Carrot

Stages affected: Seedling · Vegetative · Maturation · Harvest window

A small, shiny black fly with a reddish head whose larvae tunnel through carrot roots and leave them riddled and rust-coloured. It is the pest that spoils more home carrot crops than any other, and it is invisible until you pull the roots: UC IPM notes there are no reliable above-ground symptoms. It also attacks parsnip, parsley, celery and celeriac, which is why the whole carrot family rotates together. The adult finds the crop by scent, which is what makes thinning and handling the foliage a risk.

Visual signs

Shallow meandering tunnels in the outer root, filled with rusty brown frass, often near the tip first · Stiff creamy white legless maggots up to about 8 mm long inside the tunnels (UC IPM gives about 8 mm) · Roots that look sound until peeled, then show brown tracks just under the skin · Tops may redden or wilt on a badly tunnelled root, but usually look normal · The adult is a slender black fly about 6 to 8 mm long with a reddish brown head and yellow legs, rarely noticed

Damage pattern

Worst on the outer rows nearest hedges and long grass, where the adults shelter, and on crops left in the ground late. Damage builds through the season as the generations overlap, so a crop pulled young often escapes what a crop left to store in the ground does not. Tunnelled roots are still edible once peeled but keep poorly, since the damage lets rot in.

Stages affected: Seedling · Vegetative · Maturation

Small wedge-shaped sap-sucking insects that hop or fly sideways off a leaf the moment you disturb it. The feeding damage on its own is minor on most garden crops. They matter because one of them, the aster leafhopper, carries the phytoplasma that causes aster yellows, and once a leafhopper has picked that up it spreads it for the rest of its life. On carrot, celery, lettuce and the whole carrot family, the insect is a nuisance and the disease it carries is not.

Visual signs

Slim wedge-shaped insects 3 to 6 mm long, green, tan or mottled, held close to the leaf · They move sideways as readily as forwards, which almost nothing else does · They spring off and fly a short distance when a leaf is turned, so a plant that seems to twitch when touched is worth a closer look · Pale stippling or speckling on the upper leaf surface where the sap has been drawn out · Cast white skins left on the undersides · Wingless nymphs of the same shape on the undersides, moving in the same sideways scuttle

Damage pattern

Fine pale stippling spreading across older leaves first, and on heavy numbers a general dullness and slight curling. This rarely justifies treatment on its own. What justifies attention is a single plant that turns yellow, throws distorted or unusually narrow growth and a cluster of small shoots while its neighbours stay normal, which is aster yellows arriving rather than leafhopper feeding.

Stages affected: Germination · Seedling · Vegetative · Maturation

Not a disease. A root crop that forks, twists, splits or grows a mass of hairs has met something on the way down, and the cause is almost always in the soil preparation rather than in anything that happened later. Stones, clods, a compacted layer, lumps of fresh manure, too much nitrogen, drought followed by a soak, and crowding all do it, and each leaves a slightly different shape. The roots are entirely edible; they are just more work to clean and less of them is usable. It is the commonest disappointment with carrots and parsnips and the one most completely under the grower control.

Visual signs

Root forked into two or more legs from one point, often with a stone or clod found at the fork when you dig · Root bent, twisted or stubby, stopping short at a hard layer · A dense coat of fine white side roots along a root that is otherwise normal, usually from fresh manure or heavy nitrogen · Lengthwise splits in an otherwise sound root, from a heavy soak after dry weather close to harvest (Illinois) · Thin, tangled, intertwined roots along a whole row, from seedlings never thinned · Green, bitter shoulders where the top of the root stood out of the soil in the light

Damage pattern

Whole-bed or whole-row rather than scattered, and tied to the ground: a bed that was dug deep and sieved grows straight roots beside one that was not. Forking from stones follows the stony patch; splitting follows a dry spell broken by rain; hairiness follows the manure. Pull a few and the pattern tells you which it was.

Stages affected: Seedling · Vegetative · Maturation

Not a fungus and not quite a bacterium: a phytoplasma, a wall-less organism that lives in the plant sap and is injected by a feeding leafhopper. It cannot be cured, it does not spread from plant to plant by touch or splash, and an affected plant never recovers. Utah State lists carrot, celery, lettuce and wheat among its hosts and the range is far wider, taking in most of the carrot family and many ornamentals. The characteristic result is not rot or spotting but distortion: growth that comes out yellow, narrow, oddly upright and multiplied into a broom of small shoots.

Visual signs

One plant affected while its immediate neighbours stay normal, which is the first clue that this is insect-delivered rather than soil or splash borne · New growth yellow or almost white while older leaves stay green · Leaves narrow, strap-like and often twisted · A cluster of many small shoots from one point instead of a single stem, sometimes called a witches broom · Flowers that come out green, leafy or deformed, which is close to diagnostic once seen · On carrots, red or purple leaves and a taproot furred with fine hairs

Damage pattern

Scattered single plants rather than a spreading patch, and the pattern does not follow drainage, shade or the direction of watering. Affected plants stay alive but are useless: the leaf is bitter or off-flavoured and the root or head never fills. Because the leafhopper transmits in a single feed and then carries the phytoplasma for life, the number of affected plants tracks how many infective leafhoppers arrived, not how wet or crowded the bed was.

Stages affected: Germination · Seedling

The caterpillars of several drab night-flying moths. They live at the soil surface and chew through the stems of young plants at ground level, so you almost always find the damage before you find the caterpillar.

