Black Stain of Western red-cedar by Aureobasidium pullulans and its Relationship with Tropolone Depletion
R Chedgy, R Daniels, P I Morris, C Breuil
Western red-cedar is valued for its natural durability conferred by fungicidal tropolone chemicals. However, weathered surfaces of WRC products are still susceptible to ‘black stain’ caused by fungi such as Aureobasidium pullulans. The effect of weathering on the tropolone content of heartwood was characterized and correlated with the ability of this fungal species to colonize the same weathered surfaces. UV plus water spray severely reduced tropolone content but did not lead to increased fungal colonization compared to un-weathered wood. When WRC was treated with UV only, the tropolone content was less affected but the fungal colonization increased significantly. A.pullulans exhibited high tolerance to the tropolone β-thujaplicin in vitro; thus loss in tropolone content may not be required for colonization. In addition to UV resistance from melanization and ability to use lignin breakdown products as a carbon source, resistance to tropolones may confer considerable competitive advantage to A.pullulans growing on WRC exposed to weather. The application of water spray most likely washed away products of lignin photo-degradation, leaving the wood surface void of accessible carbon sources which resulted in decreased colonization.
Keywords: Aureobasidium pullulans, ?-thujaplicin, black stain, blue stain in service, extractives, Western red cedar, wood products