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This document provides the conclusions of the pest survey card that was prepared in the context of the EFSA mandate on plant pest surveillance (M‐2020‐0114) at the request of the European Commission. The full pest survey card for non‐EU Arceuthobium spp. is published and available online in the EFSA Pest Survey Card gallery at the following link and will be updated whenever new information becomes available: https://efsa.europa.eu/plants/planthealth/monitoring/surveillance/non-eu-arceuthobium

1. Interspecific interactions have a fundamental role in plant population dynamics, as they may set the conditions for species coexistence. Parasitic plants, like dwarf mistletoes, offer the opportunity to study competition for resources that are different from those consumed by most plants, allowing for a better understanding of the interaction. 2. We explored how interspecific interactions between two dwarf mistletoe species (Arceuthobium), co-infecting the same host species (even sharing the same individual tree of Pinus hartwegii) affect their infection dynamics at two different stages of population development (colonization of new hosts and subsequent growth), and if heterogeneity in resource availability (host density and size structure) affects these interactions. For that purpose, we integrated these processes into a spatially explicit model of density-dependent population growth. 3. We found that self-regulation (density-dependence) was strong for both species; however, the intensity and sign of interspecific interactions changed depending on host size and demographic process. Population growth in Arceuthobium globosum was reduced by competition, except for smaller hosts where A. globosum growth was facilitated by Arceuthobium vaginatum. Arceuthobium vaginatum was facilitated by A. globosum regardless of host size. Colonization of new hosts by A. globosum was enhanced by previous infection by the other species, showing intraguild facilitation. 4. Demographic importance of interactions depended on stand structure: in homogeneous, low-density forests, facilitation predominates, increasing the population sizes of both species, whereas the opposite occurs in heterogeneous and dense forests. Both species achieved stable coexistence, fulfilling the invasibility criterion because each mistletoe species can invade a forest that is already infected by the other species. 5. Synthesis. Despite the fundamentally different mechanisms underlying the interactions between mistletoes compared with non-parasitic plants, our results reveal that their behaviour at the population level is similar. Stabilizing mechanisms, like strong self-limiting population growth, allow dwarf mistletoe coexistence. Interactions shift as populations develop, and they depend largely on environmental factors such as forest structure. Intraguild mutualism is shown as a relevant process for colonization of new spaces, highlighting the complexity of competitive/facilitative interactions between parasitic plants, a formerly unexplored subject. Interactions can only be fully understood when integrating all their components at the population level. Analysing these interactions may contribute to the understanding of plant–plant interactions in general, and convey interesting implications for forest management.

The genus Arceuthobium (dwarf mistletoes, Viscaceae) comprises 42 species that parasitize hosts in Pinaceae and Cupressaceae in the Old and New Worlds. Maximum parsimony analyses were conducted on two data partitions (separately and combined): nuclear ribosomal internal transcribed spacer (ITS) sequences for all 42 currently recognized species and chloroplast trnT-L-F sequences for 34 New World species. The Old and New World species were phylogenetically distinct using ITS, thus making subgenus Arceuthobium paraphyletic. Arceuthobium pendens and A. guatemalense comprise the basalmost clade of subgenus Vaginata, characterized by the presence of flabellate secondary branching. The trnT-L-F sequences, which vary widely in length depending upon taxon, contain three times less phylogenetic signal than ITS, although homoplasy for this partition is lower. Several of the clades obtained from analysis of nuclear ITS sequences are also recovered using trnT-L-F sequences such as A. guatemalense and A. pendens, the A. rubrum group, the A. vaginatum group, and the A. campylopodum group. The ITS + trnT-L-F tree is well resolved except for four internal nodes. A revised classification of the genus is discussed that recognizes only monophyletic species that are well differentiated by molecular data.

