cypress canker

Symptoms

Dead branches scattered across an otherwise green canopy (flagging), resin oozing from cankers on stem and branches, bark sunken and discolored.

Plants affected

• arborvitae (thuja)
• Mediterranean cypress

Easily confused with

• thuja tip blight (Kabatina)

  How to tell them apart: Kabatina thujae kills only the tips of the previous year's shoots: in spring you see scattered brown-to-ash-grey dead tips a few centimetres long, sharply cut off from healthy green foliage, often with a thin grey band and tiny black fruiting bodies (acervuli) right at the boundary where the dead tip meets the living shoot — there is no resin and no sunken bark. Seiridium cardinale instead causes whole twigs and branches to flag rust-red and die back from larger girdling cankers on the woody stems: look for an elongated, slightly sunken canker with cracked bark oozing sticky clear-to-amber resin, with dark pustules on the canker face, killing everything beyond it. In short: small dead shoot-tips with a black-dotted margin and no resin point to Kabatina, while resin-bleeding sunken stem cankers with whole rust-red flagging branches point to Seiridium.

• cypress aphid

  How to tell them apart: Seiridium cardinale (cypress canker, a fungus) and Cinara cupressi (cypress aphid, an insect) both brown out Thuja foliage but the cause differs. With Seiridium, look on woody twigs and branches for sunken, resin-bleeding cankers with a sharp margin between living and dead bark; girdled branches show foliage that fades from yellow to fox-red beyond the lesion, and tiny black fruiting bodies (pycnidia) dot the dead bark. With Cinara, there is no canker or resin flow: you find clusters of large grey-brown soft-bodied aphids in the inner crown along twigs, sticky honeydew, sooty black mould on foliage, and diffuse browning patches that start lower and inside the canopy where the colonies fed. Always check the bark for a defined resinous lesion (canker) versus live aphids and honeydew (aphid).

• honey fungus (Armillaria root rot)

  How to tell them apart: On Thuja occidentalis both Seiridium cardinale (cypress canker) and Armillaria mellea (honey fungus) cause whole-plant browning and scattered dead branches, but the tell-tale signs are in different places. Seiridium shows discrete sunken cankers on twigs, branches and the upper stem, often cracked with reddish-brown bark and a characteristic bleeding of clear-to-amber resin around the lesion; foliage beyond a girdled branch turns rusty-brown while the rest of the plant may stay green, giving a patchy 'flagging' look. Armillaria attacks from below: peel back the bark at the root collar and base of the trunk and you find a creamy-white fan of mycelium smelling of mushrooms, often with black bootlace rhizomorphs in the surrounding soil and honey-coloured toadstools clustered at the base in autumn; decline here is more uniform and the whole plant fades together rather than branch by branch.

Common questions

How do I recognise cypress canker on arborvitae (thuja)?

Dead branches scattered across an otherwise green canopy (flagging), resin oozing from cankers on stem and branches, bark sunken and discolored.

Do I have to report cypress canker?

No. cypress canker is not a notifiable organism, so there is nothing to report to an authority. Treat it as a normal garden problem.

How is cypress canker treated?

Mainly with biological and cultural measures — removing affected material, changing the conditions that favour it, and treating with agents approved in your country. The exact steps and their timing depend on the plant, the season and where you live: Dr. Phyto works them out in a free diagnosis.