A method for estimating white pine blister rust canker age on limber pine in the central Rocky Mountains

Abstract

SummaryEpidemiological studies of white pine blister rust on limber pine require a temporal component to explain variations in incidence of infection and mortality. Unfortunately, it is not known how long the pathogen has been present at various sites in the central Rocky Mountains of North America. Canker age, computed from canker length and average expansion rate, can be used to estimate infestation origin and infection frequency. To investigate relationships between canker lengths and canker ages for limber pine, we collected live white pine blister rust branch and stem cankers from three locations in Wyoming and two locations in Colorado. We quantified relationships between various measures of canker length and an estimate of canker age based on dendrochronological analysis. Total branch canker length was strongly, negatively correlated (r = −0.79) with the first year of incomplete, annual ring formation (canker age). Mean longitudinal canker expansion rate was 8.4 cm year−1 for branch and stem cankers where branches distal to the canker were either dead or alive. Annual longitudinal canker expansion, however, was significantly greater on a stem or branch where the portion distal to the canker was alive (11.5 cm year−1) rather than dead (7.1 cm year−1). For branches or stems, proximal expansion rate (i.e., toward or down stem) averaged 4.9 cm year−1. The circumferential canker expansion rate (around branch or stem) was greater for stem cankers (8.3 cm year−1) than for branch cankers (6.2 cm year−1). Additional site and host tree covariates did not improve prediction of canker age. Two simple linear equations were developed to estimate a canker age from total length of a canker with the distal portion either alive or dead. An appropriate sample of canker ages can be used to determine how long a limber pine stand has been infested with white pine blister rust and how frequently infections have occurred.