During the last few years severe shoot blight and defoliation of Atlantic cedar has been reported from a range of locations in the UK.Forest Research’s Tree Health Diagnostic and Advisory Service (THDAS), identified the fungus Sirococcus tsugae as the cause of the disease.
Until recently, the fungus responsible for dieback of cedar in Britain would have been regarded as belonging to the species Sirococcus conigenus (syn Sirococcus strobilinus) However, research published in 2008 revealed that S. conigenus consisted of a group of closely related species as follows:
S. conigenus sensu stricto occurring in Europe and North America with many conifer hosts throughout the northern hemisphere. It was first described on Norway spruce in Central Europe in 1890 and has subsequently been reported on a wide range of conifers, mainly in the genera Picea, Pinus, Larix and Cedrus. There have also been reports from Morocco and North Africa. This is a well-known pathogen in nurseries but can also occur in young plantations and mature stands.
S. tsugae hitherto only recorded in western North America where it occurs on two species of Tsuga and species of Cedrus. 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 Tsuga as early as 1966.
S. piceicola, known to occur on P. abies, P. glauca and P. sitchensis and reported from three widely distributed geographic areas: eastern Canada, western Canada and Switzerland.
Distribution and susceptible species
Sirococcus tsugae has been confirmed from the Pacific Northwest as well as the Northeastern and Southeastern United States on both cedars (Cedrus Atlantica (Atlas cedar)and C. deodara) and hemlocks (Tsuga heterophylla and T. mertensiana). 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 T. canadensis than on T. heterophylla.
In 2014, the pathogen was reported from Germany where it was found on C. atlantica (EPPO Reporting Service 2015 no.4).
Since the first report on C. atlantica in England, S. tsugae has been detected at several locations in England, Scotland, Wales and Northern Ireland. It has also been detected on C. libani, C. deodara, C. brevifolia, T. heterophylla and T. mertensiana.
How it spreads
The conidia 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 S. conigenus, but there is no information about the potential for transmission of S. tsugae via seeds.
Pathways for spread: Planting stock and possible also cut foliage and seeds of Cedrus and Tsuga species from countries where S. tsugae occurs.
Cedrus has 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 (Figure 1- below left), cankers (Figure 2- below right) and gum exudation.
The dead needles are very distinctive as they have a characteristic ‘pink’ colour (Figure 3- below left) and only become brown as the season progresses. The fruiting bodies of S. tsugae may be observed on the dead needles (Figure 4- below right).
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 (Figure 5- below left). Resin bleeding from the bark can also accompany these symptoms in some cases (Figure 6 -below right).
The fruiting bodies of S. tsugae may be seen on the surfaces of cankers during the winter months and into the spring (Figure 7 - below left). Branches can die if they are girdled (Figure 8 - below right).
Brown lesions are evident in the phloem tissue of the bark (Figure 9 - below left) and may extend from affected shoots into the subtending branches and the main stem, where they can spread longitudinally (Figure 10 - below right).
On western hemlock (T. heterophylla) the disease is especially apparent in natural stands on advanced regeneration in the understorey (Figure 11 - below, upper left). It can affect one or many shoot tips on a single tree (Figure 12/14 - below, upper right and lower right). On T. mertensiana the fungus causes shoot blight (Figure 13 - below, lower left).
Cedrus and Tsuga species are valuable ornamental and forestry species in UK. Although much uncertainty remains concerning the geographical distribution of biology and potential impact S. tsugae in Britain, it might cause considerable damage to valuable ornamental trees in public and private gardens and economic losses, in particular for the nursery sector.
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 (mainly hygiene methods).
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.
If you suspect a case of Sirococcus, please report it via TreeAlert
Ana Perez-Sierra (England/Wales)
Steven Hendry (Scotland)
Forest Research Pathology advisory note 17 on Sirococcus blight
JKI website. Express PRA on Sirococcus tsugae (in German).
USDA. Forest Service (2010) Pest Alert. Siroccocus tsugae. Tip blight on Eastern hemlocks.
Rossmann AY, Castlebury LA, Farr DF, Stanosz GR (2008) Sirococcus conigenus, Sirococcus piceicola sp. nov.and Sirococcus tsugae sp. nov.on conifers: anamorphic fungi in the Gnomoniaceae, Diaporthales. ForestPathology 38 (1), 47-60.
Smith DR, Stanosz GR (2008) PCR primers for identification of Sirococcus conigenus and S. tsugae, and detection of S. conigenus from symptomatic and asymptomatic red pine shoots. Forest Pathology 38 (3), 156-168.
Stanosz GR (2012) Sirococcus Shoot Blight. In: USDA Forest Nursery Pests Agricultural Handbook No. 680, 68-70.
Stanosz GR, Smith DR, Sullivan JP, Mech AM, Gandhi KJK, Dalusky MJ, Mayfield AE, Fraedrich SW (2013) Expansion in the known geographic distribution and host range of the shoot blight pathogen Sirococcus tsugae. Poster presented at the 24th USDA Interagency Research Forum on Invasive Species (Annapolis, US, 2013-01-08/11), p 95.
Stanosz GR, Smith DR, Sullivan JP, Mech AM, Gandhi KJK, Dalusky MJ, Mayfield AE, Fraedrich SW (2011) Shoot blight caused by Sirococcus tsugae on Eastern hemlock (Tsuga canadensis) in Georgia. Plant Disease 95 (5), 612-612.