Mating type ratios and pathogenicity in Diplodia shoot blight fungi populations: Comparative analysis

Diplodia sapinea and Diplodia scrobiculata are opportunistic pathogens of Pinus species. Several studies about taxonomy, impact and epidemiology of these fungi have been conducted in previous years, which have provided useful information and have raised new issues. These diseases produce a considerable impact on plantations resulting in significant economic losses. The main aims of this study are to increase the knowledge of the potential of genetic exchange and the relative aggressiveness of these organisms that can persist in healthy tissues of asymptomatic trees. A collection of 250 isolates among which are 149 strains collected from Pinus radiata plantations in Basque Country (Spain) and 101 strains from different countries was included in this work. Mating type ratios were analysed and compared using the structure of the MAT locus (MAT1‐1‐1 and MAT1‐2‐1). Inoculations of Pinus radiata seedlings were performed in a biosafety greenhouse (P2) to confirm pathogenicity of isolates and compare their aggressiveness. The frequency of occurrence of both idiomorphs of D. sapinea in Basque Country isolates was close to 1:1, however, for collection of isolates of this fungus from around the world, the ratio was 1:2. Furthermore, the spatial distribution of the two mating types in the Basque Country was random. Despite no detection of a sexual state, these results could suggest sexual reproduction behaviour. The pathogenicity of all strains in the collection was confirmed. Although aggressiveness (in terms of lesion lengths resulting from inoculation) varied greatly, no statistically significant effects of MAT type or pathogen species were detected.     

The pine pathogen Diplodia sapinea is associated with the death of large Douglas fir trees

Climate change poses severe pressures to European conifer forests. Using non-native tree species, such as Douglas fir (Pseudotsuga menziesii), is one proposed strategy to circumvent adverse effects for forest management. However, novel forest health risks can impair the cultivation of non-native trees. In 2022, we observed large Douglas fir trees (approximately 40–50 years old, diameter at breast height (dbh) 21–41 cm) that had recently died in spring or summer 2022 in three forest stands in Eastern Austria. Intensive resin flow, blue-staining of the sapwood and the absence of bark- and wood-boring insects indicated a fungal infection. Isolations from blue-stained sapwood of the dead trees consistently yielded cultures of the opportunistic pathogen Diplodia sapinea. In a greenhouse wound inoculation experiment, seven D. sapinea isolates obtained from Douglas fir caused phloem necrosis, blue-staining of sapwood and mortality and thus displayed pathogenicity towards seedlings of both Ps. menziesii and its common host, Scots pine (Pinus sylvestris). Diplodia sapinea produced significantly longer areas of blue-stain as well as higher and faster mortality in Ps. menziesii compared to P. sylvestris. We conclude that D. sapinea substantially contributed to the death of seven of the 13 examined large Douglas fir trees. While this fungus has been described as a pathogen of young Douglas fir trees before, this is the first report that it can potentially kill large individuals of this conifer species under drought conditions. Thus, our results indicate that D. sapinea could represent a severe threat to the cultivation of Ps. menziesii in European forestry.     

Control of Hymenoscyphus fraxineus,the causal agent of ash dieback,using composting

Viability of Hymenoscyphus fraxineus inocula following temperature treatments for different exposure times was examined in vitro and in aerated flask‐ and large‐scale composting tests using green waste. After an exposure for up to 10 days at 20°C, 97.3% of H. fraxineus mycelium and pseudosclerotia plate cultures remained viable. No viability was detected following a 3‐day exposure to 40°C or a 1‐day exposure to 45°C although pseudosclerotia were more tolerant than mycelium to an exposure to 35°C. Primordial apothecia of H. fraxineus emerged from 62%–100% of infected ash rachises collected from two infected sites and stored at 4°C for 0–5 months; exposure to compost for up to 10 days at 20°C did not affect this emergence. No emergence of H. fraxineus apothecia was observed from ash rachises that were exposed to compost at 45°C for 1 day or at 35°C or 40°C for 3 days in flasks or at 40°C for 1 day or at 30°C for 5 days in a large‐scale composting system. Based on a fitted model, estimates of the survival of H. fraxineus inoculum in infected ash rachises exposed to compost at 50°C for 1 day were 0.081% of that in the untreated H. fraxineus ash rachis inoculum. Increasing loss in viability of H. fraxineus inoculum in infected ash rachises during longer and warmer exposures to compost at 35°C–45°C corresponded with a reduced concentration of pathogen DNA detected in the rachises using real‐time PCR. However, exposure of rachises to compost at >53°C resulted in a smaller reduction in pathogen DNA detected than exposure to compost at lower temperatures, possibly due to the inhibition of enzymatic degradation of DNA at elevated temperatures.     

