Sweet Potato Pests and Diseases: Wireworm, Weevil, Voles and Rot

Almost everything that matters is underground

You can lose leaves to caterpillars and beetles all summer and still dig a fine crop, because the vine grows back faster than anything eats it. What actually costs you the harvest happens where you cannot see it: soil-dwelling larvae chewing the skin, rodents hollowing the roots out, and fungal infections that only reveal themselves in the store weeks after you dig.

This changes how you should spend your effort. Do not spray the leaves. Put the work into rotation, into keeping the soil free of fresh organic matter, and into handling the roots gently at harvest. Below is what you are likely to meet and how to tell one from another when you turn the fork over.

Wireworm: the price of recently turned grass

Wireworms are the thin, tough, orange-brown larvae of click beetles, and they live for two or three years in the soil. They are overwhelmingly a problem of ground that was grass, a grassy cover crop or a weedy fallow within the last year or two, because that is where the adults lay. The damage is characteristic: small round entry holes and shallow grazed tunnels just under the skin, which spoil the appearance and open the root to rot in storage.

There is no rescue once the roots are in the ground. The controls are all preventive, and they work. Avoid planting into recently broken sod, keep fresh manure and half-rotted material out of the bed, and cultivate the bed a couple of times in the weeks before planting so birds can take the larvae you turn up.

1. Check the ground before you commit Bury a piece of carrot or potato two inches (5 cm) deep in several places a fortnight before planting. Dig them up and count the larvae attached. Several per bait means choose another bed.
2. Give broken sod a year first Ground that was lawn or cereal cover crop should grow something else for one season before sweet potato goes in.
3. Cultivate and let the birds work Turn the bed over two or three times in the weeks before planting. Exposed larvae get eaten and the population drops noticeably.
4. Use damaged roots first Wireworm holes are cosmetic if shallow, but they let rot in. Sort damaged roots out at harvest and eat them within a few weeks rather than storing them.

Sweet potato weevil and vine borers

In warmer regions the sweet potato weevil is the most serious pest of the crop. The adult is a small ant-like insect with a long snout and a reddish body; the grubs tunnel through the flesh of the roots and through the base of the stems. Infested roots develop a bitter, unpleasant taste that no amount of cooking removes, so even lightly attacked roots are not worth keeping.

The weevil survives from one year to the next in leftover roots and in volunteer plants, which makes clearing up the single most effective control. Dig every root out at harvest, including the small ones, and take all the vines off the plot. Never save your own slips from roots grown in an infested bed, and where the pest is established locally, rotate at least a few hundred feet away from last year's ground.

1. Clear the bed completely Remove every root, including the pencil-thin ones, and all the vine. Anything left behind carries the population through to next season.
2. Start with clean planting material Buy certified slips rather than sprouting a supermarket root, particularly if you have seen weevil damage before.
3. Keep soil over the crowns Weevils reach the roots through cracks in dry soil. Draw soil up over any crown that becomes exposed as the ridge settles.
4. Destroy infested roots Do not compost them. Bag them and remove them from the garden, or the grubs simply complete their development in the heap.

Voles, deer and other animals

Voles do more damage to home-garden sweet potatoes than any insect. They tunnel along the loose soil of the ridge, which is exactly the easy digging they prefer, and hollow out the roots from below while leaving the vine looking perfectly healthy. Straw mulch and a thick vine canopy give them the cover they want, so a garden with a vole history should use black plastic instead of straw.

Deer and groundhogs graze the foliage. Light grazing does surprisingly little harm because the vine regrows, but repeated stripping in late summer costs you root size, since those leaves are what fill the roots. A low fence is worth it only if grazing is happening repeatedly, not for one visit.

1. Look for runways along the ridge Vole runs show as narrow paths of flattened soil or grass at the base of the ridge. Finding them in midsummer gives you time to act.
2. Take away the cover Remove straw mulch and keep the grass short around the bed. Voles avoid crossing open ground.
3. Harvest promptly once ready Most vole damage happens in the last few weeks. If you find fresh tunneling, dig the crop rather than waiting for size.

Scurf, black rot and the rots that appear in storage

Scurf shows as dark gray-brown blotches on the skin. It is entirely cosmetic, does not enter the flesh and does not spread in store, though it does make roots dry out faster. It builds up in soil, so a bed that produced scurfy roots will do it again; rotate away for three or four years and avoid liming that ground.

Black rot is the serious one. It produces firm, dark, sunken circular patches, the flesh underneath tastes bitter, and it spreads between roots in the store. Any root with a dark sunken patch should be thrown out, not trimmed and kept. Both diseases come in on infected planting material as often as from the soil, which is the argument for buying certified slips rather than propagating from a root of unknown origin.

1. Sort ruthlessly before storing Inspect every root as it goes into the crate. Anything with a sunken dark patch, a soft area or a deep cut goes out. One bad root can take a whole box.
2. Do not wash the roots Washing spreads spores across the whole batch and drives moisture into wounds. Brush dry soil off after curing instead.
3. Cure properly, and quickly Curing seals wounds and is the strongest defense against storage rot. Get the roots into curing conditions within a few hours of digging.
4. Note which bed it came from Record where scurf or black rot showed up. Those beds need three or four years off sweet potato, and that is impossible to remember without writing it down.