In this study, a novel positive single-stranded RNA (+ ssRNA) virus named wheat yellow stripe associated virus (WYSAV) was identified in wheat plants in China. Molecular characterization revealed that the complete genome of WYSAV is divided into two segments, RNA1 and RNA2, which are 6,460 and 4,935 nucleotides (nt) in length, excluding their respective poly(A) tails. RNA1 contains one large opening reading frame (ORF), encoding a replication-associated protein. RNA2 contains six ORFs, encoding a coat protein (CP), a coat protein readthrough domain protein (CP-RTD), triple gene block protein 1 (TGB1), triple gene block protein 2 (TGB2), triple gene block protein 3 (TGB3), and a cysteine-rich protein (CRP). Phylogenetic analysis showed that WYSAV is related to members of the genus Benyvirus in the family Benyviridae. Thus, WYSAV is proposed to be a new member of the genus Benyvirus. Wheat (Triticum aestivum L.) is one of the most important food crops and ranked third in the world in terms of production, only behind rice and maize [1]. During its growth cycle, wheat faces several biotic and abiotic stresses. Wheat soil-borne virus disease is an important disease that is difficult to control and causes severe yield loss in China each year [2]. The main pathogens causing wheat soil-borne virus disease are Chinese wheat mosaic virus (CWMV) and wheat yellow mosaic virus (WYMV), and their transmission vector is Polymyxa graminis [3–5]. Members of the viral family Benyviridae usually have two to five genomic RNA segments and are transmitted by root-infecting vectors belonging to the family “Plasmodiophoridae”. Although few members of the family Benyviridae, of which beet necrotic yellow vein virus is the type member, have been identified [6], several recently identified viruses have been found to be phylogenetically related to benyviruses but are not classified as members of the family Benyviridae. These “unclassified benyviruses” include red clover RNA virus 1, Arceuthobium sichuanense virus 3, Dactylorhiza hatagirea beny-like virus, goji berry chlorosis virus [7], Guiyang benyvirus 1, Guiyang benyvirus 2, Mangifera indica latent virus [8], Rhizoctonia solani beny-like virus 1 [9], Sanya benyvirus 1 [10], and Sclerotium rolfsii beny-like virus 1 [11].In this study, we identified a novel + ssRNA virus in symptomatic leaf samples collected from cultivated wheat in the city of Zhumadian, Henan Province, China. We propose to name this virus \"wheat yellow stripe associated virus\" (WYSAV), and we have deposited its full-length sequence in the GenBank database under the accession numbers OQ547804 (RNA1) and OQ547805 (RNA2).

Insects, fungi, and diseases play an important role in forest stand development and subsequently, forest management decisions and treatments. As these disturbance agents commonly occur within and across landscapes, modeling has often been used to inform forest planning and management decisions. However, models are rarely benchmarked, leaving questions about their utility. Here, we assessed the predictive performance of a Bayesian hierarchical model through on–the-ground sampling to explore what features of stand structure or composition may be important factors related to eastern spruce dwarf mistletoe (Arceuthobium pusillum Peck) presence in lowland black spruce (Picea mariana (Mill.) B. S. P.). Twenty-five state-owned stands included in the predictive model were sampled during the 2019 and 2020 growing seasons. Within each stand, data related to the presence of eastern spruce dwarf mistletoe, stand structure, and species composition were collected. The model accurately predicted eastern spruce dwarf mistletoe occurrence for 13 of the 25 stands. The amount of living and dead black spruce basal area differed significantly based on model prediction and observed infestation, but trees per hectare, total living basal area, diameter at breast height, stand age, and species richness were not significantly different. Our results highlight the benefits of model benchmarking to improve model interpretation as well as to inform our understanding of forest health problems across diverse stand conditions.

