Two pathogens, Phacidium abietis (mainly on fir) and Lophophacidium hyperboreum (mainly on spruce) cause most of the snow blight in America. Phacidium infestans is present mostly in Europe.
Damage, symptoms and biology
Snow blight is typically observed at snow melt. Needles that were under the snow are discoloured and an ephemeral mycelium resembling a spider’s web can be seen.
The most conspicuous symptoms of needle blights are red, yellow or brown discoloured needles that may later turn grey. This discolouration is frequently restricted to a particular annual increment of foliage. Discoloured infected needles are often intermingled with a few healthy green needles, unlike frost or drought discolouration that uniformly discolour whole branches or portions of the crown.
Needle and shoot blights are caused by a related group of fungi (Ascomycetes), which cause more damage to coniferous foliage in North America than any other group of fungi. Severe infection can result in significant growth reduction. They also affect stored forest plants.
The life cycle of Phacidium sp. and Lophophacidium sp. is similar. The fungus spends the summer as mycelium in the affected needles. The fruiting bodies (apothecia) appear in the fall; they are circular or oval and dark in colour, and form a row on each side of the midrib. They distribute their spores in wet weather between September and November. Infection occurs beneath the snow. At snowmelt the following spring, the mycelium emerges from the diseased needles and invades adjacent healthy foliage beneath the melting snow. Once the snow disappears, the fungus ceases to spread.