Kathy J. Lewis

Characterization and distribution of mating type genes in the Dothistroma needle blight pathogens

ABSTRACT Dothistroma septosporum and D. pini are the two causal agents of Dothistroma needle blight of Pinus spp. in natural forests and plantations. Degenerate primers amplified portions of mating type genes (MAT1-1-1 and MAT1-2) and chromosome walking was applied to obtain the full-length genes in both species. The mating-type-specific primers designed in this study could distinguish between the morphologically similar D. pini and D. septosporum and between the different mating types of these species. Screening of isolates from global collections of D. septosporum showed that only MAT2 isolates are present in Australian and New Zealand collections, where only the asexual form of the fungus has been found. In contrast, both mating types of D. septosporum were present in collections from Canada and Europe, where the sexual state is known. Intriguingly, collections from South Africa and the United Kingdom, where the sexual state of the fungus is unknown, included both mating types. In D. pini, for which no teleomorph is known, both mating types were present in collections from the United States. These results provided new insights into the biology and global distribution of two of the worlds most important pine pathogens and should facilitate management of the diseases caused by these fungi.

Sexual reproduction and gene flow in the pine pathogen Dothistroma septosporum in British Columbia.

Dothistroma septosporum has caused a serious needle blight epidemic in the lodgepole pine forests in northwest British Columbia over the past several years. Although ascocarps had been observed in British Columbia, nothing was known about the contribution of sexual reproduction, gene flow and long-distance dispersal to the epidemic. Amplified fragment length polymorphism and mating-type markers in 19 sites were used to generate population and reproductive data. Overall, evidence suggests a mixed mode of reproduction. Haplotypic diversity was high, with 79 unique and 56 shared haplotypes (possible clones) identified from 192 fungal isolates. Overall, mating-type segregation did not differ significantly from 1:1; however, random mating was rejected in most populations in the index of association and parsimony tree-length permutation analyses using the full data set and, when using clone-corrected data sets, more of the smaller populations showed random mating. Two of the smaller populations consistently showed random mating for both tests using both clone-corrected and noncorrected data. High gene flow is suggested by no differentiation between 14 of the 19 sites, several of which came from young plantations where the pathogen was not likely present prior to the current outbreak. The remaining five sites showed some level of divergence, possibly due to historic separation and endemic pathogen populations. Results indicate a high evolutionary potential and long-distance dispersal in this pathogen, important to consider in future forest management.

Ecosystem, Location, and Climate Effects on Foliar Secondary Metabolites of Lodgepole Pine Populations from Central British Columbia

Lodgepole pines, Pinus contorta Douglas ex Louden var. latifolia Engelm. ex S. Watson, are encountering increased abiotic stress and pest activity due to recent increases in temperature and changes in precipitation throughout their range. This tree species counters these threats by producing secondary metabolites, including phenolics and terpenoids. We examined foliar levels of lignin, soluble phenolics, monoterpenoids, sesquiterpenoids, and diterpenoids in 12 stands in British Columbia, Canada. We used these data to assess associations among foliar secondary metabolite levels and ecosystem, geographic, and climatic variables. Regressions were also performed to observe which combinations of variables best explained secondary metabolite variance. Stands of P. c. latifolia in the Coastal Western Hemlock and Interior Cedar/Hemlock biogeoclimatic zones had consistently greater foliar levels of almost all measured secondary metabolites than did other stands. Lignin was present in greater amounts in Boreal White/Black Spruce ecosystem (i.e., northern) stands than in southern stands, suggesting a role for this metabolite in pine survival in the boreal forest. Attempts to develop regression models with geographic and climatic variables to explain foliar secondary metabolite levels resulted in multiple models with similar predictive capability. Since foliar secondary metabolite levels appeared to vary most between stand ecosystem types and not as much due to geographic and climatic variables, metabolic profiles appeared best matched to the stress levels within local environments. It is unknown if differences in secondary metabolite levels are the result of genetic adaptation or phenotypic plasticity, but results from this and other studies suggest that both are important. These results are interpreted in light of ongoing efforts to assist in the migration of certain populations of P. c. latifolia northward in an effort to counter predicted effects of climate change.

Influence of decay fungi on species composition and size class structure in mature Picea glauca × engelmannii and Abies lasiocarpa in sub-boreal forests of central British Columbia

