Where does pathogenicity come from?
The scientist is now puzzling over whether and why H. pseudoalbidus developed into a pathogen in the space of a few decades, and why it acquired its pathogenic properties. In other words: why a harmless endophyte, a fungus that lives in plants, turned into a germ that is deadly for ash trees. One hypothesis is that climate change is weakening the trees and that H. pseudoalbidus happens to be the stronger of the two species of fungus. After all, H. pseudoalbidus hasn’t changed anything in its life cycle and is thus akin to H. albidus. After overwintering in the litter, the fungus forms fruiting bodies on the partially decomposed midribs of ash leaves when summer comes. The fungus looks like a little white mushroom, no bigger than a thumb tack. It produces spores that are dispersed by the wind; the spores find their way onto the still-green leaves of standing ash trees, germinate and form a mycelium that spreads inside the leaf. When the leaves drop off the tree in the fall, the fungi re
