Needlecast 1-celled bone shaped 7x19 microns. Swiss Needlecast 2-celled spores 4x14 microns Ascospores of R. pseudotsugae are windblown and infect the young needles of opening buds. Fruiting Stage Fungus fruiting bodies develop on infected needles on the tree. At first they appear as round cushions. Ascospores are released during wet weather and are wind disseminated to other trees.
Spores can only infect elongating young needles at bud break.
Fruiting bodies will develop and mature on these infected needles during the following winter and early spring. The fruiting bodies of R. pseudotsugae rupture the needle epidermis in spring exposing the spore bearing areas. When magnified, these fruiting bodies appear as lines of black dots emerging from the normal white stomates. Fruiting bodies are very similar to those found on blue spruce trees infected with Rhizosphaera needlecast. These fruiting bodies also produce airborne ascospores that are released in wet weather.