Kale

Stages affected: Seedling · Cut and come again

The velvety green caterpillar of the small white butterfly, and the commonest pest of every brassica from kale and cabbage to broccoli and radish. It chews ragged holes from the leaf edge inward, leaves green droppings on everything, and arrives in three to five waves a season from the first warm days of spring until frost.

Visual signs

White butterflies with one or two black spots on the forewings fluttering low over the plants by day. Single pale yellow, bullet-shaped eggs about 1 mm long stood on end on leaf undersides, sometimes on top. Caterpillars velvety green with a faint yellow line down the back, up to 25 mm, lying along the midrib where they are hard to see, sluggish when touched. Green pellet droppings in the leaf axils.

Damage pattern

Small caterpillars scrape windows in the leaf without breaking the top surface; larger ones chew large ragged holes from the edge and between the veins, leaving the big veins standing. On heading crops they bore into the heart and foul it with droppings. Damage builds through summer and stops with the first frosts.

Stages affected: Germination · 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: Seedling · Cut and come again

A soil-dwelling organism that makes brassica roots swell into distorted clubs so the plant cannot take up water. It is neither a fungus nor strictly a water mould but a plasmodiophorid, a separate single-celled lineage; what matters in practice is that fungicides do nothing against it. It is the most serious long-term disease of the cabbage family, and the reason is persistence: once a bed has it, the resting spores stay viable for up to twenty years. There is no cure, so the whole of management is keeping it out.

Visual signs

Roots swollen into distorted clubs spindles or knobbly galls instead of a normal taproot and fine roots · Above ground the plant wilts on warm afternoons and recovers overnight at first then stops recovering · Outer leaves yellowing and often taking on a red or purple cast · Stunted growth and a plant that is easy to pull because there are no working fine roots · Galls eventually going soft and rotting with a foul smell

Damage pattern

Affects only the cabbage family including radish turnip swede kale broccoli cauliflower and their weedy relatives · Appears in patches within a bed and those same patches recur and widen in later years · Worst in acidic poorly drained soil and in wet warm conditions · A root crop is unsellable and uneatable rather than merely reduced · Spread on soil carried by boots tools wheelbarrows and on the roots of bought-in transplants

Stages affected: 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

The larva of a small grey fly that tunnels into the roots of brassicas. On a root crop such as radish or turnip it ruins the part you eat, and on a young cabbage or broccoli it can kill the plant outright. It works entirely below ground, so the first sign above ground is a plant that wilts in the middle of the day and does not recover.

Visual signs

White legless maggot up to 8 mm long blunt at one end and tapering at the other found in the root or just under the skin · Brown winding tunnels through the root often with the tissue around them rotting and smelling sour · Adult a grey fly a little smaller than a housefly with dark stripes on the thorax hovering low over the soil · Outer leaves of a young plant turning bluish or purple and wilting in the middle of the day · Seedlings that fall over with the stem eaten away at and below soil level

Damage pattern

Damage is all below ground; nothing on the leaves explains it · Worst in cool wet springs and in soil rich in undecomposed organic matter · Concentrated in the first weeks after sowing or planting out · A radish or turnip can look perfect from above and be riddled inside, so cut one open rather than judging by the top · Recurs in the same bed year after year because the pupae overwinter in the soil beneath the crop

Stages affected: Cut and come again

A seed-borne bacterial disease of every brassica, from kale and cabbage to turnip and radish, that enters through the leaf edges and travels down the veins. It thrives in warm wet weather, spreads on splashing water and hands, and one infected seed in ten thousand is enough to start an outbreak. Infected plants cannot be cured.

Visual signs

Dull yellow wedges at the leaf margin that widen into a V with its point toward the midrib. Inside the wedge the veins turn brown or black in a net-like pattern, the clearest single sign. The centre of the V dies and browns while the edge stays yellow. Cut a stem or leaf stalk across and the ring of vascular tissue is black. Lower, older leaves go first and drop.

Damage pattern

Starts on one or a few plants and spreads along the row in the direction of water splash and foot traffic. Worst after warm rain or overhead watering at 25 to 30 °C. Leaves yellow and fall from the bottom up, plants are stunted, and heading crops rot in the heart. Symptoms can take weeks to show after infection, so plants that looked clean at planting can collapse in midsummer.

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.