Sweet Corn Pests and Problems: Earworm, Borer, Smut and Rust

Work out where the damage is

Corn problems sort themselves neatly by location. Damage in the tip of the cob is almost always a caterpillar that came down the silk channel. Holes in the stem with sawdust-like frass are a borer. Marks on the leaves are fungal. A grotesque gray swelling anywhere on the plant is smut. Knowing which of those you are looking at tells you immediately whether to act, and how urgently.

The other useful question is whether the plant is still going to give you a cob. Leaf disease late in the season is cosmetic. A borer in the stem below the cob is not, because it cuts off the supply line. Judge everything by what it does to the cob.

Corn earworm: it comes in down the silks

A moth lays her eggs on the fresh silks, the caterpillar hatches and crawls down the silk channel into the tip of the cob, and it eats kernels from the tip downwards. You usually find one fat caterpillar per cob, because the first one in eats any rival. By the time you notice chewed silks or a hole at the tip, it is already inside.

The damage is limited and local. Snap the top two inches (5 cm) off the cob and the rest is perfectly good. If you want to prevent it, the practical home garden method is a clothes peg or an elastic band closing the tip of the husk tightly once the silks have browned, which physically blocks the entry route. Do not close it before pollination is finished.

1. Wait until the silks have browned Pollination has to finish first. Once the silks are dry and brown, they have done their job and the channel can be sealed.
2. Clip the husk tip shut Gather the husk over the tip of the cob and hold it closed with a clothes peg or a rubber band. It blocks the caterpillar's route in.
3. Cut off damaged tips at harvest Snap or cut the chewed top off the cob and use the rest. Do not compost the cut tip with the caterpillar still in it.
4. Clear stalks at the end of the season Pupae overwinter in debris and in the top inch of soil. Removing stalks and turning the bed reduces next year's first generation.

Stem borers: check for frass at the leaf joints

Borers tunnel into the stem, usually just above a leaf joint, and the sign is a small round hole with a pile of pale, sawdust-like frass beneath it. A plant with a borer below the cob often snaps in wind, and even if it stands, the cob above the tunnel fills badly because the tunnel interrupts the flow of sugars and water.

There is no cure once one is inside the stem. Where a plant has already snapped, cut it out and take it off the plot; splitting the stem lengthways lets you find and destroy the larva rather than leaving it to pupate. Prevention is mostly hygiene: borers overwinter in old corn stalks, so a plot cleared every autumn has far fewer of them.

1. Look for frass at the joints Run your eye down each stem at the leaf joints. A dusty brown pile on the leaf below a small hole means a larva is inside.
2. Remove and destroy affected stems If the plant has snapped or the tunnel is below the cob, pull it out. Split the stem, find the larva, and put the debris in the bin rather than the compost.
3. Clear every stalk in autumn Chop and remove all stalks and stubble after the last harvest. Leaving stalks standing over winter is what keeps a borer population going year to year.

Rust and leaf blight: usually too late to matter

Rust shows as small orange-brown pustules scattered over the leaves that rub off as powder on your fingers. Leaf blight shows as long gray-green lesions running along the leaf. Both spread in humid weather, and both usually arrive after the cobs are already filling, at which point they cost you nothing worth acting on.

They only matter when they appear early, before tasselling, and cover a large share of the leaf area. Then the plant loses the leaves it needs to fill the cob. Improve air movement by keeping the block spaced properly, avoid watering over the leaves in the evening, and remove the worst affected lower leaves. Do not grow corn on the same ground two years running.

Corn smut and animal damage

Corn smut produces swollen silvery galls on the cob, the stem or the tassel that eventually burst into a cloud of black spores. Remove galls while they are still firm and silvery and bag them; once they burst, the spores go into the soil and survive there for years. It is eaten as a delicacy in some cuisines, but if you do not want it back, take it off the plot.

Larger animals are often the real problem. Birds tear at ripening cobs, and where raccoons, badgers or deer are around they will strip a whole block in one night, usually the night before you planned to pick. Nothing deters a determined mammal except a physical barrier, so if you have lost a crop that way before, fence or net the block before the cobs start to fill.

1. Bag smut galls before they burst Cut the gall off with a good margin of tissue while it is still firm and pale. Put it in the bin, never the compost, and wash your hands and tools.
2. Net against birds as cobs fill Birds start on the tips as soon as the kernels sweeten. Netting draped over the block from silking onward is enough for birds.
3. Fence against mammals before you need to A raccoon or deer raid takes one night. Put the fence up when the silks brown, not after you have found the damage.
4. Note what appeared and when Record which problem showed up, on what date, and at what growth stage. Pests arrive at remarkably consistent points in the season, and a two year record tells you exactly when to have the netting and clips ready.