The emergence of beech leaf disease in Ohio: Probing the plant microbiome in search of the cause

American beech (Fagus grandifolia) is the target of a newly emerging disease in North America called beech leaf disease (BLD) that affects and disfigures leaves and which can lead to tree mortality. Beech leaf disease may be caused by a newly recognized subspecies of the anguinid nematode Litylenchus crenatae subsp. mccannii, but the associations of this nematode with bacterial and fungal taxa are unknown. We examined microbial communities associated with beech leaves affected by BLD in a 16‐year‐old American beech plantation using molecular methods. We detected L. crenatae subsp. mccannii in anywhere from 45% to 90% of leaves depending on the degree of visual BLD symptoms. Approximately 37% of asymptomatic leaves contained L. crenatae subsp. mccannii, whereas 90% of buds associated with symptomatic leaves contained L. crenatae subsp. mccannii. We found that fungal communities on leaves and buds were unaffected by BLD, but bud and leaves had significantly different fungal communities. Bacterial communities on buds also were unaffected by BLD, but bacterial communities were significantly different between symptomatic and asymptomatic leaves suggesting that the nematode could be altering the community of bacteria on the leaves. Clone libraries indicate that Wolbachia, an intracellular endosymbiont of arthropods, was found only on symptomatic leaves and buds associated with either symptomatic or asymptomatic leaves. In addition, only symptomatic leaves contained taxa in the genus Mucilaginibacter, which previous studies suggest could produce exopolysaccharides. These bacterial taxa could represent a marker for the vector of L. crenatae subsp. mccannii that enables spread between trees and a possible endosymbiont that could facilitate nematode feeding and establishment on nematode infested leaves. Our results are the first to examine changes to the leaf microbiome of this newly emerging pest and may aid identification of mechanisms associated with the spread and success of L. crenatae subsp. mccanni.     

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.     

Beech leaf disease: An emerging forest epidemic

Beech leaf disease (BLD) is a currently undiagnosed and seemingly lethal disease that was discovered in 2012 on American beech trees (Fagus grandifolia) in north‐east Ohio in the United States. Since its discovery, BLD has spread rapidly and can now be found in forests in 10 counties in Ohio, eight counties in Pennsylvania and five counties in Ontario, Canada. The initial symptoms of the disease appear as a dark green, interveinal banding pattern on the lower canopy foliage. These initial symptoms typically occur in the shrub or sampling layer of a beech stand. The later symptoms result in solidly darkened leaves that are shrunken and crinkled. The symptoms appear to progress through the buds as the affected buds are eventually aborted and no new leaves are produced. We fear this disease has the potential to drastically alter the Eastern deciduous forests of the United States on its own and through potential compounding disease effects. In addition, BLD poses a threat to global forests as symptoms of the disease were detected on European (F. sylvatica) and Oriental (F. orientalis) beech species in nurseries in north‐eastern Ohio. Due to its rapid spread and variability in environmental conditions where it has been detected, it seems unlikely that BLD is an abiotic disorder. Thus, intense efforts are underway to determine the causal agent of BLD. Relevant stakeholders are advised to be alert for BLD symptoms in beech forests in the Northern Hemisphere, and substantial resources should be invested in understanding this emerging forest disease.     

The effect of bagging branches on levels of endophytic fungal infection in Japanese beech leaves

The species composition of the endophytic mycobiota in leaves of Japanese beech trees (Fagus crenata) and the sources for leaf infections were studied in a forest reserve situated in central eastern Honshu, Japan. To clarify the mechanism of infection of leaves, half of the branches were covered with polyethylene bags and species composition and levels of endophytic fungal infection were then compared with those of unbagged controls. Isolations were carried out from the leaves, petioles, and current‐year twigs of both, bagged and unbagged branches. Additionally, species composition was detected in overwintered terminal buds of beech trees and in the leaves of potted seedlings that had been placed in the field in different seasons. The species assemblage of the unbagged leaves, petioles, and current‐year twigs was dominated by Mycosphaerella buna, Ascochyta fagi, Periconiella sp., and Tritirachium sp. Other frequently recovered species were Xylaria sp., Phomopsis sp., and Tubakia dryina. Mycosphaerella buna and A. fagi were never isolated from leaves on bagged branches. A. fagi was, however, detected on both bagged and unbagged petioles and current‐year twigs at comparatively low isolation frequencies. The detection of Periconiella sp. on all occasions in both bagged and unbagged leaves was a characteristic feature that differs from those of the other three dominant endophytic fungi. The fungus was also detected without significant differences in bagged and unbagged petioles and current‐year twigs on most sampling dates. Furthermore, Periconiella sp. was isolated from immature twigs inside the bud scales. Tritirachium sp. was frequently detected in unbagged leaves and petioles and in both bagged and unbagged current‐year twigs, and rarely in bagged leaves and petioles, but was never recovered from terminal buds. The results of the potted seedling experiments revealed that all four dominant species had airborne inocula. The infection of leaves by M. buna occurs exclusively by airborne propagules, i.e. ascospores in spring and conidia in autumn. In Periconiella sp. hyphal growth of the fungus from immature twigs inside the buds into the leaf tissues was suggested in addition to infection by airborne inocula. Tritirachium sp. hyphae were suggested to grow from previous‐ to current‐year twigs. Ascochyta fagi was present in the outermost scales of overwintered terminal buds, but no systemic growth of the fungus into the petioles and current‐year twigs was observed. Our technique of covering the branches before new leaves unfolded was effective in preventing infection by airborne inocula of endophytic fungi.     

