Transmission of the Invasive Pathogen Phytophthora ramorum from Symptomatic to Healthy Host Plants During a Five-Year Period in California

Reliable data on the transmission of airborne plant pathogens are crucial for the development of epidemiological models and implementation of management strategies. The short-distance vertical transmission of the forest pathogen Phytophthora ramorum from a symptomatic California bay laurel (Umbellularia californica) to healthy containerized rhododendrons (Rhododendron caucasicum × R. ponticum var. album) was monitored for five winters (2016/17 to 2020/21) in a field experiment in Northern California. Transmission events were observed during four winters at a frequency of 1 to 17 per season, but not during the extremely dry winter of 2020/21, and were positively correlated to total rainfall rates. The first leaf symptoms were detected around mid-December and reached the highest numbers in January of most years. Only limited symptom development was observed in the spring, with the last detections in May. The exposure time (the time between the first rainfall after placing a bait plant under the bay laurel and development of symptoms) varied between 3 and over 150 days, with an average between 14 and 21 days. P. ramorum was detected from water samples collected from the canopy of the symptomatic California bay laurel. No horizontal pathogen spread was detected from symptomatic to healthy rhododendrons placed at a distance of 2 to 6 m.

Apoplastic effector candidates of a foliar forest pathogen trigger cell death in host and non-host plants

Forests are under threat from pests, pathogens, and changing climate. A major forest pathogen worldwide is the hemibiotroph Dothistroma septosporum, which causes dothistroma needle blight (DNB) of pines. While D. septosporum uses effector proteins to facilitate host infection, it is currently unclear whether any of these effectors are recognised by immune receptors to activate the host immune system. Such information is needed to identify and select disease resistance against D. septosporum in pines. We predicted and investigated apoplastic D. septosporum candidate effectors (DsCEs) using bioinformatics and plant-based experiments. We discovered DsCEs that trigger cell death in the angiosperm Nicotiana spp., indicative of a hypersensitive defence response and suggesting their recognition by immune receptors in non-host plants. In a first for foliar forest pathogens, we developed a novel protein infiltration method to show that tissue-cultured pine shoots can respond with a cell death response to a DsCE, as well as to a reference cell death-inducing protein. The conservation of responses across plant taxa suggests that knowledge of pathogen–angiosperm interactions may also be relevant to pathogen–gymnosperm interactions. These results contribute to our understanding of forest pathogens and may ultimately provide clues to disease immunity in both commercial and natural forests.

Apoplastic effector candidates of a foliar forest pathogen trigger cell death in host and non-host plants

Forests are under threat from pests, pathogens, and changing climate. One of the major forest pathogens worldwide is Dothistroma septosporum, which causes dothistroma needle blight (DNB) of pines. D. septosporum is a hemibiotrophic fungus related to well-studied Dothideomycete pathogens, such as Cladosporium fulvum. These pathogens use small secreted proteins, termed effectors, to facilitate the infection of their hosts. The same effectors, however, can be recognised by plants carrying corresponding immune receptors, resulting in resistance responses. Hence, effectors are increasingly being exploited to identify and select disease resistance in crop species. In gymnosperms, however, such research is scarce. We predicted and investigated apoplastic D. septosporum candidate effectors (DsCEs) using bioinformatics and plant-based experiments. We discovered secreted proteins that trigger cell death in the angiosperm Nicotiana spp., suggesting their recognition by immune receptors in non-host plants. In a first for foliar forest pathogens, we also developed a novel protein infiltration method to show that tissue-cultured pine shoots can respond with a cell death response to one of our DsCEs, as well as to a reference cell death-inducing protein. These results contribute to our understanding of forest pathogens and may ultimately provide clues to disease immunity in both commercial and natural forests.

