Ash dieback
Ash dieback is caused by the fungus Hymenoscyphus fraxineus, which probably came from eastern Asia and is highly damaging to common ash. Forest Research says the disease is present in most of the UK. Look for blackened, wilting leaves and shoots from July to September and dark, diamond-shaped lesions on twigs and branches. Lesions near the base of the trunk can weaken the tree and make it more likely to fall. There is no known cure, although some trees show genetic tolerance, so the advice is generally not to fell living ash unless it's needed for safety or timber. Public safety comes first: trees next to roads, paths and buildings need monitoring and may need pruning or felling. Forestry Commission Operations Note 46a covers managing individual ash trees, including whether a felling licence is needed. See also tree felling.
Honey fungus
Honey fungus kills roots. According to the RHS, signs include dieback, small pale leaves, early autumn colour, clumps of honey-coloured mushrooms in autumn, white fungus sheets under bark at ground level that smell of mushrooms, and black bootlace-like cords in the soil. There are no chemicals for control. Infected roots and stumps should be dug out and destroyed, and a buried barrier, deep cultivation and resistant plants can limit spread. Stump grinding deals with the stump itself, but infected roots can stay in the ground, so ask the firm what they will do about the roots.
Bracket fungi and decay
Bracket-shaped or mushroom-like growths on a trunk, branch or root flare are fruiting bodies of fungi that rot wood. Their presence doesn't always mean the tree is dangerous, but some decay fungi weaken structure over time. Don't climb or work on a tree you suspect is decayed, and don't judge it by the fungus alone: a tree surgeon or consultant arborist can identify it, assess how much sound wood is left and recommend monitoring, pruning or removal. A tree survey is the usual route.
Phytophthora
Phytophthora is a group of water-mould organisms that rot roots and bark. The RHS lists poor, yellowing or wilting foliage, dieback and, in severe cases, discoloured bark at the stem base. Bleeding cankers, where bark weeps dark sticky fluid, are a sign of other Phytophthora species such as P. ramorum and P. kernoviae on some trees. These are notifiable, so report them via TreeAlert. Improving drainage reduces the risk, and there are no chemical treatments for gardeners. The symptoms overlap with waterlogging, drought and honey fungus, which is why a professional diagnosis is useful.
Oak processionary moth
Oak processionary moth caterpillars have hairs that can cause itchy rashes and eye and breathing problems in people and animals. According to GOV.UK, the moth is established in south-east England, with management zones around it. The GOV.UK page does not name Worcestershire as part of the established area, so check the current zone map if you're concerned. Don't touch nests or caterpillars. Report suspected cases through TreeAlert.
What to ask a firm
If you think a tree is diseased, ask the firm what they think is wrong and how they know. Visible symptoms may be enough for some problems, but others need a closer inspection or lab confirmation. Ask whether the tree is a safety risk now, what monitoring they would suggest, and whether removal is needed or a prune or wait-and-see approach is reasonable. Many tree work companies are not plant pathologists, so for unusual symptoms you can also report to TreeAlert yourself. Infected material may need careful disposal so it doesn't spread, so ask how the arisings will be handled.
How to report
TreeAlert, run by Forest Research, takes reports of suspected tree pests and diseases anywhere in Great Britain, including ash dieback, oak processionary moth, acute oak decline and Phytophthora lateralis, plus general reports where you don't know the cause. Add a clear photo.
Treatment or removal?
Pruning out dead wood, improving soil conditions and monitoring will manage many problems. Removal is for trees that are unsafe. A protected tree still needs consent, though dead or dangerous trees are treated differently: see the TPO guide.