A golden mushroom that grows in clusters and can attack and kill trees has increased by 200% in the UK in a year because of the hot summer and damp autumn.
Recorded sightings of honey fungus are up by almost 200% compared with the same period last year, according to iNaturalist.
Armillaria, or honey fungus, is not a single species but a group of closely related ones. “As their name suggests, they are a honey-brown colour, often with greenish tinges when young,” said David Gibbs, a field mycologist. “Large clumps often develop a frosty appearance and become dusted by their white spores.”
The clusters that appear in gardens and woodlands are temporary fruiting bodies of the main part of the fungus, the mycelium.
Dr Daniel Henk, a mycologist at the University of Bath, said the mycelium was underground or in the wood “doing the hard work of finding nutrients, fending off competitors, growing, and staying around long term”.
Honey fungus can also form root-like black ropes called rhizomorphs. “They form these physically tough conduits for transport and mass movement within a mycelial network- they’re like super-highways,” Henk said. This makes them highly effective at invading tree roots and bark.
Honey fungus is notorious among gardeners and is the UK’s most reported plant disease, the Royal Horticultural Society (RHS) said.
“We started getting emails with photographs of honey fungus mushrooms for us to identify much earlier this year than we usually do,” said a RHS plant pathologist, Dr Jassy Drakulic. She said the damage was caused by Armillaria mellea, which “kills off the water-conducting tissue in roots, preventing plants from taking up water so they die back above ground”.
Drakulic said that although healthy plants could be affected, they had ways of walling off infection, so it was “far more likely to cause problems to plants that are already stressed”. The hot, dry summer of 2025 could have provided that stress.
The UK’s hottest summer on record may have caused trees to be vulnerable to colonisation of honey fungus mycelium. “We usually see higher numbers of honey fungus root rot cases following drought years,” said Drakulic.
This was followed by a warm, damp autumn, ideal for mushroom fruiting. The mushrooms’ role is to release spores and spread to new areas.
“Early indications from ongoing RHS research suggest that spores from the mushrooms of Armillaria mellea are more important in spreading the fungus than previously thought,” said Drakulic, in addition to the underground spread by rhizomorphs or root-to-root contact.
Should people be concerned? In gardens, honey fungus can devastate trees and shrubs, but a bumper year of honey fungus may reflect broader ecological changes.
“Over the last two decades, climate is altering the fruiting patterns of fungi,” said Henk, adding that mushrooms “are a key part of habitats for invertebrates and food for larger animals, too”.
The “concern is about the drought itself”, Drakulic said, “with the volume of mushrooms being a sign of the stress trees are under from climate change and from poor management practices that fail to recycle dead wood back into the soil”.
“Research on the biology, ecology and sustainable management of Armillaria species is needed to discover how we can curb their potential for harm now and in future climates,” Drakulic said.