Pseudodidymella fagi in Slovakia: First detection,morphology and culture characteristics

Pseudodidymella fagi, described from infected leaves of Fagus crenata in Japan, has recently been spreading in Europe, where it causes leaf blotch of European beech Fagus sylvatica. In the years 2008–2016, an outbreak of P. fagi was observed on European beech in Switzerland, Germany and Austria, and it is reported here for the first time from Slovakia where it was detected in 2017. The identification of the pathogen is supported by the morphological analyses of material observed in vivo and in vitro and molecular analyses based on ITS sequences. Detailed morphological characterization of cultures based on European material, on F. sylvatica, is provided here for the first time.     

Scale insects,decay and canker fungi in American beech

Tripartite interactions among phytophagous insects, pathogens and their host plants provide insight into the role of host physiology in determining susceptibility to attack. American beech (Fagus grandifolia) often is simultaneously attacked by beech scale (Cryptococcus fagisuga), one or more Neonectria pathogens and Xylococculus betulae that can result in beech bark disease (BBD). Additionally, beech is frequently infected by heartrot‐decay fungi. Cursory observations in 2011 suggested that beech scale and Neonectria lesion densities were lower and greater, respectively, on trees with decay. In 2012, digital image analysis was used to quantify densities of these organisms on 123 beech from the Adirondack region of New York. Three groups of study trees (n = 41) were used: Inonotus glomeratus‐infected, Phellinus igniarius‐infected and non‐decay trees. Trees infected by decay pathogens supported lower densities of beech scale and higher densities of Neonectria. Densities of X. betulae did not significantly vary among decay groups. These results may be explained by decay‐induced changes in host physiology. Additional work is needed to elucidate the potential role of host bark chemistry in the BBD complex.     

Amino nitrogen and phenolic constituents of bark of American beech,Fagus grandifolia,and infestation by beech scale,Cryptococcus fagisuga

Concentrations of amino acids, total amino nitrogen, and phenols, ratio of phenol to amino nitrogen, and pH in the bark of American beech, Fagus grandifolia, were determined to see if these chemical constituents were correlated with susceptibility of trees to infestation by the beech scale, Cryptococcus fagisuga. The relationship of these bark constituents with scale infestation levels (none, light, and moderate), tree size (large vs. small trees), and bark shading was determined. Levels of some individual amino acids and total amino acid content in moderately infested trees were significantly higher than in uninfested trees. Large uninfested beech trees, which tend to be more readily infested by scale, had significantly higher concentrations of aspartic acid. Concentrations of phenol in the outer bark of both infested and uninfested trees were significantly higher than in the inner bark. However in bark infected by Nectria, phenol levels in the inner bark were significantly higher than in the outer bark.     

Detection of the endophyte Discula umbrinella in buds and twigs of Fagus sylvatica

Endophytic fungi have been isolated from buds and twigs of beech trees collected at four different sites in Switzerland. Discula umbrinella. was recovered at high frequencies from the bud scales and the twig pieces contiguous to the buds, but was virtually absent from the rolled up leaves enclosed by the scales. In addition to infection by air-borne inoculum, thalli of D. umbrinella may grow from the twigs into the leaf tissues.     

Resistance against oxidative stress in leaves of young beech trees grown in model ecosystems with different soil qualities, elevated CO2, and lachnid infestation

Summary The aim of the present study was to investigate whether the resistance of beech foliage(Fagus sylvatica) against oxidative stress was affected by soil quality, nitrogen or CO₂ fertilisation, or lachnid infestation(Phyllaphis fagi). For this purpose young beech trees were grown for four years in reconstructed calcareous or acidic forest soils in open top chambers under ambient or elevated CO₂ concentrations with two levels of nitrogen fertilisation. At harvest lachnid colonisation was observed, preferentially on leaves from trees in calcareous soil and on leaves from trees fertilised with the high nitrogen level. General leaf characteristics such as pigment concentrations, dry mass, and leaf mass ber area were not affected by the soil type, nitrogen fertilisation or CO₂ regime. Leaves colonised with lachnids displayed slightly increased leaf mass per area. When the stress resistance was challenged by exposure to paraquat — a herbicide inducing oxidative stress — leaves from trees grown on calcareous soil maintained significantly longer membrane integrity and, thus, were better protected against stress than leaves from trees on acidic soil. Other experimental variables had negligible or no effects on the resistance against oxidative stress.      

