First Report of Armillaria sinapina, a Cause of Armillaria Root Disease, Associated with a Variety of Forest Tree Hosts on Sites with Diverse Climates in Alaska.

In August of 2007, a preliminary survey was conducted in Alaska to evaluate potential impacts of climate change on forest trees. Armillaria sinapina, a root-disease pathogen, was isolated from conifer and hardwood hosts on climatically diverse sites spanning 675 km from the Kenai Peninsula to the Arctic Circle. Seven isolates (NKAK1, NKAK2, NKAK5, NKAK6, NKAK9F, NKAK13, and NKAK15) were identified as A. sinapina by using intergenic spacer-1 nucleotide sequences (GenBank Accession Nos. EU665175-EU665181) and somatic pairings. Of particular note is that one isolate (NKAK9F) was obtained from a declining Salix sp. (willow) growing in a flood plain near the Arctic Circle (66°32.316'N, 150°47.717'W). This isolate was collected from mycelial bark fans that were intercalated within multiple bark layers, a sign of disease. All other isolates were derived from rhizomorphs attached to and/or embedded within roots and root collars, but most host trees showed no clear indication of disease. Two isolates were collected from dead trees within a small mortality center (62°08.703'N, 150°04.593'W) that included an isolate from Picea glauca (white spruce; NKAK13) and another isolate from Betula sp. (birch; NKAK15). Additional isolates came from a beetle-killed P. glauca (NKAK1) 120 km northwest of Anchorage (61°48.079'N, 148°16.983'W) and a suppressed (overtopped by other trees in the stand) Tsuga mertensiana (mountain hemlock; NKAK2) 58 km southeast of Anchorage (60°50.679'N, 149°03.742'W). The two remaining isolates originated from the Kenai Peninsula (approximately 60°29.629'N, 149°45.465'W) and were derived from a root-diseased Populus tremuloides (trembling aspen; NKAK5) and a suppressed P. glauca (NKAK6). Although A. mellea sensu lato was previously reported on willow in interior Alaska (1) and A. sinapina was previously reported from sites under coastal influence (4), this represents the first confirmed report of A. sinapina on P. glauca, T. mertensiana, Populus tremuloides, Salix sp., and Betula sp. in Alaska. Unfortunately, pathogenicity of A. sinapina cannot be readily verified under experimental conditions because environmental variables, host-tree status (e.g., species, population, age, and vigor), and inoculum potential are difficult to recreate. Armillaria sinapina is typically regarded as a weak pathogen of diverse hosts (3). However, A. sinapina is predicted to cause more disease on hosts predisposed by climate stress, and climate change is well-documented in Alaska (2). Because A. sinapina occurs on diverse hosts under different climates across a wide geographic range in Alaska, Armillaria root disease could become more prevalent on trees stressed by climate change. References: (1) T. E. Hinds and T. H. Laurent. Plant Dis. Rep. 62:972, 1978. (2) J. J. McCarthy et al., eds. Climate Change 2001: Impacts, Adaptation and Vulnerability. Cambridge University Press, Cambridge, 2001. (3) D. J. Morrison et al. Can. J. Plant Pathol. 7:242, 1985. (4) C. G. Shaw, III and E. M. Loopstra. Phytopathology 78:9714, 1988.

Influence of host resistance on the genetic structure of the white pine blister rust fungus in the western United States.

Cronartium ribicola, the causal agent of white pine blister rust, has been devastating to five-needled white pines in North America since its introduction nearly a century ago. However, dynamic and complex interactions occur among C. ribicola, five-needled white pines, and the environment. To examine potential evolutionary influences on genetic structure and diversity of C. ribicola in western United States, population genetic analyses of C. ribicola were conducted using amplified fragment length polymorphism (AFLP) molecular markers. The fungus was sampled at six sites. Collections for two of the six sites were from separate plantings of resistant-selected western white pine and sugar pine. Heterozygosity based on polymorphic loci among populations ranged from 0.28 to 0.40, with resistant-selected plantations at the extremes. Genetic differentiation was also highest between these two populations. Principal coordinates analysis and Bayesian assignment placed most isolates that are putative carriers of virulence to major-gene resistance into a discernable cluster, while other isolates showed no clustering by site or host species. These results indicate that C. ribicola in western North America is not genetically uniform, despite its presumed single site of introduction and relatively brief residence. Moreover, major-gene resistance appears to have imposed strong selection on the rust, resulting in reduced genetic diversity. In contrast, no evidence of selection was observed in C. ribicola from hosts that exhibit only multigenic resistance.

First Report of the White Pine Blister Rust Fungus, Cronartium ribicola, on Pedicularis bracteosa.