Lodgepole pine (Pinus contorta) forests are widely distributed throughout North America and are subject to mountain pine beetle (Dendroctonus ponderosae) epidemics, which have caused mortality over millions of hectares of mature trees in recent decades. Mountain pine beetle is known to influence stand structure, and has the ability to impact many forest processes. Dwarf mistletoe (Arceuthobium americanum) also influences stand structure and occurs frequently in post-mountain pine beetle epidemic lodgepole pine forests. Few studies have incorporated both disturbances simultaneously although they co-occur frequently on the landscape. The aim of this study is to investigate the stand structure of lodgepole pine forests 21-28 years after a mountain pine beetle epidemic with varying levels of dwarf mistletoe infection in the Deschutes National Forest in central Oregon. We compared stand density, stand basal area, canopy volume, proportion of the stand in dominant/codominant, intermediate, and suppressed cohorts, average height and average diameter of each cohort, across the range of dwarf mistletoe ratings to address differences in stand structure. We found strong evidence of a decrease in canopy volume, suppressed cohort height, and dominant/codominant cohort diameter with increasing stand-level dwarf mistletoe rating. There was strong evidence that as dwarf mistletoe rating increases, proportion of the stand in the dominant/codominant cohort decreases while proportion of the stand in the suppressed cohort increases. Structural differences associated with variable dwarf mistletoe severity create heterogeneity in this forest type and may have a significant influence on stand productivity and the resistance and resilience of these stands to future biotic and abiotic disturbances. Our findings show that it is imperative to incorporate dwarf mistletoe when studying stand productivity and ecosystem recovery processes in lodgepole pine forests because of its potential to influence stand structure.

Dwarf mistletoes (Arceuthobium spp., Viscaceae) are the most destructive plant parasites to numerous conifer species worldwide. In this study, the plastid genomes (plastomes) of Arceuthobium chinense Lecomte and A. pini Hawksworth and Wiens were sequenced and characterized. Although dwarf mistletoes are hemiparasites capable of photosynthesis, their plastomes were highly degenerated, as indicated by the smallest plastome size, the lowest GC content, and relatively very few intact genes among the Santalales hemiparasites. Unexpectedly, several essential housekeeping genes (rpoA, rpoB, rpoC1, and rpoC2) and some core photosynthetic genes (psbZ and petL), as well as the rpl33 gene, that is indispensable for plants under stress conditions, were deleted or pseudogenized in the Arceuthobium plastomes. Our data suggest that the leafless and endophytic habit, which heavily relies on the coniferous hosts for nutrients and carbon requirement, may largely relax the selection pressure on photosynthesis, as well as plastid transcription and translation, thus resulting in the loss/pseudogenization of such essential plastid-encoding genes in dwarf mistletoes. Therefore, the higher level of plastome degradation in Arceuthobium species than other Santalales hemiparasites is likely correlated with the evolution of leafless and endophytic habit. A higher degree of plastome degradation in Arceuthobium. These findings provide new insights into the plastome degeneration associated with parasitism in Santalales and deepen our understanding of the biology of dwarf mistletoes.

Fir dwarf mistletoe (Arceuthobium abietinum, Viscaceae) parasitizes true firs (Abies spp.) in the Pacific Northwest through California, the Southwest, and into northern Mexico. At present, fir dwarf mistletoe consists of two special forms (formae speciales, f. sp.) and one subspecies that are morphologically similar, but exhibit a high degree of host specificity. However, A. abietinum has been treated as a subspecies of A. campylopodum or conspecific with it in some taxonomic treatments. Therefore, we undertook this study to compare the morphologies of these dwarf mistletoes; collecting morphological data from nearly 100 A. abietinum and 60 A. campylopodum populations collected across most of their geographic ranges and analyzing these data using univariate and multivariate statistical tests. Our results demonstrated that the special forms and subspecies of A. abietinum are morphologically distinct from A. campylopodum, thereby, supporting the continued recognition of A. abietinum as a separate species. Furthermore, our analysis found significant differences for several of the characters we examined between the special forms of A. abietinum. Therefore, we recombined the special forms as subspecies: A. abietinum subsp. abietinum (formerly f. sp. concoloris) and A. abietinum subsp. magnificae (formerly f. sp. magnificae). The previously described A. abietinum subsp. wiensii was morphologically distinct from subsp. abietinum and subsp. magnificae. The distinctiveness of these taxa was supported by their host affinities as well. El muérdago enano del oyamel (Arceuthobium abietinum, Viscaceae) parasita a los oyameles o abetos (Abies spp.) en la región occidental, desde California hasta el norte de México. Hasta ahora, el muérdago enano del oyamel consiste de dos formas especiales (formae speciales, f. sp.) y una subespecie que son morfológicamente similares, pero presentan un alto grado de especificidad de hospedador. Sin embargo, A. abietinum ha sido tratado como subespecie de A. campylopodum o como conspecifico con éste en algunos tratamientos taxonómicos. En este estudio se comparan las morfologías de esos muérdagos enanos. Se recabaron datos morfológicos de casi 100 poblaciones de A. abietinum y 60 de A. campylopodum con materiales colectados en la mayor parte de área de distribución y los datos se analizaron usando pruebas estadísticas univariadas y multivariadas. Nuestros resultados demuestran que las dos formas especiales y la subespecie de A. abietinum son morfológicamente diferentes de A. campylopodum, lo que justifica que A. abietinum siga considerándose como especie separada. Adicionalmente, nuestro análisis encontró diferencias significativas para algunos de los caracteres examinados entre las formas especiales de A. abietinum. Por esta razón, recombinamos esas formas a nivel de subespecie: A. abietinum subsp. abietinum (previamente f. sp. concoloris) and A. abietinum subsp. magnificae (previamente f. sp. magnificae). Arceuthobium abietinum subsp. wiensii, previamente descrita, difiere también morfológicamente de las otras dos subespecies. La diferencia entre esos taxa es corroborada además por sus preferencias de hospedador.