Genic introgression from an invasive exotic fungal forest pathogen increases the establishment potential of a sibling native pathogen

Significant hybridization between the invasive North American fungal plant pathogen Heterobasidion irregulare and its Eurasian sister species H. annosum is ongoing in Italy. Whole genomes of nine natural hybrids were sequenced, assembled and compared with those of three genotypes each of the two parental species. Genetic relationships among hybrids and their level of admixture were determined. A multi-approach pipeline was used to assign introgressed genomic blocks to each of the two species. Alleles that introgressed from H. irregulare to H. annosum were associated with pathways putatively related to saprobic processes, while alleles that introgressed from the native to the invasive species were mainly linked to gene regulation. There was no overlap of allele categories introgressed in the two directions. Phenotypic experiments documented a fitness increase in H. annosum genotypes characterized by introgression of alleles from the invasive species, supporting the hypothesis that hybridization results in putatively adaptive introgression. Conversely, introgression from the native into the exotic species appeared to be driven by selection on genes favoring genome stability. Since the introgression of specific alleles from the exotic H. irregulare into the native H. annosum increased the invasiveness of the latter species, we propose that two invasions may be co-occurring: the first one by genotypes of the exotic species, and the second one by alleles belonging to the exotic species. Given that H. irregulare represents a threat to European forests, monitoring programs need to track not only exotic genotypes in native forest stands, but also exotic alleles introgressed in native genotypes.

Two new Nothophytophthora species from streams in Ireland and Northern Ireland: Nothophytophthora irlandica and N. lirii sp. nov.

Slow growing oomycete isolates with morphological resemblance to Phytophthora were obtained from forest streams during routine monitoring for the EU quarantine forest pathogen Phytophthora ramorum in Ireland and Northern Ireland. Internal Transcribed Spacer (ITS) sequence analysis indicated that they belonged to two previously unknown species of Nothophytophthora, a recently erected sister genus of Phytophthora. Morphological and temperature-growth studies were carried out to characterise both new species. In addition, Bayesian and Maximum-Likelihood analyses of nuclear 5-loci and mitochondrial 3-loci datasets were performed to resolve the phylogenetic positions of the two new species. Both species were sterile, formed chlamydospores and partially caducous nonpapillate sporangia, and showed slower growth than any of the six known Nothophytophthora species. In all phylogenetic analyses both species formed distinct, strongly supported clades, closely related to N. chlamydospora and N. valdiviana from Chile. Based on their unique combination of morphological and physiological characters and their distinct phylogenetic positions the two new species are described as Nothophytophthora irlandica sp. nov. and N. lirii sp. nov. Their potential lifestyle and geographic origin are discussed.

Population genomics of the emerging forest pathogen Neonectria neomacrospora

ABSTRACTThe fungal pathogen Neonectria neomacrospora is of increasing concern in Europe where, within the last decade, it has caused substantial damage to forest stands and ornamental trees of the genus Abies (Mill.). Using whole-genome sequencing of a comprehensive collection of isolates, we show the extent of three major clades within N. neomacrospora, which most likely diverged around the end of the last Ice Age. We find it likely that the current European epidemic of N. neomacrospora was founded from a population belonging to the east North American clade. All European isolates (1957-2019) had a common evolutionary history, but substantial and asymmetrical gene flow from the larger American source population could be detected. The European population shows multiple signs of having gone through a bottleneck and subsequent population expansion.

Diplodia sapinea in Swedish forest nurseries

Diplodia sapinea is a common forest pathogen on Pinus spp. in a large part of the world. In 2013, disease caused by this pathogen on Scots pine (Pinus sylvestris) trees in Sweden was reported for the first time. In this study, we report the first detection of D. sapinea on diseased seedlings of P. sylvestris from two Swedish forest nurseries. Infected seedlings were collected July–November 2019. Diplodia sapinea was identified by morphological characteristics of fungal structures on plant tissues and from culture grown on Hagem agar media, followed by sequencing of fungal ITS rDNA. The result emphasizes the susceptibility of P. sylvestris seedlings. More research is needed to better understand the risk for disease spreading within forest nurseries and into the forest through infected plant material.