THE SPORICIDAL ACTIVITY OF YELLOW-CEDAR HEARTWOOD, ESSENTIAL OIL, AND WOOD CONSTITUENTS TOWARD PHYTOPHTHORA RAMORUM IN CULTURE.

Authors: Manter, Daniel; KARCHESY, J - USDAFS PNW, CORVALLIS OR; KELSEY, R - USDAFS PNW, CORVALLIS OR

Submitted to: Forest Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/24/2005
Publication Date: 9/15/2006
Citation: Manter, D.K., Karchesy, J.J., Kelsey, R.G. 2006. The sporicidal activity of yellow-cedar heartwood, essential oil, and wood constituents toward phytophthora ramorum in culture. Forest Pathology. 36:297-308.

Interpretive Summary: Phytophthora ramorum was recently identified as the causal agent for sudden oak death, a nonlethal twig or foliar blight on ornamental plants, especially Rhododendron in various European countries, and a stem or branch pathogen that has been killing tanoak and various oak species in California since 1995. Government agencies and the nursery industry are monitoring this disease and implementing regulatory measures to minimize the distribution of its spores. This is a challenging task requiring an integrated approach, especially for infection zones in urban or native forests. In this paper we demonstrate that 140 ppm of essential oil from wood of yellow-cedar, incense cedar, Port-Orford cedar, or western juniper, all strongly inhibit zoospore germination and hyphal growth of P. ramorum in culture. Four individual compounds in yellow-cedar heartwood were also tested. Zoospore germination was reduced to 0% with 10, 100, and 1000 ppm of nootkatin, carvacrol, and valencene, respectively. Nootkatone was the least active compound, with 3.5% zoospore germination at 1000 ppm. Sporangia germination was 0 % with 500 ppm of nootkatin or carvacrol. Disruption of the zoospores outer membrane and loss of sporangial contents were often observed and indicative of sporicidal activity, as neither structure could germinate if the compounds were removed. Hyphal growth was inhibited 99.9% with 50 ppm nootkatin or 500 ppm carvacrol, but growth resumed upon removing the inhibitors. The zoosporicidal activity of yellow-cedar heartwood shavings was consistent with the quantity of extractable compounds it contained. Thus, spreading fresh shavings, or chips of yellow-cedar heartwood 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.

Technical Abstract: Phytophthora ramorum was recently identified as the causal agent for sudden oak death, a nonlethal twig or foliar blight on ornamental plants, especially Rhododendron in various European countries, and a stem or branch pathogen that has been killing tanoak and various oak species in California since 1995. Government agencies and the nursery industry are monitoring this disease and implementing regulatory measures to minimize the distribution of its spores. This is a challenging task requiring an integrated approach, especially for infection zones in urban or native forests. In this paper we demonstrate that 140 ppm of essential oil from wood of yellow-cedar, incense cedar, Port-Orford cedar, or western juniper, all strongly inhibit zoospore germination and hyphal growth of P. ramorum in culture. Four individual compounds in yellow-cedar heartwood were also tested. Zoospore germination was reduced to 0% with 10, 100, and 1000 ppm of nootkatin, carvacrol, and valencene, respectively. Nootkatone was the least active compound, with 3.5% zoospore germination at 1000 ppm. Sporangia germination was 0 % with 500 ppm of nootkatin or carvacrol. Disruption of the zoospores outer membrane and loss of sporangial contents were often observed and indicative of sporicidal activity, as neither structure could germinate if the compounds were removed. Hyphal growth was inhibited 99.9% with 50 ppm nootkatin or 500 ppm carvacrol, but growth resumed upon removing the inhibitors. The zoosporicidal activity of yellow-cedar heartwood shavings was consistent with the quantity of extractable compounds it contained. Thus, spreading fresh shavings, or chips of yellow-cedar heartwood 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.