Sirococcus blight

This recently recognised fungal disease affects cedars and hemlocks. It causes shoot tip dieback and defoliation. Young trees may be killed if branches/trunk are girdled but it is reported that infected mature trees can live for many years.

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Sirococcus tsugae on Cedrus

Quick facts

Common name Sirococcus blight
Scientific name Sirococcus tsugae
Plants affected Cedars and hemlocks
Main symptoms Pink needle discolouration, needle shedding, dieback of affected shoots, cankers and resin/gum exudation
Caused by Fungus
Timing Most visible spring and summer

What is Sirococcus blight?

Sirococcus blight is a disease caused by the fungus Sirococcus tsugae. It is mainly associated with cedars (Cedrus atlantica and C. deodora) but has also been recorded on hemlocks (Tsuga spp.). Samples from trees affected by Sirococcus blight were first received by Forest Research in late 2013 and by RHS Gardening Advice in June 2016.

S. tsugae causes shoot tip blight, characterised by a pinkish needle discolouration, first visible from spring but later turning a browner colour, which can be seen from a distance. It is important to be aware that other factors can cause foliage discolouration and dieback in aerial parts of trees.

If you suspect a case of Sirococcus blight, please report it via Tree Alert.

Symptoms

You may see the following symptoms;

  • Pink to brown needle discolouration
  • Shoot tip dieback, often affecting several shoots on the same tree
  • Partial needle shedding
  • Cankers on branches. These are not immediately obvious – look for changes in bark colour (green shades to red/purple) and a reduction in branch diameter. The cankers may be accompanied by resin exudation
  • Brown lesions in the sap conducting (phloem) tissue
  • Small fungal fruiting bodies can be found on dead needles and on the surface of cankers from summer onwards

Control

Non-chemical control

There are currently no effective control measures for Sirococcus blight. Good tool hygiene may reduce the spread of the disease on an affected tree, however, the conidia (asexual spores) of the fungus are dispersed by rain splash, with strong winds likely to disperse them over longer distances.

It is, however, reported that trees infected with Sirococcus blight can live for many years and that it is unlikely that infected trees will need to be felled unless there is a risk of structural failure. If necessary, seek professional advice from a qualified tree surgeon or arborist.

Chemical control

There are no fungicides available for home gardeners with specific recommendations for use against Sirococcus tsugae.

Biology

Sirococcus tsugae is an asexually reproducing fungus. It survives in infected dead plant material, being found in needles, stems and plant litter beneath affected trees. It produces spherical fruiting structures with apical openings (pycnidia); inside these the conidia (asexual non-motile spores) develop. Infection occurs when the conidia are released from the fruiting bodies and splash or are dispersed by wind.

Young shoots are particularly susceptible with most new infections occurring in spring and early summer when there is also a higher abundance of viable conidia.

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