Until recently, Cronartium ribicola J.C. Fisch. was thought to utilize only Ribes spp. (Grossulariaceae) as telial hosts in North America. During 2004, Pedicularis racemosa Dougl. ex Benth. and Castilleja miniata Dougl. (Orobanchaceae) were proven as natural telial hosts at a subalpine site (48.634109°N, 116.570817°W, elevation 1,800 m) near Roman Nose Lake, ID, where whitebark pine (Pinus albicaulis Engelm.) and western white pine (Pinus monticola Dougl. ex D. Don) are aecial hosts, and Pedicularis, Castilleja, and Ribes spp. are common herbs/shrubs (2). During August 2006, teliospore columns typical of C. ribicola or the morphologically indistinguishable (2) C. coleosporioides J.C. Arthur were found on two Pedicularis bracteosa Benth. plants at this site, within 3 m of a large, sporulating canker on whitebark pine. ITS/5.8S rDNA regions were sequenced using detached teliospore column samples from the two plants, ITS1F and ITS4 primers (3), and standard PCR protocols (2). One sample sequence was identified as C. ribicola and the other as C. coleosporioides (GenBank Accession Nos. EF185857 and EF185858, respectively), by exact matches in comparisons with published sequences (2). Artificial inoculation confirmed P. bracteosa's ability to host C. ribicola. Sections of leaves collected near Freezeout Saddle, ID (47.00885°N, 116.00846°W, elev. 1,600 m) were rinsed in water, placed abaxial side up on moistened filter paper in 150-mm petri plates, inoculated with seven diverse sources of urediniospores/aeciospores, misted with distilled water, and incubated at 18°C with 12 h of light. A single leaf section produced urediniospores 17 days and teliospores 26 days after inoculation with one of two Roman Nose aeciospore sources. Urediniospores from this leaf section caused infections on Ribes nigrum L., and teliospore columns yielded a DNA sequence that matched C. ribicola. Though P. bracteosa is confirmed as yet another natural host of C. ribicola in North America, it may be producing less C. ribicola inoculum for pine infection than do the P. racemosa and Ribes spp. telial hosts at the collection site. Uredinia and telia of C. ribicola on P. bracteosa were much less frequent and smaller than those on P. racemosa and Ribes spp. and those of C. coleosporioides on this same host (2). Pedicularis (but not Castilleja) spp. are significant telial hosts of C. ribicola strains at some high elevation sites in eastern Asia (1). Discovery of multiple North American telial hosts in the Orobanchaceae suggests unrecognized complexity in C. ribicola's ability to exploit ecological niches in recently established pathosystems of North America (2). References: (1) G. I. McDonald et al. Pages 41-57 in: Forest Pathology: From Genes to Landscapes. J. Lundquist and R. Hamelin, eds. The American Phytopathological Society. St. Paul, MN, 2005. (2) G. I. McDonald et al. For. Pathol. 36:73, 2006. (3) T. J. White et al. Pages 315-322 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al. eds. Academic Press, San Diego, CA, 1990.

Effect of white pine blister rust (Cronartium ribicola) and rust-resistance breeding on genetic variation in western white pine (Pinus monticola).

Western white pine (Pinus monticola) is an economically and ecologically important species from western North America that has declined over the past several decades mainly due to the introduction of blister rust (Cronartium ribicola) and reduced opportunities for regeneration. Amplified fragment length polymorphism (AFLP) was used to assess the genetic variation in northern Idaho populations of western white pine (including rust-resistant breeding stock) in relation to blister rust. A total of 176 individuals from four populations was analyzed using 163 AFLP loci. Within populations, an average 31.3% of the loci were polymorphic (P), and expected heterozygosity (H(e)) was 0.123. Genetic differentiation values (G(st)) showed that 9.4% of detected genetic variation was explained by differences among populations. The comparison between the rust-resistant breeding stock and a corresponding sample derived from multiple natural populations produced similar values of P (35% vs. 34.4%) and H(e) (0.134 vs. 0.131). No apparent signs of a genetic bottleneck caused by rust-resistance breeding were found. However, a comparison of two natural populations from local geographic areas showed that the population with low pressure from blister rust had higher polymorphism and heterozygosity than the population that had experienced high mortality due to blister rust: P (30.7% vs. 25.1%) and H(e) (0.125 vs. 0.100), respectively. In addition, the population from low blister-rust pressure had twice as many unique alleles as the blister rust-selected population. The genetic distance and Dice's similarity coefficients among the four populations indicated that the local population that survived high blister-rust pressure was genetically similar to the rust-resistant breeding stock.