Leaf spots on arbutus are usually caused by one of two fungi. Elsinoë mattiroloanum is the more common and damaging, may be found on twigs as well as leaves, and can lead to leaf loss and twig dieback. Attacks by Ruptoseptoria unedonis are usually confined to the leaves.

You may see the following symptoms:

Elsinoë mattiroloanum

• Small purple leaf spots, often enlarging to form prominent purple ring spots (an outer purple ring with a green or yellow centre)
• Purple, slightly raised, lesions on twigs
• Leaves may turn yellow and fall prematurely
• Twigs may die back

Ruptoseptoria unedonis

• Small, brown to purple leaf spots, circular to more irregular in shape
• Larger spots develop a whitish centre

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

• Remove and destroy affected leaves, or twigs with dieback, together with fallen leaves at the base of the plant
• Encourage the production of vigorous, healthy growth by feeding, mulching to conserve moisture and watering of plants during periods of extended drought

Fungicides

The RHS recommends that you don't use fungicides. Fungicides (including organic types) may reduce biodiversity, impact soil health and have wider adverse environmental effects. If you do intend to use a fungicide, please read the information given in the link and download below to ensure that use, storage and disposal of the product is done in a responsible and legally compliant manner.
  
The products listed in the ‘Fungicides for gardeners’ document below are legally available for use by home gardeners in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally. Homemade products are not recommended as they are unregulated and usually untested.

There is no specific information available as to the efficacy of any home garden fungicide product against arbutus leaf spots.

Download

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

Link

Chemicals: storing and disposing safely

The development of each of these diseases is likely to be favoured by wet weather conditions. Ruptoseptoria unedonis produces large numbers of microscopic spores from tiny fruiting bodies within the spots. The spores are splashed around by rain droplets and will infect if the leaf surface remains wet for an extended period. Little is known about the life-cycle of Elsinoë mattiroloanum.

As arbutus is an evergreen plant the fungi may be present on the leaves year-round, but could also potentially overwinter on fallen leaf debris or, in the case of Elsinoë, in lesions on the twigs.