Diseases of box
Box blight (Cylindrocladium buxicola)
Watch for leaf spots, stem dieback and bare patches which are tell-tale signs of box blight.
See RHS research into box blight.
Volutella blight is a disease of box caused by the fungus Pseudonectria buxi. Volutella blight has similar symptoms (twig and leaf death) to the more serious box blight, although the characteristic black streaks of box blight do not form and defoliation is not common. Spore masses, developing on undersides of leaves in wet conditions are pink for Volutella blight and white for box blight.
P. buxi requires wounds for infection and is associated with environmental stress or clipping in wet weather. It is not a serious disease and improving cultural conditions will usually lead to plant recovery. Diseased branches should be pruned out when the foliage is dry, and old fallen leaves removed from the interior of affected plants.
Box rust is caused by a fungus called Puccinia buxi. This is not particularly troublesome and seldom causes serious problems.
It can be seen as thickened rusty blister-like pustules on both sides of the leaves. The fungus forms only one type of spore which develops in autumn and winter and breaks through the epidermis of the leaves in spring. New leaves are infected in the spring and early summer. The fungus continues to grow during the summer and autumn and as a result the infected spots become thicker. Box rust does not have an alternate host.
To control it either clip off the affected shoots or spray with one of the fungicides labelled for rust diseases.The fungicides tebuconazole (Provanto Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus, Toprose Fungus Control & Protect), and triticonazole (Fungus Clear Ultra) are approved for the control of rust diseases on ornamental plants.
The following products contain a combination of both insecticide and fungicide, enabling the control of both insect pests and disease: myclobutanil containing cypermethrin (Resolva Rose 3 in 1, Rose Shield Bug & Fungus Killer, Roseclear Ultra Gun 2, Vitax Rosegarde) and 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 pests are not a problem on the plants treated.
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.
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.
Macrophoma leaf spot
Macrophoma leaf spot is caused by the fungus Macrophoma candollei. It is a weakly pathogenic fungus, resulting in numerous tiny black raised fruiting bodies found on dying or dead straw-coloured leaves. Like Volutella blight, it is associated with plants under stress and is easily managed by improving cultural conditions.
Both new and well established plants may fail completely for no obvious reason. This may be due to a soil-borne disease called Phytophthora root rot.