What is damaging my roots and tubers underground?
Underground damage is usually found at harvest, when it is too late to act on this crop. What the page is really for is deciding what to change for the next one, because almost everything in this group persists in the soil rather than on the plant.
The one observation that decides it
Wash one root and look at the shape of the damage. Narrow clean tunnels bored straight in are wireworm. Rusty-brown surface tunnels just under the skin of a carrot are carrot fly. Irregular swellings and galls that are part OF the root, not on it, are root-knot nematode; swollen club-shaped distortion on a brassica is clubroot. A white fluffy fungal mat with tiny black dots at the base of an onion is white rot. Raised corky brown patches on an otherwise healthy potato are common scab, which is cosmetic.
Start from the plant in front of you
13 things it could be
Ordered by how much they can cost you — if the first entry fits what you can see, rule it out before considering the milder ones.
Leaves yellow and die back from the tips while the plant is still growing, and the whole plant lifts out easily. The base of the bulb is covered in a dense white, fluffy mat studded with poppy-seed-sized black sclerotia.
Conditions: 10–24°C · humidity ≥80%First move: Remove affected material nowLeaves are small, pale and sparse, and the plant wilts in warm weather despite wet soil. Whole branches die back one at a time. Cut the bark at the base and the tissue underneath is brown or reddish instead of white-green, often with a sharp line between healthy and dead.
Conditions: 15–30°C · humidity ≥90% · wet leaves / rainFirst move: Remove affected material nowSeen on: Asparagus, Blueberries, Rosemary, Thyme, Oregano, Sage and 1 more
A line of small white dots down the leaf, made by adults feeding, is the first sign. Larvae then tunnel down into the stem and bulb, where they pupate as brown rice-grain-shaped puparia. The plant distorts, rots and is usually unusable.
Conditions: 8–25°CFirst move: Treat, then re-check in 5–7 daysA small water-soaked spot spreads into a soft, wet, cream-to-brown mush with a distinctive foul smell. In stored bulbs and tubers the inside collapses while the skin still looks intact. Standing plants wilt and topple at the rotted point.
Conditions: 20–32°C · humidity ≥90% · wet leaves / rainFirst move: Treat, then re-check in 5–7 daysYoung plants wilt in sunshine, recover overnight, then wilt worse each day until they die. Leaves take on a blue-grey cast. Pull a failing plant and the roots are ragged stumps riddled with white maggots.
Conditions: 10–25°CFirst move: Fix the cause, not the leafSeen on: Cabbage, Radishes, Broccoli, Cauliflower
Foliage turns reddish then yellow and the plant looks stressed for no obvious reason. Lifted roots carry rusty-brown tunnels just under the skin, with small cream maggots inside. Damaged roots rot in store.
Conditions: 12–26°CFirst move: Fix the cause, not the leafStunting, wilting in midday heat, and yellowing of older leaves above ground; swollen, distorted club-shaped roots below ground that rot and smell.
Conditions: 12–27°C · humidity ≥70% · wet leaves / rainFirst move: Fix the cause, not the leafSeen on: Cabbage, Radishes, Kale, Broccoli, Cauliflower, Brussels sprouts and 1 more
Plants wilt in the heat of the day despite adequate water, grow slowly and yellow between the veins. Lifted roots carry irregular swellings and galls fused into the root tissue — unlike legume nitrogen nodules, these cannot be rubbed off.
Conditions: 18–30°CFirst move: Fix the cause, not the leafSeen on: Carrots
Distinctive U-shaped notches bitten out of leaf edges of heuchera, bergenia, rhododendron and strawberry. Later, a plant wilts suddenly and lifts out of its pot with no roots at all, revealing cream C-shaped grubs with brown heads in the compost.
Conditions: 5–25°CFirst move: Treat, then re-check in 5–7 daysSeen on: Strawberries, Hydrangeas
Brown, corky patches inside beetroot, swede and celeriac roots. Cauliflower curds go brown and the stem is hollow with a brown lining. Growing points die, young leaves are brittle and distorted, and fruit sets poorly.
Conditions: dry air ≤45%First move: Fix the cause, not the leafSeen on: Beetroot, Cauliflower, Celeriac
Rounded, rough, woody galls at the soil line, on roots, or on stems, starting soft and cream-coloured then hardening and darkening; reduced vigour and stunting.
Conditions: 20–30°C · humidity ≥60%First move: Treat, then re-check in 5–7 daysSeen on: Rhubarb
Narrow, clean holes bored straight into potato tubers, carrots and other roots, often with secondary rot following. Seedlings and young transplants wilt and die from stems chewed at soil level. The larvae are thin, tough, orange-brown and up to 25 mm long.
Conditions: 8–25°CFirst move: Fix the cause, not the leafSeen on: Potatoes
Raised, corky, brown patches on the tuber skin, sometimes sunken and star-shaped in severe cases. The haulm looks completely healthy — the first sign is at lifting. Beetroot, radish and parsnip growing in the same soil can show the same rough patches.
Conditions: 18–30°C · dry air ≤60%First move: Watch it before you actSeen on: Potatoes