Induced antimicrobial activity in heat‐treated woodchips inhibits the activity of the invasive plant pathogen Phytophthora ramorum

We investigated the antimicrobial activity of heat‐treated woodchips of three woody host species against the invasive oomycete plant pathogen Phytophthora ramorum to assess the potential of heated woodchips for disease suppression. Results demonstrated that heat‐treated pine (Pinus sylvestris), Japanese larch (Larix kaempferi) and rhododendron (Rhododendron ponticum) woodchips inhibited the recovery of P. ramorum spores and mycelium compared with similar material that had only been air‐dried. Effects were most evident with pine and larch; inhibition was maintained even when larch woodchips were diluted with soil. In vitro assays using methanol crude extracts from woodchips of the three species showed they all had an inhibitory effect on P. ramorum zoospores and reduced chlamydospore germination compared with air‐dried wood extracts. Chemical analysis of the extracts revealed several induced compounds were present but in different concentrations for each species. Coniferaldehyde was the most active inhibitory against spores and mycelium, whilst the dominant resin acids, dehydroabietic and abietic acid, decreased the minimum inhibitory concentration of phenolic compounds tested against P. ramorum but were ineffective when used alone. An array of compounds, including dehydroabietic acid, methyl abietate, α‐pinene and 3‐carene, occurred at elevated levels in the living tissue of Japanese larch bark attacked by P. ramorum. These compounds may be part of the induced resistance response of larch to P. ramorum. Results of a field trial using heat‐treated and air‐dried woodchips were consistent with the crude extract bioassay results, suggesting that heat‐treated woody materials have potential to reduce the survival of P. ramorum under natural conditions.