Factors that limit the abundance of Cronartium ribicola on Ribes hosts in field environments

Cronartium ribicola, the cause of white pine blister rust, alternates between white pines and Ribes to complete its life cycle. White pines are ecologically and commercially important, while Ribes are generally wild plants with little associated value. Knowledge of factors related to the distribution and abundance of C. ribicola on Ribes is critical to understanding the complete disease system.;The interaction of C. ribicola with multiple Ribes species was compared in a common garden. Severity of C. ribicola on four species of Ribes varied within species, and differed significantly between R. hudsonianum and R. glandulosum. The variability in disease severity among plants in woodland habitats was related to the genetic background of the hosts by dividing plants of R. missouriense into two ramets, or clones, leaving one in the woods and transplanting one to a common garden. Comparisons of rust severity between the pairs of ramets provided evidence that host genetics was a significant factor explaining rust abundance among woodland hosts in sites with high disease incidence. Supporting data were obtained through inoculations of woodland plants, providing additional evidence that woodland plants that were not colonized tended to be less susceptible than colonized plants. While host genetics was significantly related to rust severity among plants, other factors clearly played a role in patterns of rust severity among sites and between stems within plants. Manipulating plant stems to bring them to a common microenvironment eliminated significant differences in rust abundance observed between stems left in their original positions.;Rain-free periods limited C. ribicola on Ribes. Fluctuations in rust severity were significantly related to daily rainfall that was lagged by 13 days. The provision of leaf wetness was sufficient for new lesion development. Thus, viable inoculum and susceptible hosts were present, and without provisional leaf wetness the ambient environment limited disease. Leaves with higher rust severity tended to detach earlier than leaves with less rust, based on results obtained from different experiments. Telia were observed from early June throughout the remainder of two seasons. Collectively, the results identify epidemiological factors related to the abundance of C. ribicola on Ribes.