Potato

Stages affected: Vegetative · Flowering · Maturation

The striped yellow beetle and its fat red larvae that can strip a potato patch to stalks in a fortnight, and will take eggplant, tomato and tomatillo when potatoes are not on offer. It overwinters in the soil of last year's bed, arrives as the shoots come up, and runs through two generations in a season. It has beaten more insecticides than any other garden pest, which is why hand-picking matters.

Visual signs

Adults 10 mm, rounded, yellow-orange with ten narrow black stripes along the wing covers and black spots on the head and thorax. Egg clusters of 10 to 30 bright orange-yellow oval eggs on the leaf undersides. Larvae humpbacked and soft, brick red when small and pink-orange when full grown at 12 to 15 mm, with a black head and two rows of black spots down each side. Ragged holes and bare stems, with black droppings on the leaves.

Damage pattern

Adults chew notches from the leaf edges as plants emerge; larvae, three to four weeks later, strip whole leaves from the top of the plant down and account for about three-quarters of the damage. A second generation of adults appears in midsummer. Potatoes tolerate about 30 % defoliation before flowering but only 10 % during tuber bulking.

Stages affected: Flowering · Maturation · Post-harvest

The disease of the Irish famine and still the one that can take a potato or tomato patch from green to black in a week of cool wet weather. A water mould whose spores blow in on the wind, it rots leaves, stems, tomato fruit and potato tubers, and it survives only in living tissue, so volunteer potatoes and leftover tubers are where it comes from each year.

Visual signs

Irregular dark green to brown water-soaked blotches on leaves, often starting at the tip or edge, not bounded by the veins, 10 to 40 mm across and enlarging daily; in humid mornings a fine white mould on the underside at the edge of the blotch. Dark brown to black greasy streaks on stems and leaf stalks. On tomato fruit, firm brown-olive greasy patches with a rough surface. On tubers, sunken purple-brown patches on the skin and a dry, rusty, granular rot beneath.

Damage pattern

Explosive in cool, wet, humid spells, roughly 10 to 24 °C with leaves wet for 10 hours or more, and halted by hot dry weather. Starts as a few blotches and takes a whole plant and then the bed within 7 to 10 days. Spores wash down from the haulm into the soil and rot the tubers in the ground or, worse, in store. Tomatoes and potatoes infect each other.

Stages affected: Vegetative · Flowering

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: Maturation

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: Flowering · Maturation

The commonest leaf disease of garden tomatoes and a regular one on potatoes, and occasionally on peppers and aubergines. It works up from the soil in warm wet weather, stripping leaves from the bottom of the plant; on tomatoes that exposes the fruit to sunscald, on potatoes it shortens the season and trims the yield. Hungry or stressed plants suffer most.

Visual signs

Irregular brown leaf spots up to about 12 mm across · Dark concentric rings inside the spot giving a target-like look, which is the deciding feature · A yellow halo around the spots and whole leaves yellowing as spots merge · Defoliation starting at the oldest lowest leaves and moving upward · On tomato fruit, sunken leathery tan-brown lesions at the stem end; on potato tubers, small dark sunken spots with a dry corky rot just beneath the skin

Damage pattern

Always starts on the oldest leaves nearest the soil · Follows warm weather with heavy rain or high humidity · Overwinters in plant debris and on nightshade and horsenettle weeds

Stages affected: Flowering · Maturation

The rough corky patches on potato skins that make a crop ugly without making it inedible. A soil-dwelling bacterium that attacks tubers through the breathing pores while they are forming, worst in dry, alkaline soil; it also marks beet, radish, carrot and turnip. Entirely cosmetic in the kitchen and almost entirely preventable in the bed.

Visual signs

Round to irregular tan-to-brown corky lesions on the tuber skin, 5 to 10 mm across, raised, flat or sunken in pits, sometimes merging into rough scabby patches over much of the surface. The flesh beneath is sound once the scab is peeled. Nothing shows on the leaves or stems; you find it at harvest.

Damage pattern

Infection happens in the two to six weeks after tubers start to form, in soil that is dry and above about pH 5.5, and worst above 6.5; a well-watered, acid bed grows clean tubers in the same garden. Lesions do not spread in store. Repeated potatoes or root crops in the same bed build it up, and fresh manure, lime and wood ash make it worse.

Stages affected: Germination · Vegetative

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 · Vegetative

The caterpillars of several drab night-flying moths. They live at the soil surface and chew through the stems of young plants at ground level, so you almost always find the damage before you find the caterpillar.

Visual signs

Smooth dull caterpillar up to 40 mm long in grey brown or near black often with faint stripes · Curls tightly into a C shape the moment it is touched or uncovered · Found 25 to 50 mm down in loose soil within a hand's width of the damaged plant · Nothing visible on the plants by day because they feed after dark

Damage pattern

A young plant cut clean through at soil level and left lying beside its own stump · Whole seedlings gone overnight with the root still in the ground · Ragged holes chewed in the leaves that rest on the soil · Damage running along a stretch of row rather than scattered across the bed