Pests
Symptoms A fat, striped caterpillar tunneling into the tip of the ear under the husk, leaving chewed kernels and dark droppings. It usually stays near the tip, but the damage can let mold in.Chewed, frass-fouled kernels at the ear tip where a fat, striped caterpillar has fed under the husk; silks may be clipped, and tip damage opens the door to mold. Cause The corn earworm, a caterpillar whose moth lays eggs on the silks. The young caterpillars crawl down the silks into the ear. It is the same pest as the tomato fruitworm.The corn earworm (Helicoverpa zea), whose moths lay eggs on fresh silks; the hatching larvae follow the silks down into the ear tip. It is the same species as the tomato fruitworm. Organic treatment The good news is the worm usually stays at the very tip, so you can just cut that part off. A classic trick is a few drops of vegetable or mineral oil on the silks after they have been pollinated, and an organic spray such as Bt or spinosad works if you time it to fresh silks.Because most feeding is confined to the ear tip, the loss is often cosmetic: cut off the damaged tip at the kitchen. A few drops of mineral oil applied to the silk channel after pollination is a traditional home tactic; a spinosad or Bt product timed to fresh silks is the labeled organic option. Prevention Pick varieties with long, tight husks that make it harder for the worm to get in, and plant early so the ears silk before the moths are at their worst in late summer.Choose varieties with tight, long husk cover, which slows the larvae, and plant early so the crop silks before peak moth flights in late summer.
Symptoms Tiny holes in the young leaves, then sawdust-like droppings where the leaves meet the stalk, and small holes bored into the stalk itself. Badly tunneled stalks can break.Small shot-hole feeding in the whorl leaves, then sawdust-like frass at leaf axils and pin-sized entry holes in the stalk and ear shank; tunneled stalks weaken and can snap. Cause The European corn borer, a caterpillar that starts by chewing the young leaves, then tunnels into the stalks, hollowing them out.The European corn borer (Ostrinia nubilalis), a caterpillar that first feeds in the leaf whorl, then bores into stalks and ear shanks, weakening the plant from inside. Organic treatment Check the young leaf whorls and rub out the egg patches and small caterpillars before they tunnel in. An organic Bt spray only works while they are still out on the leaves; once inside the stalk you cannot reach them.Scout the whorls and pick off egg masses and young larvae before they bore in; a Bt product reaches them only while they are still feeding in the open whorl. Once inside the stalk they are protected. Prevention Chop up and dig in the old cornstalks after harvest, because the borer spends the winter inside them, and move your corn to a new spot each year.Shred and turn under cornstalks after harvest, since the borer overwinters in old stalks; rotating away from last year's patch helps too.
Symptoms Young corn plants cut off right at the ground overnight, as if with scissors. If you dig near a cut plant, you often find a fat gray caterpillar curled up in the soil.Young seedlings clipped off at or just above the soil line overnight, often toppled and left; a gray, greasy-looking caterpillar curls up just below the surface nearby. Cause Cutworms, fat caterpillars that live in the soil and come out at night to chew through young stems at the base.Cutworms, the soil-dwelling larvae of several night-flying moths, which sever tender seedling stems at the base as they feed after dark. Organic treatment Go out at night with a flashlight and pick them off, or dig around a freshly cut plant to find the culprit. A little collar of cardboard or foil pushed into the soil around each stem keeps them out.Handpick at night or just under the soil around fresh damage; a cardboard or foil collar pushed an inch into the soil around each seedling blocks them. Prevention Clear weeds from the bed before you plant, because the moths lay their eggs in weedy soil, and plant into warm soil so the seedlings grow quickly past the tender stage.Keep the bed weed-free before planting, since cutworm moths lay in weedy ground, and delay sowing until the soil is warm and seedlings grow fast through the vulnerable stage.
Symptoms Tiny dark beetles that hop away when you get close, leaving the young leaves peppered with little pinholes. Bad early damage stunts small plants and can spread a corn disease.Tiny dark beetles that jump when disturbed, leaving fine pinholes and shot-hole pitting in the leaves of young plants; heavy early feeding stunts seedlings and can transmit Stewart's wilt. Cause Small beetles called flea beetles that chew young corn leaves. One kind also carries the germ that causes Stewart's wilt, a corn disease, spreading it as it feeds.Several flea beetle species that feed on corn seedlings; the corn flea beetle also carries the bacterium that causes Stewart's wilt from plant to plant. Organic treatment Small plants usually grow past light damage. If the beetles are thick, cover the young plants with a lightweight row cover, taking it off when the tassels form so the wind can pollinate, and use an organic spinosad spray only if you must.Seedlings usually outgrow light feeding; where beetles are thick, a floating row cover over young plants (removed at tasseling for pollination) protects them, and spinosad is labeled if needed. Prevention Pull weeds that shelter the beetles, plant into warm soil so the corn grows quickly, and pick varieties that resist Stewart's wilt if it is a problem where you live.Control weeds that harbor the beetles, plant after the soil warms so seedlings grow fast, and choose Stewart's-wilt-tolerant varieties where the disease occurs.
