Spinach

Stages affected: Seedling · Cut and come again

A small grey fly whose maggots tunnel inside the leaves of spinach, beet and chard, leaving pale blotches that ruin the leaf for eating. It rarely kills a plant but can spoil most of a spring crop, and it moves freely between the three host crops and lambsquarters.

Visual signs

Rows of 4 to 8 tiny white cigar-shaped eggs, about 1 mm long, laid side by side on the underside of a leaf. A few days later pale green, then brown, blotches appear where the maggots feed between the leaf surfaces; hold the leaf to the light and you can see the white maggots and dark frass inside. The adult is a greyish fly, 6 mm long, similar to a small house fly.

Damage pattern

Mines start as narrow winding tunnels and widen into irregular blotches that may take most of the leaf. Damage is from inside the leaf, so there are no holes and no chewed edges. Worst on spring sowings; three to four generations a year, with the first and heaviest in mid spring.

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: Cut and come again

The most important disease of spinach, a water mould that spreads in cool damp weather and can take out a bed in a week. It only attacks spinach, so it does not move to your beet, chard or lettuce, but new races keep appearing that overcome resistant varieties.

Visual signs

Pale yellow, slightly angular patches on the upper leaf surface, 5 to 20 mm across, that enlarge and turn tan. On the underside of each patch, in damp weather, a soft grey to purple fuzz of spores. Older leaves go first; badly hit leaves curl and die.

Damage pattern

Starts on the lower, older leaves of the densest part of the row and spreads upward and outward through the bed over days. Worst after cool nights with dew or fog, and in autumn and overwintered crops. Whole leaves become inedible; plants rarely die outright but produce nothing worth picking.

Stages affected: Flowering · 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: Flowering · 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: Seedling

Tiny dark beetles that jump like fleas when disturbed and pepper leaves with small round holes. Established plants shrug them off, but on seedlings and on a crop grown for its leaves the damage matters, and on radish and other brassicas they arrive in numbers early in the season.

Visual signs

Beetle 1.5 to 3 mm long usually shiny black bronze or metallic blue and sometimes with a pale stripe down each wing cover · Jumps away instantly when you lean over the plant which is the surest identification · Enlarged hind legs visible if you can get one to hold still · Found on the upper leaf surface in sunshine and most active on warm still days · Pale larvae feed unseen on roots and do little damage on most crops

Damage pattern

Many small round holes 1 to 2 mm across giving the leaf a shot-through or peppered look · Damage heaviest on the youngest leaves and at the edges of a bed nearest cover · Seedlings can be killed outright or checked so badly they never catch up · Older established plants carry the damage without losing yield, so the holes alone are not a reason to act · On root crops the holes cost nothing directly; the risk is a seedling set back at the stage when the root should be forming

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.