Abstract Disturbance patterns in the sub-boreal spruce forests of central British Columbia have long been thought to result from frequent stand-initiating fires. However, recent evidence suggests that fires in the wetter areas of this region are infrequent (>500 years) and the uneven-aged stand structures have been shown to be self-maintaining in the absence of fire. The importance of decay fungi as agents of gap-formation and facilitators of uneven-aged stand structure was investigated. Three plots on each of two recently clearcut sites were established. Each stump in the plots was stem mapped and the species and diameter recorded. Decay at the stump top was recorded as white or brown rot, and the area occupied by decay was measured. Spruce stumps dominated the larger diameter classes but had less butt rot than sub-alpine fir stumps. Decay fungi contribute to stem breakage in living trees with heart rot, and as saprots of dead trees. Therefore, the results suggest that decay fungi play an important role in removing sub-alpine fir trees from the canopy of these wetter sub-boreal ecosystems, and in enabling spruce recruitment. The type of decay observed in sub-alpine fir suggests that breakage is predominantly due to white rot fungi causing heart rot, such as Echinodontium tinctorium and Stereum sanguinolentum. In spruce, brown rot fungi, which are predominantly saprot or wound-entry decay fungi, became more common in larger spruce, and may result from wounding as sub-alpine fir fall from the canopy. Spatial analysis indicated the stumps with stem decay were clumped. However, this is more likely due to strong clumping of the host, rather than spreading of butt rot pathogens through the roots.

Spore Dispersal by Dothistroma septosporum in Northwest British Columbia

We studied spore dispersal by Dothistroma septosporum, causal agent of a serious outbreak of red band needle blight in lodgepole pine plantations in northwest British Columbia. Spore abundance was assessed at different distances and heights from inoculum sources and microclimatic factors were recorded during two consecutive years. Conidia were observed on spore traps from June to September during periods of rainfall. It was rare to detect spores more than 2 m away from inoculum sources. The timing and number of conidia dispersed were strongly tied to the climatic variables, particularly rainfall and leaf wetness. Should the trend toward increased spring and summer precipitation in the study area continue, the results suggest that disease spread and intensification will also increase. Increasing the planting distances between lodgepole pine trees through mixed species plantations and overall reduction in use of lodgepole pine for regeneration in wet areas are the best strategies to reduce the spread of the disease and enhance future productivity of plantations in the study area.

Northern range limit mountain pine beetle outbreak dynamics in mixed sub‐boreal pine forests of British Columbia

Recent widespread mountain pine beetle (Dendroctonus ponderosae) (MPB) outbreaks in western North America have caused unprecedented tree mortality across much of the beetles known historic range and into novel habitats near the range margins. Our current understanding of outbreak dynamics and their relationship to forest structure is based almost wholly on research from climatically optimal habitats where infestations have historically been most severe. As range expansion continues marginal habitats may become increasingly susceptible to outbreaks, yet little is known about the factors driving outbreak development in these forest types or how the disturbance regime differs. This study uses dendroecology and forest structure analysis to reconstruct historic outbreak dynamics at the northern MPB distribution limit in British Columbia to extend the disturbance record beyond the scope of documented history and to evaluate the long-term relationships between stand structure, host suitability, and regime characteristics at the range margin. We sampled 22 sites with evidence of past infestations, and using mortality dates, survivor growth releases, and tree vigor analyses we reconstructed five probable outbreak periods predating the current epidemic. At least two of these events fit the profile of large epidemics occurring in the 1900s–1910s and again in the 1940s–1950s, with evidence of two 19th century infestations and chronic localized mortality beginning around the 1980s. Stand age appears to be the most important factor limiting outbreaks prior to 1870, indicating a strong relationship with fire history at centennial time scales. Outbreak return intervals were comparable to those reported in other locations, yet beetle preference for larger and older trees suggests that provincial susceptibility models may not accurately represent northern outbreak risk, particularly in older stands. Following stand maturation, host availability does not appear to limit outbreak severity or recurrence. Here density-independent factors associated with additional climate pressures are likely more important. With enhanced climate suitability, outbreak scale and severity should increase dramatically in this landscape which is currently not limited by host distribution. Our reconstruction may inform management strategies as this occurs and outbreaks move into previously uninfested forests that share many structural and ecological similarities with our sites.

Pathogens, Invasive Species, and Prognosis for the Future

Organisms such as fungi and insects, can attack trees and cause reductions in growth due to reallocation of resources to defense responses, or reductions in photosynthetic capacity. These organisms can also kill trees, which can enhance the growth of neighbor trees due to release from competition. Changes in growth brought about by such attacks can be detected and studied using tree-ring analysis. Three case studies are presented in this chapter. The first is on Dothistroma needle blight in British Columbia, Canada where it is a native pathogen that causes growth reduction of pine. Dendrochronology is used to reconstruct past outbreaks of the pathogen, to identify weather variables that enhance disease development and spread, and to link changes in outbreak dynamics with climate change. The second case study is on white pine blister rust in North America, where the fungus was introduced into high elevation forests of whitebark pine. Dendrochronology enabled approximation of the date of introduction, disease intensification and spread dynamics, interactions with other insects and climate, as well as projections of future stand composition and structure. This is an important application of tree-ring analysis given the importance of whitebark pine for ecosystem function. The third case study is on emerald ash borer, a wood boring insect recently introduced to North America. In this case, tree-ring analysis is used to determine the efficacy of applications of insecticide in reducing growth loss and mortality rates. Collectively these case studies demonstrate that dendrochronology enables integrated analysis of host, climate and pathogen or insect effects on tree growth and survival, and ultimately on ecosystem function and resilience.