Cedar Leaf Blight

Didymascella thujina (E. J. Durand) Marie

Host(s) in Alaska:

Western redcedar (Thuja plicata)

Habitat(s): foliage

Current Status & Distribution in Alaska (2023 Update)

Cedar leaf blight is a foliage disease of western redcedar that occurs throughout the range of its host in Southeast Alaska (see Detection Map). Affected foliage appears dull-reddish-brown to grey in mid-summer. There was elevated incidence and severity of this disease in 2023, especially on Prince of Wales Island. Thirty observations of this disease were recorded during ground detection surveys. In Southeast Alaska, impacts from this disease are usually insignificant; however, this disease is considered one of the most important diseases of western redcedar in British Columbia, where it can reduce growth of mature trees in coastal environments and kill seedlings. Warm, wet conditions are conducive to disease development, so this will be an important disease to track under climate change. Disease-climate models produced for British Columbia (Gray et al. 2013) could be extended to Alaska to predict potential change in disease occurrence.

Symptoms, Biology & Impacts

Infected one-year-old foliage becomes bleached or reddish-brown. Fruiting structures develop on the upper surface of cedar foliage in June and gradually darken. Following sporulation, fruiting structures often fall from foliage, leaving circular pits or holes. Twigs with heavily infected foliage may be shed in fall. 

This disease is very common on western redcedar in the forest and in ornamental settings, but seldom causes mortality of western recedar trees. Reduced growth may result when foliage is killed. Seedlings and saplings are the most vulnerable to heavy damage and can be killed. Disease tends to be most severe in lower tree crowns, and at sites with high humidity and high tree density. Fungicides are sometimes applied in nurseries or to ornamental trees to control disease.

Survey Method

Cedar leaf blight damage is observed through ground surveys. Systematic surveys have not been conducted to evaluate disease extent or severity in Alaska.

Detection Map 

Cedar leaf blight occurs throughout the range of western redcedar in Alaska. The northern extent of western redcedar is near Kake on Kupreanof Island in the central Panhandle. The disease is found elsewhere in the Pacific Northwest where the host occurs, especially in low-elevation, coastal environments.

Links to Resources & Publications

Gray, L.K., Russell, J.H., Yanchuk, A.D. and B.J. Hawkins. 2013. Predicting the risk of cedar leaf blight (Diymascella thujina) in British Columbia under future climate change. Agricultural and Forest Meteorology 180: 152-163. Available here.

Content prepared by Robin Mulvey, Forest Health Protection, robin.mulvey@usda.gov.

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