Advice
Algae on leaves
Broad-leaved evergreen plants like camellias, laurel and holly can sometimes get a green powdery coating on their leaves. This is not in fact a disease but a covering of algae. Algae prefers conditions that are shaded and moist and so commonly the problem is seen on plants that are in a shady, sheltered situation, or the leeward side of hedges, where there is little wind to encourage adequate air movement. It is especially prevalent in wet seasons.
Although
algae generally doesn't harm the plant (you can check this
by scraping the algae off with your nail - the leaf should
be healthy underneath) it can appear unsightly if the growth
builds up. There are no spray that can be safely applied to
evergreen plants to control algae, so instead cultural practices
should be adopted. By encouraging vigour and improving air
movement around the affected plant(s) the problem should decrease.
This can be achieved by feeding regularly with an appropriate
fertiliser and pruning out congested growth to reduce humid,
stagnant conditions.