Woody plant infections of the pathogenic oomycete Phytophthora ramorum Werres, DeCock, & Manin'tVeld have brought an uncertainty to the future of western forests. Two understory hardwoods, tanoak (Notholithocarpus densiflorus (Hook. and Arn.) Manos, C.H.Cannon, and S.Oh) and California bay (Umbellularia californica (Hook. & Arn.) Nutt.), are commonly infected with P. ramorum leaf infection, the nonfatal foliar form of this pathogen. As the fatal bole infection form of this pathogen is most commonly studied, comparatively little research has been conducted on the nonfatal foliar infection form. This study measured physiological characteristics (midday water potential, stomatal conductance, net photosynthesis, and water-use efficiency) of healthy and infected foliage from tanoak and California bay trees in Redwood National Park to determine the effects P. ramorum foliar infection on leaf-level productivity. For both species, midday water potential, stomatal conductance, and net photosynthesis were all lower in infected samples compared to healthy samples. There was no significant difference in instantaneous water-use efficiency between infected and healthy foliage. Results suggest that P. ramorum leaf infection can lower leaf-level productivity and water status. Further research on the subject is still needed to better support informed management decisions in infected forests and to predict the long-term effects of P. ramorum leaf infections across a diverse suite of host species in the Pacific Northwest.
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Vol. 68 • No. 2