Quantitative Disease Resistance to White Pine Blister Rust at Southwestern White Pine’s (Pinus strobiformis) Northern Range

White pine blister rust, caused by the non-native, invasive fungal pathogen Cronartium ribicola, is a significant cause of mortality in white pines (Pinus subgenus Strobus) in North America. Along with climate-driven range contraction, mortality from blister rust can seriously impact the abundance and distribution of the nine white pine species native to the United States and Canada. Very little evaluation of this disease in southwestern white pine (Pinus strobiformis) has been previously undertaken, but genetic resistance to the disease has been documented, including major gene resistance (MGR) conferred by a dominant R gene. Data is emerging suggesting that the species also has quantitative disease resistance (QR). Our results suggest QR occurs at low frequency, with perhaps 10% of trees having a moderate level (> 35% survival). We assessed progeny arrays from 40 P. strobiformis families (1873 seedlings), originating from three populations, inoculated with C. ribicola. Subsequently, the seedlings were assessed for signs, symptoms and resulting impact in a common garden trial over a 7.5-year period to determine the types and frequency of resistance in a portion of this species’ range. There was a high incidence of both stem symptoms and mortality in the P. strobiformis families tested, and families ranged in survival from 0 to 84.6%. Three families had > 70% survival, representing perhaps the highest documented QR to date in a North American white pine species. Approximately 29.1% of the 441 surviving seedlings showed no stem symptoms, and of the approximately 70.8% of seedlings surviving with infections only few (24 of 316) had infections of moderate to high severity. QR traits associated with improved survival were primarily related to lower severity of infection, a reduced number of stem symptoms, and an increased number of bark reactions. Despite the high overall susceptibility, the presence of QR appears to be at a frequency and level useful to forest managers involved in restoration and reforestation efforts.

Comparative association mapping reveals conservation of major gene resistance to white pine blister rust in southwestern white pine (Pinus strobiformis) and limber pine (P. flexilis)

All native North American white pines are highly susceptible to white pine blister rust (WPBR) caused by Cronartium ribicola. Understanding genomic diversity and molecular mechanisms underlying genetic resistance to WPBR remains one of the great challenges in improvement of white pines. To compare major gene resistance (MGR) present in two species, southwestern white pine (Pinus strobiformis) Cr3 and limber pine (P. flexilis) Cr4, we performed association analyses of Cr3-controlled resistant traits using SNP assays designed with Cr4-linked polymorphic genes. We found that ~ 70% of P. flexilis SNPs were transferable to P. strobiformis. Furthermore, several Cr4-linked SNPs were significantly associated with the Cr3-controlled traits in P. strobiformis families. The most significantly associated SNP (M326511_1126R) almost co-localized with Cr4 on the Pinus consensus linkage group 8 (LG-8), suggesting that Cr3 and Cr4 might be the same R locus, or have localizations very close to each other in the syntenic region of the P. strobiformis and P. flexilis genomes. M326511_1126R was identified as a non-synonymous SNP, causing amino acid change (Val376Ile) in a putative pectin acetylesterase (PAE), with coding sequences identical between the two species. Moreover, top Cr3-associated SNPs were further developed as TaqMan genotyping assays, suggesting their usefulness as marker-assisted selection (MAS) tools to distinguish genotypes between quantitative resistance (QR) and MGR. This work demonstrates the successful transferability of SNP markers between two closely related white pine species in the hybrid zone, and the possibility for deployment of MAS tools to facilitate long-term WPBR management in P. strobiformis breeding and conservation.

Recovery Plan for Scots Pine Blister Rust Caused by Cronartium pini

This Scots pine blister rust (caused by Cronartium pini) recovery plan is one of several plant disease-specific documents produced as part of the National Plant Disease Recovery System (NPDRS) requested by the Homeland Security Presidential Directive Number 9 (HSPD-9). The purpose of the NPDRS is to ensure that the tools, infrastructure, communication networks, and capacity required for mitigating impacts of high-consequence, plant-disease outbreaks are implemented so that a reasonable level of crop production is maintained. This recovery plan is intended to provide a brief summary of the disease, assess the status of critical recovery components, and identify disease management research, extension, and education needs. These documents are not intended to serve as stand-alone documents that address all of the many and varied aspects of plant disease outbreaks, all of the critical decisions that must be determined, or all of the actions needed to achieve effective response and recovery. These plans are, however, documents that will help the USDA to guide further efforts directed toward plant disease recovery.

