Pests and diseases affecting Hertfordshire
The Forestry Commission explains how to recognise them, how to report them and what other action to take:
A fungal disease that kills ash trees through leaf loss, crown dieback and bark lesions.
Its caterpillars strip the leaves from oak trees and their hairs cause rashes and allergic reactions for people and animals.
Affected trees have dark fluid oozing from cracks in the bark and will die within 4-5 years.
Causes bark cracks and cankers, as well as bleeding from from the trunk and branches.
This insect's larvae damages leaves, so they discolour and fall before autumn.
The wasp's larvae cause growths called 'galls' on a tree's leaves, as well as their buds and stalks.