Tree nurseries play a key role in the Canadian economy and reforestation efforts, producing over 600 million seedlings annually. Despite rigorous management practices, nurseries are not exempt from pathogen outbreaks, which can be devastating on many levels. In October 2022, the public forest nursery of St-Modeste (Canada) noted an unusually high mortality rate among their 2-year-old balsam fir (Abies balsamea ) seedlings. Phytophthora abietivora, recently identified as responsible for the Phytophthora root rot (PRR) in Christmas tree plantations, was suspected to be the causative agent of the outbreak. The objectives of this study were to identify the pathogen(s) responsible for the outbreak in the nursery and determine its pathogenicity and transmissibility to other seedlings. After the isolation of the pathogen and molecular detection, it was confirmed that the epidemic was caused by P. abietivora. The pathogen was not only found on healthy-looking balsam seedlings, but also on many other tree species grown in the nursery showing no above-ground PRR symptoms, such as spruce seedlings. The strain isolated in the nursery proved to be highly infectious to Fraser fir seedlings, and results were exacerbated by artificial flooding of seedlings. More worryingly, the disease could be transmitted to susceptible recipient seedlings from asymptomatic donor seedlings. The pathogen could be detected in soil and roots from both donor and recipient seedlings. Together, these findings indicate the first report of an outbreak of P. abietivora on balsam fir seedlings under nursery conditions. Efforts must be increased to minimise economic losses and to manage future outbreaks better in order to protect Christmas trees and forests.
Beech leaf disease (BLD) is rapidly spreading throughout beech forests in the northeastern regions of North America, posing a significant ecological threat to these ecosystems. The etiological agent in this disease is the foliar nematode Litylenchus crenatae. To unravel the molecular mechanisms governing L. crenatae parasitism and pathogenicity, it is essential to analyse its gene expression profiles. Accurate quantification of gene expression using reverse transcription quantitative PCR (RT-qPCR) requires stable internal reference genes for normalisation. To date, no comprehensive studies have identified or validated suitable internal reference genes for L. crenatae across relevant stages of BLD. In this study, we evaluated nine candidate reference genes from L. crenatae and systematically evaluated their expression stability across various developmental stages and plant-interaction conditions. Our analysis identified two genes coding an EF-hand domain-containing protein (EF-hand) and a Ubiquitin-conjugating enzyme 2 (UBQ2) that exhibited the most stable expression profiles, indicating their suitability as internal controls for RT-qPCR assays in this nematode. Utilising these reference genes, we further characterised the expression profiles of four parasitism-related genes. These target genes were assessed across different nematode developmental stages collected from infected buds and leaves. Overall, our results provide suitable reference genes for accurate gene expression studies in L. crenatae, contributing to a better understanding of the molecular interaction between this nematode and its beech hosts.
Larch (Larix) and, to a lesser extent, rhododendron (Rhododendron ponticum) have emerged as key drivers of the Phytophthora ramorum epidemic in Britain because of the copious numbers of sporangia that are released from their foliage following infection; the inoculum then initiates stem and foliar infections on nearby broadleaf and conifer species. This has raised concerns that other conifer species could play a similar role to larch, with most concern centred on Sitka spruce which currently comprises > 50% of all commercial conifer plantations in Britain. To address this, we assessed the potential of the EU1 lineage of P. ramorum to sporulate on foliage of conifers already known to have some susceptibility (Sitka spruce (Picea sitchensis), grand fir (Abies gradis), Douglas fir (Pseudotsuga menziesii), western hemlock (Tsuga heteophyllum) and coastal redwood (Sequoia sempervirens)). Our findings suggested these conifers are much less effective at sustaining sporulation (means ranging from ~16 to ~49 sporangia per cm2 of needle) compared with larch (nearly 600 per cm2). Lesions formed by both EU1 and EU2 lineages of P. ramorum in Sitka spruce bark were significantly smaller than those in larch, indicating that spruce is a less susceptible host. However, substantial zoospore concentrations (50,000 to 500,000 mL−1) were required to cause infections through intact bark of both larch and spruce. Outcomes of these comparisons are discussed in the context of the frequency with which larch and Sitka spruce were found to be naturally infected during surveys of P. ramorum in Britain. Overall, we conclude that in contrast to larch, Sitka spruce and several other conifers grown in Britain do not have potential to sustain an epidemic of P. ramorum, although they may possibly act as low-level reservoirs of infection.
Aesculus is a genus of woody plant species that contains multiple ecologically and ornamentally important species. Powdery mildew is common on this host genus and is particularly virulent on the economically significant horse chestnut, Aesculus hippocastanum. Previously, all Erysiphe species found on Aesculus spp. were identified as Erysiphe flexuosa. Recent genomic research has indicated that powdery mildew species frequently show a high degree of host specificity which was not captured by traditional morphological approaches. As such, we proceeded to collect and sequence multiple regions of the powdery mildew genome to determine the causal agents of disease on different Aesculus spp. The results of our multilocus phylogenetic analyses revealed the existence of a genetically distinct species, which has been found only on Aesculus spp. native to North America. Erysiphe aesculi-sylvaticae sp. nov. is proposed for this cryptic species. In the past 25 years, there have been multiple reports of an E. flexuosa epidemic on Aesculus hippocastanum in Europe, while E. aesculi-sylvaticae has yet to be identified outside of the United States. The discovery of this new cryptic powdery mildew species as well as analysis of additional sequences from collections of E. flexuosa from North America, including from the type specimen collected in 1872, are discussed in detail with an emphasis on the implications for the co-evolution, worldwide spread, and invasion dynamics of these powdery mildews. Future research should continue to collect and sequence powdery mildews on different Aesculus spp. to better understand the diversity and spread of these economically important pathogens.
