I always think of Dulwich as a green oasis in (more or less) the middle of London. But within this earthly paradise there lurks a silent killer - Armillaria mellea, honey fungus - that often explains why seemingly healthy bushes and trees (and sometimes herbaceous perennials) die unexpectedly. More widely, it’s top of the list of disease enquiries to the Royal Horticultural Society along with box blight (now extensive in Dulwich) and Phytophthora.

The most obvious symptoms are death of the tree or shrub, with early signs being foliage discolouration and dieback of shoots. It can be diagnosed by peeling back the bark on roots or the stem base, which will reveal a thin white layer of fungal tissue (mycelium) and/or thin black strap-like “bootlaces” (rhizomorphs) between the dead bark and the wood. Fruiting bodies - “mushrooms” - may also appear on the surface. These are variable in appearance but come in shades of brown and usually have a conspicuous ring around the stem below the cap. The fungus spreads by air-borne spores emerging from the fruiting bodies and by root to root contact between plants and by underground rhizomorphs. Left untreated, plants will continue to be affected.

The best treatment is to dig up and remove the diseased plant and as much of its roots as possible, as well as those of any adjacent dead or dying plants; in hedges, the RHS recommends taking out plants a metre either side of the dead plant. Then leave the area fallow for 6 to 12 months, digging over regularly to break up and starve any fungal fragments. Sinking an impermeable plastic barrier around the affected area is an alternative. Stress such as the recent drought makes plants more susceptible, so water, fertilise and mulch in dry weather - but do not pile up mulch or soil around the stems of plants as this allows the fungus to spread throughout the stem collar.

The RHS publishes lists of genera of trees and shrubs that are susceptible and those that show resistance (www.rhs.org.uk/honeyfungushosts).