HALLOWEEN is coming. In our street, the kids have already put up a “enter if you dare” streamer, and the bright orange pumpkins are being carved into hideous, gap-toothed faces.

In previous years, I’d always assumed that we’d be alright without such garish items to ward off evil spirits because in our back garden, we have a large rowan tree.

The rowan, or mountain ash, has many mythological connections. The ancient Greeks thought that Hebe, the goddess of the prime of life, had her magical cup of eternal youth stolen by demons. The gods despatched eagles to retrieve it, which they did – but at great cost to the eagles, which is why the leaves of a rowan are shaped like feathers plucked from an eagle’s body, and its berries are as red as an eagle’s droplets of blood.

In Norse mythology, the rowan was thought to ward off evil spirits – some sources say that its name comes from an old Norse word, “ruan”, meaning “a charm”, although the Oxford English Dictionary says that “ruan” means “red” which seems much more likely.

Neverthless, rowans were planted outside houses and inside churchyards to ward off witches. In fact, rowans seem particularly efficacious as a deterrent to female spell-mongers who at this time of year would be flying about the sky on broomsticks making the cows run dry. But a sprig of rowan placed over a cowhouse door kept the witches at bay and the milk flowing.

So having a rowan in our back garden seemed to protect us from the worst of Halloween, although it didn’t deter trick or treaters from knocking at the door in search of easy sweets.

But this year, after its feathery leaves turned an extraordinary bronze, for the first time in 25 years, the rowan has sprouted an amazing crop of mushrooms at its foot. It looks like there are dozens of little nuclear explosions going off beneath the soil around the tree, each producing a honey-coloured cloud above the grass.

At first we thought it was honey fungus, an evil fungus that spreads diabolically unseen under the ground, seeking out a tree’s roots and then strangling its way up the trunk. Once honey fungus has taken hold, there’s no future for the tree.

Now, though, we think it is shaggy scalycap. Shaggy scalycap is also a honey-coloured mushroom, but, as its name suggests, its distinctive feature is that it is covered in warts.

Some people reckon you can eat it, but only if you cover it in baking soda for hours, blanch it in boiling water and then fry it in butter by which time, unsurprisingly, it tastes very bland. Other people reckon it is poisonous, particularly if taken in conjunction with alcohol. Ten hours after consumption, the effect can be explosive from both ends – not necessarily fatal but far from pleasant.

We haven’t yet worked out whether the tree needs to come down – any tips on treating shaggy scalycap would be very welcome.

But as the bewitching season nears, it is very worrying that the tree we depended upon to ward off evil spirits has itself fallen under the spell of a saprophyte that lives on the dead and decaying. Perhaps we should spend the weekend carving out a gap-toothed orange pumpkin just to be on the safe side.