Visual signs

Smooth dull caterpillar up to 40 mm long in grey brown or near black often with faint stripes · Curls tightly into a C shape the moment it is touched or uncovered · Found 25 to 50 mm down in loose soil within a hand's width of the damaged plant · Nothing visible on the plants by day because they feed after dark

Damage pattern

A young plant cut clean through at soil level and left lying beside its own stump · Whole seedlings gone overnight with the root still in the ground · Ragged holes chewed in the leaves that rest on the soil · Damage running along a stretch of row rather than scattered across the bed

Stages affected: Germination · Seedling

Soft-bodied molluscs that feed at night and in wet weather, rasping holes in leaves. Seedlings and low soft foliage are the most at risk, and a single night can take out a row of young plants.

Visual signs

Ragged holes with smooth edges chewed through the middle of leaves rather than at the margins · Silvery dried slime trails on leaves soil and pots, which is the deciding feature · Damage appears overnight and stops in dry sunny spells · Seedlings grazed off at soil level · Slugs or snails found under boards pots and mulch during the day

Damage pattern

Worst after rain irrigation and in humid shaded beds · Feeds at night; plants look untouched by day · Concentrated on low soft leaves near the soil

Stages affected: Vegetative · Maturation

Microscopic soil roundworms that enter young roots and make the plant build swollen galls around them. The galls block water and nutrient flow, so the plant above ground looks thirsty and starved in soil that is neither. On carrot the symptom is forked, stubby, knobbly roots covered in small beads and side roots. The host range is huge — tomato, pepper, aubergine, cucurbits, lettuce, beet, beans and most other garden crops — and the worst damage is in warm, sandy soil. Nothing available to a home grower kills them in the ground; management is rotation to non-hosts, resistant varieties where they exist, and containers of fresh mix.

Visual signs

On carrot: roots forked, stubby and knobbly, with many hairy side roots, and small round swellings along them like beads on a string · On other crops: distinct round or spindle-shaped galls on the roots, part of the root itself rather than attached to it, which does not rub off · Above ground, plants stunted and wilting on warm afternoons despite moist soil, recovering overnight · Yellowing and poor vigour in patches of the bed rather than across it · Damage worst in the warmest, sandiest part of the plot

Damage pattern

Patchy, following the warm light soil where the nematodes thrive and spreading slowly as soil is moved. Builds year on year where host crops are repeated. A bed that grew poor tomatoes last year and gives knobbly carrots this year is the classic sequence.

Stages affected: Germination · Seedling

The name covers several different soil organisms doing the same thing: killing seedlings at or just below the soil line, either before they emerge or in the first fortnight after. Most of the damage in cold wet conditions comes from Pythium and Phytophthora, which are oomycetes rather than true fungi, but Rhizoctonia and Fusarium do the same job in warmer soil. The organisms are present in almost every garden soil and most reused potting mix; what turns them from harmless to lethal is cold, wet, airless conditions and a seedling that takes too long to get up. Utah State names it as one of the three problems on cilantro, which is a slow germinator sown into cool spring soil.

Visual signs

Seedlings that were upright yesterday lying flat this morning, still green · A dark water-soaked pinch in the stem right at soil level, thinner than the stem above and below it · Seedlings that never emerge at all, with the seed soft and brown when you dig one up · A bare patch in an otherwise even row, often roughly circular and spreading outward day by day · Sometimes a fine grey or white web on the soil surface in the affected patch · Roots brown and stringy, with the outer layer sliding off the core when pulled between finger and thumb

Damage pattern

Spreads outward from a centre, so an expanding bare patch rather than scattered single losses. It is a seedling disease and stops mattering once the stem has woody tissue at the base, usually two to three weeks after emergence. A sowing that fails patchily and never recovers, where digging finds soft brown seed, is usually this rather than poor seed.

Stages affected: Vegetative · Maturation

Not a disease and not a mistake: the plant switching from making leaves to making seed, which is what it was always going to do. It matters because in a leaf or root crop the switch ends the harvest — the plant pours everything into a flower stem, the leaves turn narrow and bitter, and a root goes woody. Lengthening days, heat and any check to growth all bring it forward, and which of those dominates depends on the crop. Cilantro and spinach are driven mainly by day length, lettuce and radish mainly by heat; biennials such as parsley and celery bolt after a spell of cold followed by warmth, so an overwintered plant runs to seed in spring. A dry spell or a transplant check will trigger any of them. Once a plant has committed there is no way back, so everything useful happens before it starts.

Visual signs

A stem lengthening from the centre of a plant that had been low and spreading, often surprisingly fast once it starts · Leaves higher up the stem narrower, more deeply cut or more feathery than the ones below · Spacing between the leaves opening out so the plant looks stretched · Flower buds in a tight cluster at the tip · In lettuce, the head going conical and firm and the milky sap running bitter · In radish and brassicas, a slender stalk with four-petalled flowers and the root turning pithy

Damage pattern

Usually the whole sowing at once, since every plant in it is the same age and met the same weather. That is the clearest way to tell it from a disease: a problem that arrives on all the plants simultaneously and is worse on the oldest sowing is developmental rather than infectious. Scattered individual plants bolting well ahead of the rest are usually the ones that were checked — transplanted, dried out, or left too long in a module.