Inoculum Survival and Infectivity of Arceuthobium americanum Infecting Pinus banksiana in Manitoba

Aspects of the pathology and reproductive biology of Arceuthobium americanum Nuttal ex Engelman were examined with respect to microclimate and as they related to the inoculum potential and population dynamics of this pathogen. Fruit survival was measured over time to maturity and in successive years. Overwinter fruit survival was high in both years, averaging 94.9% and 87.5% respectively. Rates of fruit mortality increased through the following spring and summer months in both years of the study. As a result, fruit survival approximated a type I survivorship curve. Aerial shoot loss accounted for the greatest proportion of fruit mortality, ranging from 0.43 to 0.66 in the first year of the study and 0.32 to 1.00 in the following year. Shoot loss showed a significant correlation with vapour pressure deficit (r=0.87 in 1992/93; r=0.92 in 1993/94). Individual fruit loss ranged from 0.21 to 0.46 and from 0.18 to 0.61 respectively, in 1992/93 and 1993/94. Individual fruit loss showed a significant correlations with vapour pressure deficit in 1992/93 (r=0.91). The infection process, including overwinter seed retention, germination, factors influencing seed mortality and successful establishment of an infection, was also examined with respect to microclimate. Overwinter seed retention was 86% and 67% over all tree and tissue ages in respective years. Seed retention was significantly lower on tissues subsequently covered by snow when no needles were

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