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.

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

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

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 amateur gardeners with specific recommendations for use against Thuja blight. However, the fungicides tebuconazole (Provanto Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus, Toprose Fungus Control & Protect), and triticonazole (Fungus Clear Ultra) are labelled for the control of other diseases on ornamental plants, and could therefore be used legally on Thuja (at the owner’s risk) to try and control Thuja blight.

The following products contain a combination of both insecticide and fungicide, enabling the control of both damaging invertebrates and disease: triticonazole containing acetamiprid (Roseclear Ultra, Roseclear Ultra Gun). When a proprietary product contains an insecticide as well as a fungicide it would be preferable to use an alternative product if invertebrate damage is not a problem on the plants treated.

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.

Inclusion of a product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.

Download

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

Links

Chemicals: using a sprayer
Chemicals: using safely and effectively
Chemicals: storing and disposing safely

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.