Life cycle and in vitro sporulation dynamics of Corinectria constricta,the causal agent of Pinus radiata stem canker,in Chile

In 2008, a canker disease caused by the fungus Corinectria constricta was detected in southern Chile. The causal agent was previously identified as Neonectria fuckeliana (now Corinectria fuckeliana), which has been associated with stem cankers in Pinus radiata plantations in New Zealand since the 1990s. Many basic aspects of the life cycle of C. constricta remain unknown. The current study aimed to (a) document the periods during which C. constricta fruiting bodies are present in P. radiata plantations and associated factors; (b) determine the C. constricta life cycle in P. radiata plantations in southern Chile; and (c) evaluate, under in vitro conditions, the sporulation dynamics of ascospores. The first and second aims were carried out by evaluating affected plantations every 15 days, identifying asexual and sexual fungal structures, and recording the time periods when the structures were present. The third aim was achieved with in vitro tests in Petri dishes simulating humidity chambers. The life cycle was characterized by the presence of sporodochia from the Cylindrocarpon‐like (asexual form of C. constricta) morph during the autumn of 2012 (March–May). Subsequently, perithecia began to form on the sporodochia during April of 2012, taking approximately 3 months to mature (May–July), persisting for the rest of the year and providing inoculum to infect new trees. The development of perithecia in winter demonstrates that this is the most important period for dispersal and infection. In terms of sporulation dynamics, perithecia can release ascospores up to eight days following a wetting event; without this event, the spores are not released.     

Identification and pathogenicity of Alternaria alternata causing leaf spot and blight disease of Ailanthus excelsa in India

Alternaria leaf spot of Ailanthus excelsa is generally considered as minor disease in India. Recently, severe disease outbreaks were recorded in the nursery of the Forest Research Institute, Dehradun, progeny trial at Jhumpa, Haryana, and in a nearby farm field. Leaf symptoms included small circular, brown, necrotic spots with a chlorotic halo. With severe infections, leaf spots coalesced and resulted in leaf blight. A small‐spored Alternaria with concatenated conidia was isolated consistently from the leaf samples with spot symptoms. Sequence analyses of the internal transcribed spacer (ITS) region of ribosomal DNA (rDNA) and the translation elongation factor 1‐alpha (tef‐1α) gene region of two fungal isolates confirmed the species as A. alternata. In detached leaf assays, both isolates produced symptoms similar to those observed on the nursery/field‐grown plants. To validate the detached leaf assay result, pathogenicity was also demonstrated on whole plants in a glasshouse. Koch's postulates were fulfilled by re‐isolating A. alternata from the inoculated leaves. This work is the first to confirm that A. alternata is associated with leaf spot and blight disease of A. excelsa in India.     

Mechanisms and metabolomics of the host–pathogen interactions between Chestnut (Castanea species) and Chestnut blight (Cryphonectria parasitica)

Chestnut blight is a stem‐girdling disease of Castanea caused by the fungal pathogen Cryphonectria parasitica. Chestnut blight affects all Castanea species to some degree. In Asian species, chestnut blight is a commercially relevant disease which primarily affects nut production. In American and European species, chestnut blight has caused significant declines in wild populations and continues to negatively affect nut production in the European chestnut (C. sativa). Despite the profound effect of this disease in the Castanea genus, very little is known concerning the factors involved in the host–pathogen interaction between C. parasitica and its Castanea hosts. This review summarizes information on known mechanisms and metabolites involved in the host–pathogen interaction and contributes original information on the pathogen in relation to susceptible and putatively resistant genotypes with a view to furthering research that will promote a better understanding of this devastating disease and enable its control.     

First report of Lecanosticta acicola on non‐native Pinus mugo in southern Sweden

In 2017, severe symptoms of brown spot needle blight, similar to those caused by Lecanosticta acicola, were observed on needles of non‐native Pinus mugo var. Hesse planted in an arboretum in southern Sweden. Microscopic characterization and molecular diagnostics of isolates obtained from diseased needles confirmed the identity of L. acicola. This is the first report of the quarantine pathogen L. acicola in Sweden. Subsequent surveys are urgently needed to assess the presence and extent of the L. acicola invasion in Scots pine (Pinus sylvestris) stands in Sweden and other Scandinavian countries.     

Foliar nematode,Litylenchus crenatae ssp. mccannii,population dynamics in leaves and buds of beech leaf disease‐affected trees in Canada and the US

A foliar nematode, Litylenchus crenatae ssp. mccannii, is associated with beech leaf disease (BLD) symptoms. Information about the types of tissues parasitized and how nematode populations fluctuate in these tissues over time is needed to improve surveys as well as understand the nematodes role in BLD. During this study, the nematode was detected throughout the known range of BLD by researchers at both Canadian and US institutions using a modified pan method to extract nematodes. Monthly collections of symptomatic and asymptomatic leaves during the growing season (May–October), and leaves and buds between growing seasons (November–March), revealed that nematodes were present in all tissue types. Progressively larger numbers of nematodes were detected in symptomatic leaves from Ohio and Ontario, with the greatest detections at the end of the growing season. Smaller numbers of nematodes were detected in asymptomatic leaves from BLD‐infected trees, typically at the end of the growing season. The nematode was detected overwintering in buds and detached leaves. The discovery of small numbers of nematodes in detached leaves at one location before BLD was detected indicates that nematodes may have been present before disease symptoms were expressed. Other nematodes, Plectus and Aphelenchoides spp., were infrequently detected in small numbers. Our findings support the involvement of the nematode in BLD and indicate that symptoms develop only when certain requirements, such as infection of buds, are met. We also found that the nematode can be reliably detected in buds and leaves using the modified pan extraction method.     

