No abstract

Research Highlights: Two genets of Armillaria altimontana Brazee, B. Ortiz, Banik, and D.L. Lindner and five genets of Armillaria solidipes Peck (as A. ostoyae [Romagnesi] Herink) were identified and spatially mapped within a 16-year-old western white pine (Pinus monticola Doug.) plantation, which demonstrated distinct spatial distribution and interspecific associations. Background and Objectives: A. solidipes and A. altimontana frequently co-occur within inland western regions of the contiguous USA. While A. solidipes is well-known as a virulent primary pathogen that causes root disease on diverse conifers, little has been documented on the impact of A. altimontana or its interaction with A. solidipes on growth, survival, and the Armillaria root disease of conifers. Materials and Methods: In 1971, a provenance planting of P. monticola spanning 0.8 ha was established at the Priest River Experimental Forest in northern Idaho, USA. In 1987, 2076 living or recently dead trees were measured and surveyed for Armillaria spp. to describe the demography and to assess the potential influences of Armillaria spp. on growth, survival, and the Armillaria root disease among the study trees. Results: Among the study trees, 54.9% were associated with Armillaria spp. The genets of A. altimontana and A. solidipes comprised 82.7% and 17.3% of the sampled isolates (n = 1221) from the study plot, respectively. The mapped distributions showed a wide, often noncontiguous, spatial span of individual Armillaria genets. Furthermore, A. solidipes was found to be uncommon in areas dominated by A. altimontana. The trees colonized by A. solidipes were associated with a lower tree growth/survival and a substantially higher incidence of root disease than trees colonized only by A. altimontana or trees with no colonization by Armillaria spp. Conclusions: The results demonstrate that A. altimontana was not harmful to P. monticola within the northern Idaho planting. In addition, the on-site, species-distribution patterns suggest that A. altimontana acts as a long-term, in situ biological control of A. solidipes. The interactions between these two Armillaria species appear critical to understanding the Armillaria root disease in this region.

A conceptual framework for evaluation of climate effects on tree diseases is presented. Climate can exacerbate tree diseases by favouring pathogen biology, including reproduction and infection processes. Climatic conditions can also cause abiotic disease—direct stress or mortality when trees’ physiological limits are exceeded. When stress is sublethal, weakened trees may subsequently be killed by secondary organisms. To demonstrate climate's involvement in disease, associations between climatic conditions and disease expression provide the primary evidence of atmospheric involvement because experimentation is often impractical for mature trees. This framework tests spatial and temporal relationships of climate and disease at several scales to document climate effects, if any. The presence and absence of the disease can be contrasted with climate data and models at geographic scales: stand, regional and species range. Temporal variation in weather, climate and climate change is examined during onset, development and remission of the disease. Predisposing factors such as site and stand conditions can modify the climate effects of some diseases, especially at finer spatial scales. Spatially explicit climate models that display temperature and precipitation or derivative models such as snow and drought stress provide useful data, and however, information on disease extent at different spatial scales and monitoring through time are often incomplete. The framework can be used to overcome limitations in other disease causality approaches, such as Koch's postulates, and allow for the integration of vegetation, pathogen and environmental data into causality determinations.