Maintaining healthy forests requires multiple individuals, including foresters who develop timber sale silvicultural prescriptions and loggers who implement those prescriptions, resulting in the transplantation of forest health science into workable management plans. However, data on the experiences, attitudes, and opinions of these two groups are often missing when developing or refining forest health treatment strategies. To explore the role that these groups play in sustaining forest health, we examined timber sale administrators’ and loggers’ perspectives on treatment approaches for eastern spruce dwarf mistletoe (Arceuthobium pusillum) (ESDM), a parasitic plant native to Minnesota that increases mortality and reduces growth rate and regeneration success of black spruce (Picea mariana). While ESDM has been managed for decades in black spruce stands in Minnesota, little is known about the effectiveness of the management approaches. Data were gathered through interviews and focus groups with loggers, as well as an online survey and focus groups with foresters who administer timber sales. Study participants identified a range of field-based barriers, knowledge gaps, and uncertainties that hamper the ability to effectively implement ESDM treatment strategies as designed, including financial, administrative, informational, policy-related, and environmental factors. These factors have a significant bearing on the ability to effectively implement ESDM treatment approaches; yet may be factors that were not known or considered when developing treatment strategies. This case study underscores the value of nurturing a science–management partnership to ensure that a broad set of voices are considered when developing or revising forest health treatment strategies.

Arceuthobium sichuanense is an aerial parasitic plant (dwarf mistletoe) which causes severe damage to spruce forests on the Qinghai-Tibet Plateau. Picea crassifolia and Picea purpurea , two main host species of A. sichuanense , have different growth characteristics and elevational distributions. The effects of A. sichuanense infection on P. crassifolia and P. purpurea trees were evaluated by examining needle and current-year shoot morphology, needle water use efficiency and needle nitrogen concentration, with 30 needle samples, 30 current-year shoot samples and 10  A. sichuanense aerial shoot samples for each host species. The most apparent effects were significant reductions in both needle size and current-year shoot length. The high degree of correlation in foliar δ 15 N values between the dwarf mistletoe and its host trees indicated that nitrogen in the dwarf mistletoe was derived entirely from its host. Percent reductions in needle and current-year shoot length were smaller for P. purpurea than for P. crassifolia , possibly suggesting that P. purpurea exhibits a lesser capacity to accommodate the parasitic effects of mistletoe infection by adjusting the growth of needles and current-year shoots. The reductions in needle nitrogen concentration and δ 13 C values were lower in P. purpurea than in P. crassifolia , indicating that P. purpurea suffered more nitrogen and water stresses and exhibited lower vigor compared to P. crassifolia . Our results demonstrated that P. purpurea may suffer more negative effects induced by dwarf mistletoe infections at needle and branch levels than P. crassifolia .