Thuja blight

What is Thuja blight?

Thuja blight is a fungal disease caused by Didymascella thujina (syn. Keithia thujina) that attacks the leaves and shoots of Thuja species, particularly Western red cedar (Thuja plicata). Young trees can be badly damaged, and the disease can also be unsightly on larger specimens. Lower branches tend to be worst affected, particularly in dense plantings with poor air circulation.

Symptoms

You may see the following symptoms:

• Individual scale leaves turn yellowish and then brown in late spring and early summer
• Large, oval, brown-black fungal fruiting bodies, visible to the naked eye, develop in the leaf. There is usually one fruiting body per scale leaf, but sometimes two or three
• The fruiting bodies fall out once they have released their spores, leaving a cavity in the leaf (which could be mistaken for pest damage)
• Dead leaves can persist on the tree throughout the winter
• Heavy infections can lead to widespread browning of leaves, and sometimes twig dieback. Seedlings and young plants (up to four or five years old) can be killed

Control

Non-chemical control

• Avoid planting Thuja plicata (particularly seedlings and young plants) in areas where air circulation is poor
• Remove and dispose of any twigs that are shed as a result of infection by Thuja blight
• Cut out heavily-infected shoots, if this can be done without spoiling the overall shape of the tree

Chemical control

There are no fungicides available to home gardeners with specific recommendations for use against Thuja blight. However, the fungicides difenoconazole (Westland Plant Rescue Fungus Control), myclobutanil (Bayer Garden Systhane Fungus Fighter and other formulations), tebuconazole (Bayer Garden Multirose Concentrate 2) and triticonazole (Scotts Fungus Clear Ultra, Roseclear Ultra) are labelled for the control of some other diseases on ornamental plants, and could therefore be used legally on Thuja (at the owner’s risk) to try and control Thuja blight.

Some formulations also contain insecticides but these are best avoided if no insect pest problem is specifically identified. For example, some formulations of myclobutanil (Westland Rose Rescue) contains cypermethrin, the formulation of tebuconazole (Bayer Garden Multirose Concentrate 2) contains deltamethrin and some formulations of triticonazole (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun) contain acetamiprid to control insect pests.

There is no specific information available as to the efficacy of these products against Thuja blight. It would be impractical to treat large, mature specimens, but treatment could be attempted on young trees or trees grown as hedging. Research carried out on commercial plantations of Thuja (using a product unavailable to gardeners) has shown that two treatments, in late March and late April, can be effective, with possibly a third spray in late June in wet seasons.

Download

Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

Biology

Infection is favoured by leaf wetness so on larger trees the lower branches, where air circulation is poorer, tend to be the worst affected. On smaller, younger trees the entire plant can be affected, particularly in dense plantings.

The black fruiting bodies develop just below the leaf surface, and when they are mature the epidermis of the leaf splits open and spores are released. The spores are produced from May to October. They are resistant to desiccation, and spores shed in the autumn can overwinter on the leaf surface to germinate the following spring.