Nonlinear shifts in infectious rust disease due to climate change

Range shifts of infectious plant disease are expected under climate change. As plant diseases move, emergent abiotic-biotic interactions are predicted to modify their distributions, leading to unexpected changes in disease risk. Evidence of these complex range shifts due to climate change, however, remains largely speculative. Here, we combine a long-term study of the infectious tree disease, white pine blister rust, with a six-year field assessment of drought-disease interactions in the southern Sierra Nevada. We find that climate change between 1996 and 2016 moved the climate optimum of the disease into higher elevations. The nonlinear climate change-disease relationship contributed to an estimated 5.5 (4.4–6.6) percentage points (p.p.) decline in disease prevalence in arid regions and an estimated 6.8 (5.8–7.9) p.p. increase in colder regions. Though climate change likely expanded the suitable area for blister rust by 777.9 (1.0–1392.9) km2 into previously inhospitable regions, the combination of host-pathogen and drought-disease interactions contributed to a substantial decrease (32.79%) in mean disease prevalence between surveys. Specifically, declining alternate host abundance suppressed infection probabilities at high elevations, even as climatic conditions became more suitable. Further, drought-disease interactions varied in strength and direction across an aridity gradient—likely decreasing infection risk at low elevations while simultaneously increasing infection risk at high elevations. These results highlight the critical role of aridity in modifying host-pathogen-drought interactions. Variation in aridity across topographic gradients can strongly mediate plant disease range shifts in response to climate change.

Influence of Forest Conditions on the Spread of Scots Pine Blister Rust and Red Ring Rot in the Priangarye Pine Stands

Scots pine blister rust and red ring rot are common on Scots pine throughout its entire range. Specialists do not explain a significant variation in the prevalence of the diseases uniquely since it depends on complex ecological and silvicultural factors. The aim of this research is to study the influence of forest growth conditions on the incidence of Scots pine blister rust and red ring rot in pine stands of the Priangarye (territory located along the lower reaches of the Angara within the Krasnoyarsk Krai). The research methods included a detailed forest pathological examination of prevailing pine forest types, specific symptom-based macroscopic diagnosis of the diseases, data analysis using parametric and non-parametric statistical tests. Forest growth conditions indicators included type of forest, habitat conditions, and bonitet class of forest stands. The incidence of Scots pine blister rust and red ring rot in pine forests of the Priangarye reaches the extent of moderate and severe damage, respectively. The prevalence of Scots pine blister rust is significantly higher in low-bonitet lichen pine forests; the incidence rate increases along the gradient of decreasing fertility and soil moisture level. The incidence of red ring rot is significantly higher in herb-rich pine forests, in gradations of maximum soil fertility and medium soil moisture. The revealed patterns are explained by the bioecological characteristic features of pathogens (for red ring rot—additionally by factors of structural immunity in pine trees). The results of the research should be recognized in the organization of forestry practice.

Cronartium flaccidum (Scots pine blister rust).

C. flaccidum is a heteroecious rust fungus, completing different stages of its life cycle on different plants. Mating of haploid strains occurs on species of Pinus, followed by the production of aeciospores, which infect various species of herbaceous dicotyledons. An asexual stage producing urediniospores occurs on the dicotyledonous plants, followed by the production of teliospores, the sexual stage, that germinate to form basidiospores that infect pines thus completing the cycle. A closely-related autoecious rust, Endocronartium (Peridermium) pini, only infects Pinus hosts. C. flaccidum is known from Europe and parts of northern and eastern Asia; it is a Regulated Pest for the USA (USDA/APHIS, 2008). As an invasive in other temperate areas, this rust could be damaging on native and introduced pines or the alternate host species. The infections on pines develop slowly, therefore the fungus might be overlooked, such that accidental introduction of the rust could occur through the importation of conifer [Pinopsida] seedlings or trees.