ETH-Zurich researcher Valentin Queloz describes the species of fungus responsible for the ash dieback recently observed in Switzerland as a "familiar stranger." However, researchers still don't know why a harmless leaf colonizer could turn into an aggressive pathogen. The uncanny phenomenon began in eastern Poland. In the nineties ash trees started dying apparently out of the blue, and entire forests shrank. Then, in 2006 a Polish scientist succeeded in isolating a previously unknown fungus from 70 percent of the dead ash trees. Only consisting of hyphae, he called his discovery Chalara fraxinea -- a vegetative form that produces sticky spores, known as conidia, asexually. It wasn't until later that researchers in Poland found little yellow-white fungal fruiting bodies on the leaf remains of ash trees that they were able to identify as Hymenoscyphus albidus. It later transpired that Chalara fraxinea and the fruiting bodies were one and the same species but, as is often the case in mycology, carry two different names: one for the anamorph and one for the teleomorph.H. albidus wasn't a stranger after all, having first been characterized in 1851. But why should this fungus, which had always been regarded as a harmless litter decomposer, suddenly turn on its host, the ash?