Mint

Stages affected: Vegetative · Flowering · Cut and come again

The one disease that regularly spoils a mint patch, and the reason not to overwater or overfeed it. A rust fungus specific to mint and its close relatives, it overwinters on last year stubble and on wild or escaped mint nearby, then works through three visible spore stages across the season. Leaves that carry it should not be eaten, dried or composted. It rarely kills an established clump but it can strip one bare by late summer, and once it is in a planting it tends to return each year unless the old growth is cleared.

Visual signs

Small dusty pustules on the undersides of the leaves, bright orange to rust brown, that mark your fingers when rubbed · Yellow blotching or stippling on the upper surface directly above each pustule, so the leaf shows the problem on both sides at once · Early in spring, reddish swollen blisters on young shoots carrying a yellow spore stage · Later in the season, dark brown to black pustules on stems and regrowth · Leaves yellowing and dropping from the bottom of the stem upward · Systemically infected shoots come up thickened and distorted with stretched discoloured sections of stem

Damage pattern

Starts on the lower, shaded, slowest-drying leaves inside a dense clump and works upward, so a thick unpruned plant loses its lower half first. Worst in a wet season, in shade, and on plants that have been fed and watered generously into soft crowded growth. Rarely fatal, but a clump stripped by August gives nothing for the rest of the year and carries the fungus into the next one on its stubble.

Stages affected: Vegetative · Cut and come again

A soil fungus that enters through the roots and grows up inside the water-carrying tissue, so the plant wilts with the soil still wet. Its host list is enormous — mint, tomato, aubergine, potato, strawberry, raspberry, many trees and shrubs — and the fungus persists in soil for years as tiny resting bodies, which is why there is no cure and why rotation has to be long. On mint it is the reason commercial plantings are moved and the reason a home clump is worth starting from clean stock rather than a scrap from a neighbour.

Visual signs

Wilting on a plant whose soil is plainly moist, worst on hot afternoons and partly recovered by morning until it stops recovering at all · Yellowing that often takes one side of the plant or one shoot while the rest stays green, which is the clearest single sign · Leaves browning from the margins inward and hanging on the stem rather than dropping cleanly · On mint, stunted shoots with shortened joints and leaves that curl upward and turn bronze · Cut a wilting stem across near the base and look for a ring or streak of brown staining in the wood just under the skin

Damage pattern

Individual plants or patches rather than a whole bed at once, spreading slowly outward over seasons as the soil load builds. Symptoms appear in warm weather when the plant is working hardest, and a mildly affected plant can look almost normal in spring and collapse in July. The brown staining inside the stem is what separates it from everything else, because a plant short of water, root-rotted or chewed through at the base has clean tissue inside.

Stages affected: Vegetative · 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: Vegetative · Cut and come again

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: 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: Vegetative · At planting

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