lines running along the length of the body.

Treatment

Action should be taken when damage occurs on 3% of the crop, (or 3 out of every 100 plants). Consult your local Extension office for current pesticide recommendations. In addition grass and weeds near the field edges should be mowed early in the season to reduce habitat for the insect.

Wilting plants with ragged holes around hte base of stem in late July or August


Cause

European corn borer (Ostrinia nubilalis), the larval stage of a moth. In most of Minnesota, only one generation occurs per year.

Treatment

Infected sites are sometimes difficult to spot until infestation is underway. Larva enter stems through small holes, then begin tunneling throughout the plant. Wilting and broken stems often follow this tunneling activity.

Pesticide treatments are difficult since the larva is only active outside the plant for a few days between egg hatch and tunneling activity--although labeled formulations of Orthene, Sevin, Bt, and permethrin are commercially available. Watch for flat white egg masses on leaf surfaces, or use black-light insect traps to scout for adult females in mid-June. Pesticides should be applied immediately after egg hatch, or 7 to 10 days after females begin to appear in June.

For additional control remove overwintering sites by eliminating crop residue from corn, pepper, potato, tomato, bean, and beet fields.

Chewing damage to the tips of ears


Cause

The corn earworm (Helicoverpa zea), the larval form of a grayish-brown moth. Corn earworms are green, brown, maroon, or black in color with four black stripes along the back and a yellow head. At their latest stage they measure nearly 2 inches in length. Few corn earworms survive the winter in Minnesota. Instead adult moths migrate north from southern states beginning in early May. Damage in Minnesota is often noticed between mid-August and early September.

Treatment

Insecticides are not effective once larvae are protected within the corn ears. Therefore treatment should begin when more than 5 adult moths are captured per nigh, over several nights using commercially available pheromone traps. Contact your local Extension office for current information about which traps to use, and what pesticides are currently approved for this insect.

Holes in leaves or ragged, chewed leaves


Cause

Armyworms, the larva of several species of moths. Armyworms generally do not overwinter in Minnesota. Instead adult moths migrate north each season from southern states. Significant outbreaks occur every few years.

Treatment

Many insecticides are available, including permethrin, carbayl, and various Bt formulations. Treatment should be initiated when more than 25% of the crop is partially defoliated.

Diseases

Large, greasey or powdery galls on ears or tassels, often black or purple


Cause

Corn smut (Ustilago zeae), a fungal organism.

Treatment

Control insect pests and avoid injury to plants during cultivation. If possible remove galls before the break open and discard by burning or place in the garbage.

Brown or orange pustules on leaf surfaces


Cause

Common rust (Puccinia sorghi), a fungal disease.

Treatment

Spray plants when there is an average of 1 or 2 pustules or lesions per plant. Approved fungicides include chlorothalonil, copper formulations, mancozeb, and propiconazole.

Pale green or yellow streaks with wavy margins that extend the length of the leaf


Cause

Stewart's bacterial wilt (Xanthomonas stewartii), a bacterial disease often spread by small flea beetles.

Treatment

Plant resistant corn varieties when possible. In addition weeds in field edges and surrounding areas should be controlled to eliminate flea beetle habitat. Contact your local Extension office for current flea beetle control recommendations.

Small, tan leaf spots surrounded by a yellow halo


Cause

Eyespot (Aurreobasidium zeae), a fungal disease. Outbreaks are often worse during wet conditions, with symptoms first appearing on lower leaves then progressing up the plant.

Treatment

Plant resistant varieties and rotate with other crops annually. Remove crop debris at the end of each season. Fungicides may be available for severe outbreaks. Contact your local Extension office for current recommendations.