Why Does My Rose Stem Have Sunken Discoloured Canker?

A sunken, flattened or discoloured lesion on a rose stem — often dark brown, purple-edged or orange-stained, sometimes with the bark cracking, splitting or developing tiny black or orange fruiting bodies within the lesion — is a rose canker. Several fungal pathogens cause rose canker, including Leptosphaeria coniothyrium, Botrytis cinerea acting on pruning wounds, and Cryptosporella umbrina. All enter through wounds — pruning cuts, frost cracks, mechanical damage or insect feeding sites — and then colonise the stem tissue, killing a widening area of bark and the tissue beneath it.

Types of rose canker and their appearance

Brand canker (Leptosphaeria coniothyrium) produces a brown lesion that starts at a pruning cut or bud and spreads downward, often ringed with a purple or dark margin and eventually showing tiny black pycnidia (fruiting bodies) in the dead tissue. Botrytis canker is common on pruning stubs left too long and is associated with the grey mould that is visible in damp weather. Perennial canker produces a series of concentric sunken rings and progressively girdles the stem over multiple seasons. All eventually kill the portion of stem above the lesion.

Assessing the severity

A canker confined to one cane that has not reached the main stem or the union at the base of the plant can usually be removed by pruning without lasting harm to the rose. If the canker has encircled (girdled) the stem completely, everything above that point is likely to die and must be removed. If multiple canes are affected or canker has reached the crown of the plant, the rose is severely weakened. Examine the interior of cut stems — if the pith is brown rather than white and green, the infection extends further than the surface suggests.

Pruning out canker

Cut back the affected cane to at least 10 cm below the furthest point of visible discolouration, cutting to a healthy outward-facing bud and ensuring the pith in the cut surface is clean and white. Sterilise your secateurs between each cut using diluted bleach (10%) or rubbing alcohol — this prevents transferring spores from cut to cut. Dispose of all pruned material in the bin or burn it; never compost canker-affected wood. After cutting, a wound sealant is optional on large cuts but helps reduce the risk of reinfection through the fresh wound surface.

Preventing canker

The most effective prevention is sound pruning practice: cut to a healthy outward-facing bud, slope the cut away from the bud at 45 degrees so water drains off, and avoid leaving stubs that die back and provide canker entry points. Prune in dry weather where possible, as spores spread in rain. Feed roses adequately — potassium strengthens cell walls and improves disease resistance; excess nitrogen creates soft, vulnerable growth. Avoid mechanical damage to stems during gardening tasks, and tie climbing roses carefully so the ties do not bite into the bark.

Fungicide protection

There are no fungicides licensed specifically for rose canker in home gardens, but copper-based fungicides sprayed in early spring as growth begins — and again after pruning — can reduce the spore load on stems. This is a preventive measure, not a cure for established canker. Good autumn hygiene — removing fallen leaves, clearing debris and making a final clean cut on any die-back — removes the overwintering spore reservoir and reduces pressure the following season.