Symptoms Ears torn open and eaten, and stalks knocked over, appearing overnight right when the corn is ready. Raccoons seem to know the exact night before you plan to harvest.Ears pulled down, husked, and half-eaten overnight just as they reach the milk stage, with stalks knocked flat; damage arrives suddenly the night before you meant to pick. Cause Raccoons, which raid ripe sweet corn at night and can clear a small patch in a night or two. They are one of the most common reasons a home corn crop is lost.Raccoons, which raid sweet corn at the milk stage and can strip a patch in a night or two; they are one of the most common causes of a lost home crop. Organic treatment You cannot spray for raccoons; you have to keep them out. A short two-wire electric fence, switched on as the ears get close to ready, is the surest way to protect the patch.There is no spray; exclusion is the answer. A two-wire electric fence (about 4 and 8 inches high) turned on as ears near maturity is the most reliable home defense. Prevention Set up and turn on the fence before the corn is ripe, not after the first raid. Once raccoons find your corn, they come back every night.Put the fence up and energize it before the ears reach the milk stage, not after the first raid, since once they find the patch they return nightly.
Diseases
Symptoms Puffy silver-white growths on the ears or stalks that later burst into a cloud of black powder. On the ear, these galls take the place of kernels. They show up most on stressed or damaged plants.Silvery-white galls on ears, tassels, or stalks that swell, then rupture to release a mass of powdery black spores; ear galls replace kernels and are most common on stressed or injured plants. Cause A fungus that gets in through wounds and silks, especially in hot, dry spells, and forms the galls. Its spores can survive in the soil for years.The fungus Ustilago maydis, which enters through wounds and silks and is favored by hot, dry weather punctuated by injury; its spores survive years in the soil. Organic treatment There is no home spray. Cut off and throw away (do not compost) the galls before they burst, so their spores do not spread into your soil.There is no spray for the home garden; cut off and bag the galls before they rupture and blacken, to keep spores out of the soil. Do not compost them. Prevention Try not to injure the plants while weeding, keep water and feeding steady so the corn is not stressed, move your corn each year, and pick more resistant varieties if smut keeps coming back.Avoid wounding plants during cultivation, keep moisture and fertility steady to reduce stress, rotate away from infected beds, and choose more tolerant varieties where smut recurs.
Symptoms Small rusty-brown, powdery spots on both sides of the leaves that darken later in the season. A bad case yellows the leaves and can leave the ears less full.Small cinnamon-brown, powdery pustules scattered on both leaf surfaces, darkening toward the season's end; heavy infection yellows and kills leaf tissue and can cut ear fill. Cause A fungus whose spores blow in on the wind and take hold in cool, damp weather. In most gardens it arrives fresh each year rather than surviving the winter in place.The fungus Puccinia sorghi, whose spores blow in on the wind and infect under cool, moist, humid conditions; it does not overwinter in most northern gardens but arrives each year on wind currents. Organic treatment Most gardens can live with a little rust. Pull off the worst lower leaves to slow it and give the plants air. You rarely need to spray in a home garden.Most home crops tolerate rust without treatment; remove badly infected lower leaves to slow spread and improve airflow. A labeled fungicide is rarely needed in a garden. Prevention Pick rust-resistant varieties, which are easy to find, and plant early so the corn is nearly done before rust gets bad late in summer. Water at the base when you can.Choose rust-resistant hybrids, which are widely available, and plant early so the crop matures before rust builds late in the season; avoid overhead watering where practical.
Symptoms Long yellow streaks with wavy edges running down the leaves, appearing after flea beetles have been feeding. A bad early case wilts and stunts young plants and can kill them.Long, pale-green to yellow streaks with wavy margins running along the leaves, following flea-beetle feeding scars; severe early infection wilts and stunts seedlings, sometimes killing them. Cause A bacterium spread by the corn flea beetle as it feeds. After a mild winter more beetles survive, so the disease tends to be worse the next year.The bacterium Pantoea stewartii, carried and introduced by the corn flea beetle; mild winters that let more beetles survive drive worse outbreaks the following year. Organic treatment You cannot cure an infected plant; pull and throw away badly affected seedlings. The real control is keeping the flea beetles that spread it in check.There is no cure once a plant is infected; remove and destroy severely affected seedlings. Management is aimed at the flea-beetle vector, not the bacterium itself. Prevention Grow resistant varieties, which is the main defense, and keep flea beetles off young plants with a row cover or an organic treatment so they cannot pass the disease along.Grow resistant or tolerant hybrids, the primary defense, and manage flea beetles on young plants with row cover or a labeled treatment to break the transmission cycle.