Fusarium species in declining wild apple forests on the northern slope of the Tian Shan Mountains in north‐western China

Wild apple forests in the Tian Shan Mountains in north‐western China have been adversely affected by an unknown disease in recent years. Symptoms attributed to this disease that affects wild apple trees include xylem browning and dieback which are suggestive of infection by Fusarium species. Therefore, the research team conducted the first survey for Fusarium in the afflicted wild apple forests. Twig samples with symptoms of xylem browning and dieback were collected in the Xinyuan, Gongliu, Yining and Huocheng Counties of Xinjiang Uighur Autonomous Region in China. Based on phylogenetic analyses and morphological observation, sixty strains of Fusarium accounted for 48% of the total number of fungi isolated from samples were subsequently classified into six species including twenty‐four F. avenaceum, seventeen F. solani, ten F. tricinctum, five F. proliferatum, two F. sporotrichioides and two unfamiliar Fusarium sp. 1. The five previously known species of Fusarium were then tested for pathogenicity to leaves and twigs in vitro. The results indicated that all of the species, except for F. tricinctum, can cause obvious lesions on the leaves of host plants and on the twigs of Fuji and wild apple. This is the first report of Fusarium species pathogenicity in Xinjiang wild apple forests, confirming a new host for these pathogens in this study.     

Phylogeography and host range of Armillaria gallica in riparian forests of the northern Great Plains,USA

Root disease pathogens, including Armillaria, are a leading cause of growth loss and tree mortality in forest ecosystems of North America. Armillaria spp. have a wide host range and can cause significant reductions in tree growth that may lead to mortality. DNA sequence comparisons and phylogenetic studies have allowed a better understanding of Armillaria spp. taxonomic diversity. Genetic sequencing has facilitated the mapping of species distributions and host associations, providing insights into Armillaria ecology. These studies can help to inform forest management and are essential in the development of disease risk maps, leading to more effective management strategies for Armillaria root disease. Armillaria surveys were conducted on publicly owned lands in North Dakota, South Dakota, and Nebraska, U.S.A. Surveyed stands consisted of riparian forests ≥0.4 hectares in area. Armillaria was found at 78 of 101 sites. A total of 57 Armillaria isolates—associated with 12 host tree species—were used for DNA sequencing of the translation elongation factor‐1 alpha (tef1) gene. Armillaria gallica was the only species identified within the study sites. Results suggest that A. gallica is a common root pathogen of hardwood trees in riparian forests of the northern Great Plains with a wider host range and geographic distribution than previously recognized.     

Protection of spruce from colonization by the bark beetle, Ips perturbatus, in Alaska

Field bioassays were conducted in south-central Alaska in a stand of Lutz spruce, Picea × lutzii, to determine whether a semiochemical interruptant (verbenone and trans-conophthorin) and/or a defense-inducing plant hormone (methyl jasmonate, MJ) could be used to protect individual standing trees from bark beetle attack. During two experiments (initiated in May 2004 and 2005, respectively), attacks by Ips perturbatus on standing trees were induced by using a three-component aggregation pheromone (ipsenol, cis-verbenol, and ipsdienol) and prevented by using the interruptant. In 2005, treatments from 2004 were repeated and additional treatments were evaluated by using MJ spray or injection with and without the interruptant. Aggregation began before 3 or 7 June, and attack density was monitored through 3 or 16 August. During both years, tree mortality caused by I. perturbatus was recorded twice (in August, and in May of the following year). In both experiments, attack density was greatest on trees baited with the three-component attractive pheromone, but was significantly reduced by addition of the semiochemical interruptant to trees baited with the attractant. There were no significant differences in attack density between attractant + interruptant-treated trees and unbaited trees. In 2004, mortality was highest among attractant-baited trees, whereas addition of the interruptant significantly reduced the level of initial (10 week post-treatment) and final (54 week post-treatment) mortality. In 2005, no significant reduction in attack density occurred on trees baited with the attractant when MJ was sprayed or injected. The highest initial (10.6 week post-treatment) and final (49.4 week post-treatment) mortality was observed among trees that had been injected with MJ and baited with the attractant. Mortality at the final assessment was significantly lower in all other treatment groups. As in 2004, addition of the interruptant to attractant-baited trees significantly reduced the level of final mortality compared to attractant-baited trees. MJ was not attractive or interruptive to I. perturbatus or associated bark beetles in a flight trapping study. However, MJ-treated trees (sprayed or injected) exuded copious amounts of resin on the bark surface. Anatomical analyses of felled trees from four treatment groups [Tween (solvent)-sprayed, MJ-sprayed, Tween-injected, and MJ-injected + attractant baited] showed that treatment with MJ increased the number and size of resin ducts produced following treatment. These analyses also revealed a reduction in radial growth in MJ-treated trees. Our results show that during both years, treatment with a simple, two-component interruptant system of verbenone and trans-conophthorin significantly reduced I. perturbatus attack density and tree mortality on attractant-baited trees and provided a full year of protection from bark beetle attack.