Oregano

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: Vegetative · Flowering

Mites under 1.3 mm that feed on the undersides of leaves and spin fine webbing. Populations explode in hot dusty dry conditions and after broad-spectrum sprays kill their predators.

Visual signs

Fine pale stippling on leaves seen best held up to the light · Fine silk webbing on leaf undersides and later on twigs and nuts · Leaves turning yellowish or bronzed then dropping in mid to late summer · Moving specks on the leaf underside visible with a hand lens

Damage pattern

Starts on the dusty side of the tree near paths and drives · Worst on water-stressed trees in hot weather · Late-summer leaf loss costs little this season but reduces next year crop

Stages affected: Vegetative · Flowering

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: Dormant · At planting

Soil-borne water moulds that rot the roots and crown of plants growing in soil that stays saturated: fruit and nut trees, strawberries (where one species causes red stele), raspberries, peppers and many ornamentals. They need free water to swim to the roots, so this is a drainage and watering disease above all. Once the crown is rotted there is no cure; everything is prevention.

Visual signs

Plants that stay small, wilt in the first warm days although the soil is wet, and yellow or redden early, often on one side first · On strawberries, roots with few side roots and a red core when split lengthways (red stele), or a brown, rotted crown when the plant is cut through · On trees, leaves undersized and dropping early, twigs dying back, and a brown or reddish stain or canker in the wood when the bark is cut away at the base of the trunk · Feeder roots dark brown and brittle on any host · Often follows a wet winter, a blocked drain, or a dripper placed against the stem or trunk

Damage pattern

Concentrated in low spots, heavy soil and anywhere water stands after rain; spreads downhill and along the row with drainage water · Worse where soil or mulch is piled against the stem or trunk · On strawberries shows in the first warm spell of spring on plants that looked fine in winter; on trees builds over a season or two · Susceptibility varies widely by variety and rootstock, which is the main lever where the disease is known

Stages affected: 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