Abstract. Seed-source movement trials using common garden experiments are needed to understand climate, tree (host), and pathogen interactions. Douglas-fir (Pseudotsuga menziesii var menziesii) is an important tree species native to western North America influenced by the foliar fungi Phaeocryptopus gaeumannii, a biotroph and causal agent of Swiss needle cast (SNC), and Rhabdocline species, necrotrophs that cause Rhabdocline needle cast. We used the Douglas-fir Seed-Source Movement Trial, a large provenance study of Douglas-fir that consists of populations and test sites chosen to represent the range of climate conditions experienced by Douglas-fir west of the Cascade and northern Sierra Nevada Mountains, USA, to assess disease severity and symptom expression in Douglas-fir in relation to climatic differences between test sites and population sources. Using generalized linear mixed models, probability of disease severity/expression was modeled with respect to the climate variables May through September precipitation (MSP), mean winter temperature (MWT), and continentality. Stark differences in disease expression were observed in trees from different regions, especially in relation to resistance to Rhabdocline spp. and tolerance to P. gaeumannii. There were no major differences across seed-source regions at any particular site in infection levels of P. gaeumannii assessed by fruiting body abundance, yet disease tolerance followed similar geographic patterns as resistance to Rhabdocline spp. Transfers of populations from low to high MSP, and/or cool to warm MWT, increased the probability of moderate to severe Rhabdocline spp. infection and SNC disease symptoms. Our results suggest that local seed sources are adapted to local climate and pathogen pressures and that seed sources from regions with high foliage disease pressure are most resistant/tolerant to those foliage diseases. We also confirm that temperature and precipitation are important epidemiological factors in forest disease and that assisted migration must take into account trophic interactions of trees. Movement of seed sources from dry spring and summer and/or cool winter conditions to mild, mesic environments is likely to lead to increased probability of losses due to these foliage diseases.

No abstract

In 2019, a decline of Quercus emoryi (Emory oak) was observed on the Coronado National Forest located in southeastern Arizona. Symptoms associated with oak mortality included crown die-back and large cankers revealing charcoal-like stromal growth. Trunks and root collars showed girdling and many affected trees also displayed evidence of gold-spotted oak borer activity. Initial surveys in stands identified clusters of severe infections. Samples with black perithecia and stromal tissue were collected from symptomatic hosts. Morphological characterization of the fungus was completed on fresh perithecial tissue. Stromata were pulvinate and black showing embedded perithecial bumps, with ostioles visible from the surface of the stroma. Asci were short stipitate and cylindrical with visible oil drops, 6.6 to 9.4 (mean: 8.8) × 139.8 to 179.9 μm (mean: 166.4). Ascospores were smooth ovoid, brown to dark brown, with narrowed and round ends, 6.9 to 9.1 (mean: 7.7) × 13.8 to 25.9 μm (mean: 16.5). Colonies grown on ½ strength potato dextrose agar (½ PDA) (Korhonen 1980) at 25°C for 14 days were whitish-grey when viewed from the top and darkened embedded spheres were visible from the bottom. Single-spored cultures were isolated by dissecting the hymenium and placing in distilled water. The suspension was streaked onto ½ PDA plates and incubated for 12 hours, and pure cultures were grown on ½ PDA. DNA was isolated from a mycelial scrape and extracted using a 10% Chelex solution (Safaee 2017). The internal transcribed spacer region was amplified with the ITS1/ITS4 primers (White et al. 1990) and PCR products were sequenced at Eurofins Scientific (Lewisville, KY). Isolates (BSS1, BSS2, BSS3, BSS4) were compared to other Biscogniauxia mediterranea sequences in the NCBI database (MG098274, EF026134) and had 100% sequence identity. Pathogenicity assays were conducted using 8-year-old Emory oaks in a greenhouse. The oaks were drought stressed and watered at 50% pot capacity for two months prior to inoculation. Each tree received approximately 2400 ml of water every two weeks for the remainder of the experiment. Eight trees were wounded three times with a 5mm corer. A 5mm plug of mycelium of isolate BSS2 was used to inoculate wounds. Four trees were used as controls and inoculated with ½ PDA plugs. Wounded and inoculated areas were assessed after three months. All eight trees exhibited dark necrotic tissue in the vascular cambium and lesions ranged from 11.4 – 25.8 mm in length and were 3.9 – 13.3 mm wide. The controls exhibited no fungal sign, lesions, or necrotic tissues. The pathogen B. mediterranea was recovered from 13 of the 24 total inoculations and its identity was confirmed with ITS sequencing. Mortality and die-back caused by Biscogniauxia spp. are commonly associated with drought-stressed trees, with increasingly hot and dry conditions routinely noted as inciting factors (Desprez Loustau et al.). Emory oaks are among the dominant tree species in much of the Madrean oak woodlands of the southwestern US and Mexico and provide vital ecological and cultural services. As southwestern states continue to experience hotter and drier, conditions, it is likely Emory oak will become increasingly susceptible to die-back and mortality due to this Biscogniauxia species (Southern et al.).

Understanding the genetic architecture of tolerance and resistance to pathogens is important to monitor and maintain resilient tree populations. Here we investigate the genetic basis of tolerance and resistance and to needle cast disease in Douglas-fir (Pseudotsuga menziesii) caused by two fungal pathogens: Swiss needle cast (SNC) caused by Nothophaeocryptopus gaeumannii, and Rhabdocline needle cast (RNC) caused by Rhabdocline pseudotsugae). We performed a case-control genome-wide association analysis (GWAS) and found these traits to be polygenic. Significant associations with SNC resistance were found for SNPs in genes for stomatal regulation and ethylene and jasmonic acid pathways, which are known for their roles in plant defense and immunity. Top-associated SNPs for SNC tolerance were found in genes of secondary metabolite pathways. We identified a key upstream transcription factor of plant defence, ERF1, as the main candidate for RNC resistance. Our findings contribute to the understanding of the highly polygenic architectures underlying disease resistance and tolerance in Douglas-fir and have important implications for forestry and conservation as the climate changes.