Fungicidal Suppression of Swiss Needle Cast and Pathogen Reinvasion in a 20-Year-Old Douglas-Fir Stand in Oregon

Abstract

Abstract Aerial applications of chorothalonil fungicide were carried out annually over 5 consecutive years, 1996–2000, on three sets of paired, 2-ha units in a Douglas-fir plantation affected by Swiss needle cast in the Oregon Coast Range. The effect of treatment on disease control was evaluated annually from 2001 to 2004. One- and 2-year-old foliage in the fungicide-treated units had fewer fruiting bodies of the pathogen Phaeocryptopus gaeumannii compared with the unsprayed units for foliage sampled in 2001. Total needle retention was also greater in the fungicide-treated units after five consecutive annual fungicide applications. Reduced P. gaeumannii infection in the fungicide-treated units persisted for foliage produced in 2001, which did not receive direct fungicide treatment. Reduced infection levels in the 2001 foliage cohort in the treated units was presumed to be due to the effect of disease control on inoculum production. At 4 years following the final treatment application, infection levels averaged over four foliage cohorts (2000–2003) remained significantly smaller for the fungicide-treated units. Trees in fungicide-treated units retained 10–50% of the 2000 needle cohort (4-year-old needles) and 25–60% of the 2001 cohort in May 2004. However, there was no detectable effect of treatment on infection for 2002 foliage sampled in 2003 or for 2003 foliage sampled in 2004 (1-year-old needles), suggesting that the duration of disease control was relatively brief.