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Sirococcus blight of cedar

Description

Sirococcus tsugae is a fungus which particularly affects cedar and hemlock trees. Cedars (Cedrus species) are non-native, evergreen conifers which have been used as ornamental trees in gardens and estates in Britain. More recently, they have been considered as an alternative forestry species, particularly on drier sites in southern and eastern Britain. Cedars are generally pest and disease-free, but in 2014 and 2015 Atlas cedars (Cedrus atlantica) were observed with severe shoot blight and defoliation. Sirococcus tsugae was identified as being consistently associated with these symptoms.

Distribution and susceptible species

Until recently S. tsugae was only recorded in western North America, where it occurs on three species of hemlock (Tsuga species) and on cedar trees.

Sirococcus shoot blight of western hemlock was originally described from British Columbia and Alaska, with specimens now known to belong to S. tsugae having been collected on hemlock trees as early as 1966. Sirococcus tsugae has been confirmed from the Pacific northwest area of North America, as well as the north-eastern and south-eastern United States, on Atlas cedar and Himalayan cedar (C. deodara),  and on western and mountain hemlocks (Tsuga heterophylla and T. mertensiana). More recently it has also been detected on Eastern hemlock (T. canadensis) in Georgia, USA. It is reported that S. tsugae appears to be less aggressive on eastern hemlock than on western hemlock.

In 2014, it was reported on Atlas cedar in Germany (EPPO Reporting Service 2015 no.4).

S. tsugae has been detected at several locations in England, Scotland and Wales since the first report on Atlas cedar in England.

It has also been confirmed as being present on a herbarium specimen of mountain hemlock (collected in Scotland in 2004), and on young western hemlock regeneration in south-west England.

How it spreads

The conidia (asexual, non-motile spores) of the fungus are locally dispersed by rain splash, and it is probable that strong winds can disperse them over longer distances. Seed transmission has been reported for the closely related Sirococcus conigenus, but we have no information about the potential for transmission of S. tsugae via seeds. Planting stock, and possibly also cut foliage and seeds of cedar and hemlock species from countries where S. tsugae occurs, are pathways for spread.

Symptoms

Cedars have needles arranged spirally on the young leading shoots, and in whorls on short, spur-like side-shoots. In the spring, affected trees display dead needles on the shoots, dead shoots (below left), cankers (below right) and gum exudation.

Fig.1 Sirococcus dead shoots Fig 2 Sirococcus cankers


The dead needles are very distinctive because they have a characteristic ‘pink’ colour (below left), and only become brown as the season progresses. The fruiting bodies of S. tsugae might be observed on the dead needles (below right).

Fig 3 Sirococcus dead needles


Affected branches can display cankers, but these are often indistinct and characterised by a slight reduction in branch diameter, together with a change of bark colour from green to a darker red / purple (below left). Resin bleeding from the bark can also accompany these symptoms in some cases (below right).

Fig 5 Sirococcus bark colour Fig 6 Sirococcus bark


The fruiting bodies of S. tsugae might be seen on the surfaces of cankers during the winter months and into the spring (below left). Branches can die if they are girdled (below right).

Fig 7 Sirococcus fruiting bodies Fig 8 Sirococcus branches can die


Brown lesions are evident in the phloem tissue of the bark (below left), and might extend from affected shoots into the subtending branches and the main stem, where they can spread longitudinally (below right).

Fig 9 Sirococcus lesions Fig 10 Sirococcus lesions


On western hemlock the disease is especially apparent in natural stands on advanced regeneration in the under-storey (below left, upper picture). It can affect one or many shoot tips on a single tree (below right, upper and lower pictures). On mountain hemlock the fungus causes shoot blight (below left, lower picture).

Fig 11 Sirococcus western hemlock Fig 12 Sirococcus shoot tips
Fig 13 Sirococcus shoot blight  Fig 14 Sirococcus tree shoot

Impact

Cedar and hemlock species are valuable ornamental and forestry species in the UK. Although much uncertainty remains concerning the geographical distribution, biology and potential impact S. tsugae in Britain, it might cause significant damage to valuable ornamental trees in public and private gardens, and economic losses, in particular for the nursery sector.

Treatment

No effective control measures against S. tsugae in forest stands have been reported from North America to date, and information about possible control methods in nurseries or in parks and gardens is scarce, and mainly promotes hygiene methods.

Status

Sirococcus tsugae is not currently listed in the European Union’s Plant Health Directive. However, its addition to the Alert List of the European & Mediterranean Plant Protection Organization (EPPO) is under consideration.

Reporting sightings

Tree Alert iconIf you think you have spotted signs of Sirococcus tsugae, please check the symptoms section above before reporting it.

 

Science

Information about the science of, and research into, Sirococcus blight is available from our Forest Research agency.

Last updated: 11th January 2018