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... Tree and tissue age were tested for their impact on infection success by inoculating the 1, 2, and 3 year-old tissues of 3, 5, and 7 year-old trees, and the 1, 4, and 8 year-old tissues of 12, 17, and 22 year-old trees... Seed germination in the laboratory experiments negatively correlated with vapour pressure deficit (p=0.0007), ranging from 30% at 2.50 kPa to 90% at 0.6 kPa. Below 0.3 kPa and temperatures of 10C, 15C, 20C and 25C there was a decrease in the proportion of seeds that germinated in the 1992-1993 laboratory experiments. Between 0.3 kPa and 0.0 kPa, seed germination was apparently inhibited by molding. In the 1993-1994 laborarory experiments, molding did not occur, though low values below 0.2 kPa at 10