Honey fungus

Autumn is the season for mushrooms, the fruiting bodies of fungi that appear suddenly in our gardens at this time of year, along with morning mists and the smell of woodsmoke. An integral part of our environment fungi play an essential role within the ecosystem, converting dead material into nutrients required for plant growth. However, in the quasi-naturalistic setting of the garden, not all fungi are created equal. There are relatively harmless saphrophytic fungi, which live on dead or decaying organic matter, and aid the process of decomposition. These perform a vital function and one which, from a gardener’s perspective, is relatively benign. There are also beneficial micorrhizal fungi which form a codependnent relationship with the roots of plants, assisting in the uptake of nutrients from the soil in exchange for sugars and carbohydrates. But there are also pathogenic fungi, which are rather more of a nuisance, possessing as they do a penchant for living material.

Two weeks ago, several patches of cinnamon hued mushrooms, each with a darker central spot on the cap, appeared in one of my regular gardens. This was not an auspicious start to the day, as these mushrooms bore a marked resemblance to one of the three signs of the armillaria group of fungi, also known as honey fungus. Armillaria is a virulent pathogenic genus – recognised by the RHS as ‘the most destructive fungal disease in UK gardens’ – which invades the roots of trees and woody perennials, weakening the plant and then consuming the decaying organic matter. The cap of the mushroom is convex at first, like a shallow dome or half a tea cake, but as it ages the outer edges curve upwards, revealing the gills beneath. While the mushrooms do not necessarily appear each year the presence of honey fungus is also suggested by a sheet of white fungal growth beneath the bark at the base of the infected plant, and by the characteristic black rhizomorphs, or ‘bootlaces’, by means of which the organism can spread long distances through the soil. The mushrooms in this garden were concentrated around the decaying remains of some old shrubs, on which both the white mycelial sheet (which smells very noticeably of mushrooms) and the beginnings of the bootlaces were evident. Finding the fruiting bodies, with their characteristic colouring, was a fairly good indicator of what was now lurking in the lawns and borders. Finding all three signs together removed any remaining vestiges of doubt. Honey fungus, I was now confident, had arrived.

To put things in perspective, it is reputedly the case that the largest living organism is a kind of honey fungus, Armillaria ostoyae, which covers an area larger than 2,000 acres in a forest in Oregon. No wonder that I wasn’t overjoyed to see its relative manifesting in these Kentish grounds.

A pair of mature birch trees dominate this garden (there had originally been three, but one had to be felled last year when I noticed a rotten hole had developed at the base of one – mentioned in a blog post here), and one of the newly planted borders near a particularly fine crop of mushrooms features a Magnolia 'George Henry Kern', Viburnum tinus, and Hydrangea 'Annabelle'. I couldn’t have created a more sumptuous menu for the honey fungus had I tried – all of these appear on the list of plants particularly susceptible to this pathogen, so we shall have to keep an eye out for signs of stress, by which time it may well be too late. I would prefer where possible to lift the plants and containerise them in the same position with some artful planting to hide the containers, a plan that’s presently in negotiation. The first step was to dig out all the infected rotten wood – stumps and roots were well decayed by now and offered little resistance to the trusty mattock – and as much soil as possible, all of which was bound for the bonfire. The chemical control for this was banned for use as a garden herbicide in 2003, so physical destruction (burning) of infected material is the only legal option at present. The legislation wasn’t able to prevent me from disinfecting my tools with Jeyes Fluid before moving on to other areas of the garden, a sensible precaution to take.

The next step is to have a reputable tree surgeon inspect the remaining birch trees for signs of infection, particularly as the root and stump of their departed companion resembles at present some kind of mushroom gourmand’s fantasy. These trees are too tall and close to the house, and the garden too exposed and windy, to countenance any chance of structural weakness.

Should the worst transpire, we will have to look to more resistant plants – a list of which is available here – to replace those that might succumb too easily to this voracious fungus. For now, we’re pairing the measures we’ve already taken to all the optimism we can muster, and hoping it won’t come to that.

Tthe beginnnings of blue black ‘bootlace’ rhizomorphs in the middle of this rotten stump