The sporicidal activity of yellow‐cedar heartwood, essential oil and wood constituents towards Phytophthora ramorum in culture

Abstract

SummaryIn this paper, we demonstrate that 140 mg/kg of essential oil from the wood of yellow‐cedar, incense cedar, Port‐Orford‐cedar or western juniper strongly inhibits zoospore germination and hyphal growth of Phytophthora ramorum in culture. Four individual compounds in yellow‐cedar heartwood were also tested. Zoospore germination was reduced to 0% with 10, 100 and 1000 mg/kg of nootkatin, carvacrol and valencene, respectively. Nootkatone was the least active compound, with 3.5% zoospore germination at 1000 mg/kg. Sporangia germination was 0% with 500 mg/kg of nootkatin or carvacrol. The disruption of the zoospore outer membrane and the loss of sporangial contents were often observed and indicative of sporicidal activity. Hyphal growth was inhibited by 99.9% with 50 mg/kg of nootkatin or 500 mg/kg of carvacrol, but growth resumed upon removing the inhibitors. The zoosporicidal activity of yellow‐cedar heartwood shavings stored dry for approximately 10 years was consistent with the quantity of extractable compounds they contained. Thus, spreading fresh shavings or chips of yellow‐cedar heartwood with appreciable higher concentrations of the active compounds, over areas in infection zones where spores might be difficult to control such as trails and parking lots used by hikers and bicyclists, might be useful as part of an integrated program to minimize P. ramorum spore distribution.