Hot summer and damp autumn cause UK boom in destructive honey fungus | Fungi

The presence of a golden fungus which grows in clusters and can attack and kill trees has increased by 200% in the UK in one year due to the hot summer and wet autumn.

Recorded sightings of honey mushrooms have increased by almost 200% compared to the same time last year, according to iNaturalist.

Armillaria, or honey fungus, is not a single species but a group of closely related species. “As their name suggests, they are honey brown in color, often with greenish highlights when young,” explained field mycologist David Gibbs. “Large clumps often develop a frosted appearance and are dusted with their white spores. »

The clusters that appear in gardens and forests 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 “and did the hard work of finding nutrients, fending off competitors, growing and staying for the long term”.

Honey fungi can also form black, root-like ropes, called rhizomorphs. “They form physically strong conduits for transport and mass movement within a mycelial network – they are like superhighways,” Henk said. This makes them very effective at invading tree roots and bark.

Honey fungus is notorious among gardeners and is the most reported plant disease in the UK, the Royal Horticultural Society (RHS) has said.

“We started receiving emails with photographs of honey fungi to be identified much earlier this year than we usually do,” said RHS plant pathologist Dr Jassy Drakulic. She said the damage was caused by Armillaria melleawhich “kills water-conducting tissues in the roots, preventing plants from absorbing water and dying above ground.”

Drakulic said that while healthy plants could be affected, they had ways of blocking infection, so it was “much more likely to cause problems for already stressed plants.” The hot and dry summer of 2025 could have been the cause of this stress.

The UK’s hottest summer on record may have left trees vulnerable to colonization by honey fungus mycelium. “We generally see a higher number of cases of root rot caused by honey fungus after drought years,” Drakulic said.

A warm and humid autumn follows, ideal for mushrooms to fruit. The role of fungi is to release spores and spread to new areas.

“Initial indications from ongoing RHS research suggest that fungal spores from Armillaria mellea are more important in the spread of the fungus than previously thought,” Drakulic said, in addition to underground spread by rhizomorphs or root-to-root contact.

Should people be worried? In gardens, honey fungus can devastate trees and shrubs, but a bumper honey fungus year can reflect broader ecological changes.

“Over the past two decades, climate has changed the fruiting patterns of mushrooms,” Henk said, adding that mushrooms “are also a key part of invertebrate habitats and food for larger animals.”

“The concern is about the drought itself,” Drakulic said, “with the volume of fungus being a sign of the stress trees are under due to climate change and poor management practices that fail to recycle dead wood back into the soil.”

“Research on the biology, ecology and sustainable management of Armillary Species are needed to figure out how we can reduce their danger potential, now and in future climates,” Drakulic said.