Beet

Stages affected: Seedling · Vegetative

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: Vegetative · Harvest window

The commonest leaf disease of beet, chard and spinach: small round spots with purple edges that multiply across the older leaves in warm humid weather until the leaves yellow and die. The root is untouched unless the plant loses most of its leaves, but it ruins the greens and slows the root.

Visual signs

Round spots 2 to 5 mm across, tan to pale grey in the centre with a distinct reddish-purple border, scattered over the older leaves first. In humid weather the centres carry a faint grey fuzz and often fall out, leaving shot holes. Badly spotted leaves yellow, collapse and lie flat; new leaves keep coming from the centre.

Damage pattern

Starts on the lowest, oldest leaves and moves up and along the row in warm humid spells, 24 to 32 °C by day with dew or rain at night. Builds through mid and late summer; spring and autumn crops in cool weather often escape. Repeated defoliation stunts the root; a few spots do nothing.

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: 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: 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 · Harvest window

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: Vegetative

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 · Harvest window

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.