What are the sunken patches and cankers on my stems?
A lesion that encircles a stem kills everything above it, so stem problems escalate faster than leaf problems and deserve an earlier decision. On woody plants the same lesion also overwinters, which makes it next spring's source of infection.
The one observation that decides it
Decide whether the stem is woody or green, and whether the lesion sits at the soil line. At the soil line on a seedling, pinched and water-soaked, is damping-off; on an established plant with a white fan of threads and mustard-seed bodies it is southern blight. On woody stems, elliptical sunken dead bark flaking in rings is canker; amber ooze with a shepherd's-crook shoot tip is fire blight. A single raspberry cane dying while its neighbours are fine is cane blight.
Start from the plant in front of you
12 things it could be
Ordered by how much they can cost you — if the first entry fits what you can see, rule it out before considering the milder ones.
Blossoms and shoots suddenly wilt, blacken and bend into a shepherd's crook as if scorched by fire; sunken cankers ooze amber bacterial droplets in warm weather.
Conditions: 21–30°C · humidity ≥70% · wet leaves / rainFirst move: Remove affected material nowSeen on: Apple trees, Pear trees, Quince
Dark, water-soaked greasy patches on leaves that rapidly turn brown; white fuzzy sporulation on leaf undersides in humid mornings; firm brown lesions on stems and fruit. Whole plants can collapse within days.
Conditions: 10–25°C · humidity ≥90% · wet leaves / rainFirst move: Remove affected material nowDark, sunken circular lesions on fruit often oozing pink spore masses when wet; irregular dead blotches on leaves along veins; cankers on stems and twigs.
Conditions: 20–30°C · humidity ≥85% · wet leaves / rainFirst move: Treat, then re-check in 5–7 daysSeen on: Cucumbers, Beans, Raspberries, Blackberries, Walnut trees
Elliptical sunken patches of dead bark, often centred on a bud or wound, with the bark flaking away in concentric rings. Shoots above a girdling canker die back. White pustules appear on cankers in summer and red-orange ones in winter.
Conditions: 5–20°C · humidity ≥85% · wet leaves / rainFirst move: Remove affected material nowSeen on: Apple trees
On tomato, one-sided leaf wilting and scorching, light streaks on stems, and 'bird's-eye' spots with white halos on fruit; on trees, sunken oozing cankers and dieback.
Conditions: 16–28°C · humidity ≥80% · wet leaves / rainFirst move: Remove affected material nowSeen on: Cherries
Dark brown or black streaks run along young stems, leaves turn straw-brown and drop suddenly, leaving bare patches that spread outwards. In humid weather a white bloom of spores is visible on the underside of infected leaves.
Conditions: 18–25°C · humidity ≥90% · wet leaves / rainFirst move: Treat, then re-check in 5–7 daysSeeds rot before sprouting, or seedlings topple over with a thin, water-soaked, pinched stem at the soil line; roots are brown and mushy.
Conditions: 5–20°C · humidity ≥90% · wet leaves / rainFirst move: Treat, then re-check in 5–7 daysLeaves on one cane wilt and die while neighbouring canes look fine. The base of the cane turns dark brown and brittle, and the bark cracks and peels away to show grey fungal growth beneath. Affected canes snap off cleanly at the base.
Conditions: 12–26°C · humidity ≥85% · wet leaves / rainFirst move: Treat, then re-check in 5–7 daysSeen on: Raspberries
Sudden wilting and yellowing; a white fan of fungal threads at the soil line studded with tan, mustard-seed-sized resting bodies; rapid plant collapse.
Conditions: 25–35°C · humidity ≥80% · wet leaves / rainFirst move: Treat, then re-check in 5–7 daysWater-soaked stem and pod rot covered in dense cottony white mould, later embedded with hard black pellets (sclerotia); sudden wilting of branches above the rot.
Conditions: 10–24°C · humidity ≥90% · wet leaves / rainFirst move: Remove affected material nowSeen on: Lettuce, Beans, Sunflowers
Silvery-white swollen galls on cobs, tassels, stems or leaves, which enlarge, darken and finally rupture to release a mass of powdery black spores. Infected plants may also be stunted or distorted.
Conditions: 26–34°C · dry air ≤60%First move: Treat, then re-check in 5–7 daysSeen on: Sweetcorn
Rounded, rough, woody galls at the soil line, on roots, or on stems, starting soft and cream-coloured then hardening and darkening; reduced vigour and stunting.
Conditions: 20–30°C · humidity ≥60%First move: Treat, then re-check in 5–7 daysSeen on: Rhubarb