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. Major spore release and infection coincides with bud-break and new shoot growth, but some spores are released throughout the summer.
Infection is mostly on new needles but some are infected during the second year.
The disease spreads mostly within the infected tree and to adjacent trees, but spores can be disseminated over long distances. This pathogen is mainly disseminated through shipment of infected nursery stock. The Douglas-fir on the right is infected with both Rhabdocline and Swiss needlecast. The tree on the left shows resistance to these two diseases. Plant healthy stock.