Epidemiology of dwarf mistletoe in western hemlock stands in south coastal British Columbia

Abstract

Surveys of seven western hemlock (Tsugaheterophylla) stands in south coastal British Columbia using fixed-radius plots assessed dwarf mistletoe (Arceuthobiumtsugense) infection intensity level and spatial distribution patterns. Regression analysis indicated a close relationship between the plot infection index (average dwarf mistletoe rating of trees in a plot) and the percentage of infected trees. Plot infection index reached 4.0 when all trees in a plot were infected. Infected trees were either associated with widely spaced infection centers or were more or less evenly distributed throughout the stand. Spatial distribution pattern and spread rate were related to (i) the severity and distribution pattern of the initial inoculum sources, (ii) the manner of stand regeneration and resulting stand structure, and (iii) the presence of barriers to spread such as a high nonhost component and drastic slope increases. Tree mortality from dwarf mistletoe averaged 0.6% (range, 0–1.8%) and was much lower than mortality from other causes. On average, 9.3% of the trees had large dwarf mistletoe caused swellings on the lower third of the bole.