The invasive deciduous tree Ailanthus altissima, native to China, has rapidly spread worldwide and is now considered one of the most ecologically disruptive species. During a survey carried out in 2024 in Rome province, trees exhibiting severe dieback symptoms were documented, including canopy decline, internal wood discoloration and necrotic lesions on both the outer and inner bark. Morphological and molecular analyses identified Neofusicoccum parvum as the causal agent, and pathogenicity tests confirmed its role in disease development. Given the invasiveness of A. altissima and the high pathogenic potential of N. parvum, their interaction could provide new chances for natural constraint to its spread. This study represents the first report of N. parvum infecting A. altissima in Italy, emphasising its significance for ecosystem dynamics and invasive species containment.
Platycladus orientalis is widely distributed in China and is an excellent afforestation tree with uses for timber, medicinal, ornamental and landscape purposes. Branch blight was observed on cypress plants in Zhejiang Province of China. Three fungal isolates (CSAL1, CSAL2 and CSAL3) were obtained by tissue isolation from diseased samples. The isolates were identified as Alternaria alternata based on morphological characteristics and a multi-gene phylogenetic tree constructed by combining sequences of the internal transcribed spacer (ITS), large subunit ribosomal RNA (LSU), glyceraldehyde-3-phosphate dehydrogenase (gapdh), translation elongation factor 1-alpha (tef 1) and Alternaria alternata major allergen (Alt a 1). The pathogenicity of the isolates was confirmed by fulfilling Koch's postulates. To the best of our knowledge, this is the first report of A. alternata causing branch blight on P. orientalis in China.
Calonectria leaf blight is among the most prevalent diseases affecting planted Eucalyptus in Southeast Asia. Calonectria reteaudii was recently identified as the main causal agent of a localised epidemic in Malaysia, and a number of isolates were collected from diseased leaves and soils associated with symptomatic trees. This provided an opportunity to gain a better understanding of the pathogen ecology, particularly the relationship between soil and phyllosphere inoculum sources. A set of eight polymorphic SSR markers was used to genotype 21 isolates from leaves and 23 isolates from soils collected beneath infected trees. In addition, mating types of all isolates were identified using PCR-based diagnostic mating-type primers. Low genotypic diversity was found in both the leaf and soil populations of C. reteaudii. However, the isolates from diseased leaves exhibited higher levels of genotypic and genetic diversity compared to those from associated soils. All multilocus genotypes identified in the soil isolates were also represented in the leaf population. While all isolates from the soil were of the MAT1-1 mating type, both MAT1-1 and MAT1-2 were identified in isolates from the diseased leaves, but the MAT1-1/MAT1-2 ratio deviated significantly from a 1:1 ratio. The high level of clonality observed for the isolates suggests that conidia are most likely the primary source of pathogen dispersal. The results also suggest that isolates of C. reteaudii from the soils collected beneath heavily infected trees likely originated from the phyllosphere and not vice versa.
The vital activity of Erysiphe alphitoides, as a major foliar fungal pathogen, causes the most important biotic stress; being chronic disease, this fungal infection spreads throughout the oakeries in Europe. In some cases, this epiphytotic is considered as the main reason for the slowdown in the natural recovery of the common oak population in Europe. The pathogen has the greatest impact on seedlings, resulting in their mortality. We revealed a clearly confirmed unfavorable impact of powdery mildew on all critical and calculated indicators of the Kautsky curve in the infected Quercus robur leaves. Regardless of the type of oak forest, an influence of the fungal pathogen on photosynthetic processes in annual seedlings has been established. In conditions of E. alphitoides-caused biotic stress, Q. robur seedlings experienced violations of the photosynthetic apparatus, progressed over time and depended on the damage severity. The powdery mildew was confirmed to affect most of the plant physiological traits studied. In oak seedlings with the leaf surface damaged by powdery mildew more than 50%, alterations in the chlorophyll fluorescence induction were revealed, quite different in their intensity and direction, compared to the healthy (fungi-unaffected) leaves. This suggests that all the studied parameters might be used as indicators of the most stressed state of common oak seedlings.
Woodwasps of the genus Sirex are known to have an obligatory symbiotic relationship with Amylostereum fungi. During oviposition, these wasps inoculate arthrospores of the fungus along with a toxic mucous secretion. As a result, attacked conifer trees can exhibit symptoms of decline and death. The aim of this study was to evaluate the diversity of Amylostereum fungi associated with Sirex obesus. To achieve this, female S. obesus wasps were collected from Pinus plantations in Brazil. The mycangia were removed from the wasps using a stereoscope under aseptic conditions. These structures were subsequently macerated in sterile 0.85% saline solution, and aliquots of the resulting suspension were plated onto PDA medium. Fungal growth was monitored daily, and the colonies were purified and identified by sequencing the ITS-rRNA gene region. As a result, we observed that Amylostereum chailletii and Amylostereum areolatum were present in the mycangia of the dissected wasps, with A. chailletii occurring more frequently (93.3%). The association between A. areolatum and S. obesus still requires further study. This discovery has direct implications for the control of this species of wood wasp and may serve as a basis for future research aimed at developing management strategies.
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