Dill

Stages affected: Vegetative · Flowering

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 · Flowering · Maturation

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: Vegetative · Flowering

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: Flowering · Maturation

A white, talcum-like coating on leaves and stems caused by many closely related fungi, each with a narrow host range: the mildew on one plant rarely spreads to an unrelated one nearby. On herbs such as dill and rosemary and on many flowers it is mostly cosmetic, but heavy infections yellow and drop leaves. The crop-specific mildews of cucurbits and peas have their own records.

Visual signs

Greyish-white powdery patches on the upper leaf surface, stems and flowers that can be rubbed off with a finger, spreading to cover whole leaves. Leaves may curl or twist first, then yellow and brown; flowers may fail to open. Late in the season tiny dark specks can form in the white growth.

Damage pattern

Starts on crowded, shaded or overwatered plants and on soft growth from heavy nitrogen feeding. Worst when days are mild and nights humid, active once days pass about 16 °C and less common in summer heat. Indoors, still air and poor light on overwintered herbs bring it on.