Parsley

Stages affected: Vegetative · Cut and come again

The commonest disease of parsley (Wisconsin): a seed-borne fungus that peppers the leaves with small brown spots in wet weather until they yellow and die. A different species from the Septoria of tomatoes, which does not cross between the two crops. Sources differ on the species name.

Visual signs

Small greyish-brown, somewhat angular spots, under about 6 mm, often with tan centres; small oval spots on the leaf stalks. Tiny black specks (pycnidia) in the spots, visible with a hand lens, are the deciding feature. Badly spotted leaves yellow and wither from the outside of the clump inward.

Damage pattern

Appears after rain, heavy dew or overhead watering and spreads leaf to leaf in splashing water, on hands and on tools used in wet foliage. Active from about 10 to 30 °C. Infected seed gives poor, patchy germination and diseased seedlings.

Stages affected: Seedling · Vegetative · Cut and come again

Small soft-bodied sap-sucking insects that cluster on the softest new growth. They reproduce extremely fast in warm weather and are the main insect vector of plant viruses in the garden.

Visual signs

Clusters of small pear-shaped insects on shoot tips buds and leaf undersides · A pair of short tubes (cornicles) projecting back from the rear end, which is the deciding feature · Green yellow brown red or black depending on species, wingless or winged · Curled twisted or yellowing new leaves and stunted shoots · Sticky honeydew on the leaves below, often turning black with sooty mould · White cast skins left on leaves and easily mistaken for live insects · Ants running up and down the stems

Damage pattern

Concentrated on the newest softest growth and worst on heavily fed plants · Honeydew and sooty mould appear below the colony rather than at it · Virus symptoms such as mottling yellowing and distorted growth may follow long after the aphids have gone

Stages affected: Cut and come again

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.

Stages affected: Vegetative · Cut and come again

The caterpillar of the black swallowtail butterfly, which feeds only on the carrot family: parsley, dill, fennel, carrot, celery and Queen Anne's lace. Feeding is rarely enough to seriously injure a plant, and most gardeners move or tolerate it rather than kill it.

Visual signs

Young caterpillars are dark and spiky with a white patch across the middle. Older ones grow to about 38 mm, smooth and green with black bands studded with yellow to orange dots. When disturbed they push out a forked yellow-orange scent organ from behind the head. They prefer flowers, seeds and the tender leaves.

Damage pattern

Leaves and flower heads eaten back to the stalks, usually by one or a few large caterpillars rather than a swarm, from midsummer into early fall. Feeding is rarely sufficient to seriously injure an established plant (Minnesota).

Stages affected: Vegetative · Cut and come again

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.