Host of the month’ is a series of Blogs and PDF’s that highlight a tree host and their associated priority pests and diseases that are best seen and recorded in that month. For May we’re looking at true cedars (Cedrus species) and Sirococcus blight.

Four species are currently recognised: Cedar of Lebanon (C. libani), Atlas cedar (C. atlantica), Cyprus cedar (C. brevifolia) and Deodar (C. deodara). The first three are Mediterranean and very closely related, Deodar however hails from the Himalayas. Lebanon, Atlas and Deodar cedars are reasonably common across the UK, the former two in particular were extremely popular specimen trees in the grounds of stately homes and parks where older individuals display their distinctive layered appearance. Many other trees have ‘cedar’ in their common names but are not related. Species such as Western red-cedar (Thuja plicata), Japanese cedar (Cryptomeria japonica) or Incense cedar (Calocedrus decurrens) are not affected by Sirococcus tsugae.

True cedars are relatively straightforward to tell from other conifers, all are evergreen with stiff needles that have a quadrangular cross-section arranged in two ways; spiralling and single on long terminal shoots, in whorls of 20+ on short lateral shoots. Cones are always upright and disintegrate in situ when mature to release the winged seeds. Larches (Larix species) have the same needle arrangement but they are deciduous, needles are soft and flat in cross-section, and their cones are smaller and don’t shatter at maturity but remain intact and are retained on the tree for a number of years.

Priority diseaseSirococcus blight
Sirococcus blight is caused by the fungal pathogen Sirococcus tsugae and affects true Cedars and Hemlocks (Tsuga species). It was first confirmed in the UK in 2014 and has been reported affecting all three cedar species commonly found in England, Scotland, Wales and Northern Ireland.

The most obvious sign of infection with S. tsugae on Atlas cedar is the striking pink-ish discoloration of infected needles most visible in early to mid-summer, on other cedars the infected needles turn brown. Cankers are often found on infected shoots but can be difficult to spot, resin patches on shoots or black pin-head shaped fruit bodies can indicate their presence beneath the bark.
Infected needles are shed early from the tree and defoliation can be severe and often results in a thinning, patchy crown. Although tree death is common it is not inevitable, but many trees are removed for aesthetic reasons.

For more information check the Observatree resource pages for Sirococcus tsugae, or the Host of the Month. You can also test your knowledge with the Host of the month Quiz.

May is an ideal time to seek out cedar trees and see if you can identify any of the signs and symptoms of Sirococcus tsugae infection. Sirococcus tsugae is an Observatree priority disease so please report possible sightings via TreeAlert. Healthy tree data is equally important so please do report healthy cedars too.