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Effect of hot-water soluble Thuja Plicata heartwood extractives on the growth of Thuja and non-Thuja isolates of Poria Weirii. Morrison, Duncan John


Poria weirii Murr. isolates cultured from various hosts may be divided into two groups on the basis of their ability to parasitize Thuja plicata Donn and to grow on media containing water-soluble T. plicata heartwood extractives. The role of these extractives in determining whether a Poria weirii isolate can parasitize Thuja plicata was examined. Measurement of linear growth along wood micro-sections confirmed that substances inhibitory to one isolate group are hot-water soluble T. plicata heartwood extractives. The tolerance of isolates from T. plicata (Thuja) and Pseudotsuga menziesii (Mirb.) Franco (non-Thuja) to a hot-water extract of Thuja plicata heartwood and its hexane soluble (principally thujaplicins) and insoluble (thujaplicin-free) fractions was studied in detail. Dry weight of mycelium produced in liquid glucose-asparagine medium was measured. Thuja isolates were more tolerant than non-Thuja of the hot-water extract and its fractions; thujaplicins caused the greatest response differences. Thuja isolates showed greater stimulation than non-Thuja at subinhibitory thujaplicin concentrations. Higher thujaplicin concentrations were required to produce fungistatic and -toxic conditions in Thuja isolates than in non-Thuja. There were highly significant differences between isolate groups in response to 7.5 ppm thujaplicin. Tolerance was not related to date of isolation or geographical location. With sufficient inoculum potential, a non-Thuja isolate may overcome T. plicata heartwood toxicity. The resulting infection is usually localized. Unlike Thuja isolates, non-Thuja were unable to increase thujaplicin tolerance during three serial generations on liquid medium containing thujaplicins. Long exposure of non-Thuja isolates to T. plicata heartwood under field conditions did not increase their thujaplicin tolerance. Thuja isolates may be able to adapt the enzyme systems blocked by thujaplicin or to detoxify thujaplicins. These results have significance for control of Poria weirii. Planting Thuja plicata in and around infection centers caused by non-Thuja, isolates could limit their size by reducing the number of root contacts between susceptible species.

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