Incidence and impact of Swiss needle cast in forest plantations of Douglas-fir in coastal Oregon

An epidemic of Swiss needle cast, caused by the ascomycete Phaeocryptopus g aeumannii, is causing defoliation and growth reductions in Douglas-fir fore st plantations along the Oregon Coast. The area of symptomatic plantations has been monitored annually since 1996 by aerial survey; in spring 1999, 11 9,500 ha were affected. Pathogen and symptom development have also been mon itored on nine permanent plots in stands of differing disease severity. Inf ection levels and symptom severity are greatest in low elevation plantation s close to the coast. In areas of severe disease, trees retain only current year needles. Defoliation is proportional to the number of stomata occlude d by pseudothecia of the fungus, with needles being shed when about 50% of stomata are occupied, regardless of needle age. Fungus sporulation and prem ature needle abscission are greatest on the upper branches of trees. Annual application of fungicides increases needle retention significantly. Tree h eight and diameter growth and total tree volume are reduced by disease, and tree volume is significantly correlated with needle retention on our plot trees. The epidemic continues to be most severe in Douglas-fir plantations established on sites where Sitka spruce and western hemlock or red alder pr edominated in earlier times.