### Identification Keep a sharp eye on the broad leaflets of Horse-chestnut trees (*Aesculus*) for what looks like a premature, rusty autumn. Phyllosticta sphaeropsoidea manifests as large, irregular reddish-brown blotches, often surrounded by a vivid yellow "halo" of dying tissue. To distinguish it from the common Horse-chestnut leaf miner, look at the veins: while the moth larvae stay strictly between them, this fungus ignores boundaries, spreading across the leaf veins with ease. For a definitive ID, use a hand lens to spot the pycnidia—tiny, black, pinprick-sized fruiting bodies peppered across the brown patches like grains of black pepper.
### Habitat & Range This fungus is an opportunist found wherever its host, the Horse-chestnut, resides. It is common across Europe and North America, thriving in urban parks, stately avenues, and temperate deciduous woodlands. It prefers humid environments; you’ll find the heaviest "burn" on trees in sheltered valleys or crowded gardens where air circulation is poor and morning dew lingers.
### Behaviour This is a "seasonal ghost" that performs a clever vanishing act. It overwinters in the fallen leaf litter on the forest floor. When spring rains arrive, the fungus ejects microscopic spores (conidia) that hitch a ride on splashing raindrops to reach the fresh, emerging canopy. By mid-summer, the infection takes hold, and by August, the tree may appear completely scorched.
### Diet As a necrotrophic parasite, it feeds on the living cells of the leaf. It secretes specialized enzymes that dissolve the plant’s cell walls, allowing the fungus to absorb the nutrient-rich "soup" left behind. It essentially eats the leaf from the inside out, turning green vitality into brown decay.
### Fascinating Fact Despite its dramatic appearance, this fungus is rarely a tree-killer! It is a "master of moderation," stealing just enough energy to reproduce without destroying its host. It’s a cosmetic parasite that gives the tree a "bad hair day" rather than a death sentence.