Desiccation-tolerance ofFagus crenata blume seeds from localities of different snowfall regime in central Japan

In beech (Fagus crenata Blume) forests on the Pacific Ocean side in Central Japan, snowpack depth is little and xeric conditions may prevail in winter, in contrast to heavy snow in beech forests on the Japan Sea side. The effects of such conditions during winter on the viability of beech seeds were studied at a beech forest on the Pacific Ocean side. Thickness and weight ratio of pericarp of beech seeds were significantly greater in populations on the Pacific Ocean side compared to the Japan Sea side, this apparently being related to snowpack depth in winter. During the initial stage of seed drying, the drying rate of seeds from the Fuji population (the Pacific Ocean side) was less than that from the Sumon population (the Japan Sea side), possibly due to the thicker pericarp. Germination percentage of seeds which dried to a water content as low as 7% d.w. did not decrease for either population, indicating desiccation tolerance of beech seeds. In the beech forest (University Forest at Yamanaka) on the Pacific Ocean side, water content of seeds sown below litter from both populations remained sufficient for viability during winter. Germinating seeds from the populations on the Japan Sea side were more desiccation-sensitive in early spring, compared to those from the Fuji population, partially due possibly to differences in pericarp thickness.     

Photosynthesis and biomass allocation of beech (Fagus crenata) and dwarf-bamboo (Sasa kurilensis) in response to contrasting light regimes in a Japan Sea-Type beech forest

Regeneration of beech (Fagus crenata) forests depends on the formation of canopy gaps. However, in Japan Sea-type beech forests, a dwarf bamboo (Sasa kurilensis) conspicuously occupies sunny gaps. Therefore,F. crenata seedlings must escape the severe interference ofS. kurilensis in the gaps and persist beneath a closed canopy of the beech forest. We hypothesized that the growth ofF. crenata seedlings in the understory would be favored by their being more plastic thanS. kurilensis in photosynthetic and morphological traits, which would support the matter production ofF. crenata seedlings in a wide range of light availabilities. To examine this hypothesis, the photosynthetic-light response of individual leaves and the biomass allocation in aboveground parts (i.e., the culm/foliage ratio) were surveyed at sites with contrasting light availabilities in a Japan Sea-type beech forest in central Japan. InF. crenata, photosynthetic light utilization efficiency at relatively low light was greater, and the dark respiration rate was smaller in the leaves of seedlings (10 cm in height) beneath the closed canopy than in the leaves of saplings at the sunny forest edge. The culm/foliage (C/F) ratio of theF. crenata seedlings at the shady site was small, suggesting effective matter-production beneath the beech canopy. On the other hand,S. kurilensis both in the gap and beneath the beech canopy showed low plasticity in photosynthesis and the culm/foliage ratio. Because the shoot density ofS. kurilensis was smaller beneath the beech canopy than in the gap, the light availability at the bottom of theS. kurilensis layer was greater beneath the beech canopy. These results suggest that the photosynthetic productivity of theF. crenata seedlings would be enough for the seedlings to survive in the understory with a low density ofS. kurilensis shoots beneath the closed beech canopy.     

Colonization of beech leaves by two endophytic fungi in northern Japan

Leaves of Japanese beech (Fagus crenata) were collected monthly during the vegetation period at five sites in the Tohoku district in Japan to isolate endophytic fungi. Leaves were also collected only once at two additional sites. Two endophytic fungi were dominant, a Discula species and a sterile mycelium. This result strongly suggests that these two fungi are generally associated with leaves of the Japanese beech at different sites. At most sites the isolation frequency of Discula sp. was greatest in June and gradually decreased from July to October whereas the isolation frequency of the sterile mycelium increased during the vegetation period and remained at a high isolation frequency in October. Spores of Discula sp. were released for a very short time in late May, just after the disappearance of the snow cover on the forest floor. These spores may be important for the infection of newly sprouting leaves.     

The Effect of Landfill Leachate Spraying on Foliar Nutrient Concentrations and Leaf Transpiration in a Northern Hardwood Forest, Canada

The effect of municipal solid waste (MSW) leachate spray irrigationon a mature northern hardwood forest was investigated. Canopyfoliar samples and stem increment cores were collected fromtwo indicative species, sugar maple (Acer saccharum Marsh.)and American beech (Fagus grandifolia Ehrh.), within each ofa heavily sprayed, lightly sprayed and control area. Foliarconcentrations of N and P were significantly higher in bothmaple and beech foliage within the sprayed areas when comparedto an unsprayed area (control). Levels of Mg and K were markedlyhigher in maple but not beech foliar samples within the heavilysprayed areas when compared to foliage sampled within the unsprayedcontrol. While no significant trends were observed within themaple foliage, both Fe and B levels increased significantlyin beech foliar samples obtained from within the heavily sprayedarea in comparison to foliage samples from the control. Directporometric measurements of the transpiration rate and diffusive(stomatal) resistance of canopy and understory plant leavesrevealed a significant increase in diffusive resistance anda decline in transpiration rate with leachate spraying. Afterfour years of spraying a significant effect of leachate applicationon radial stem growth of both maple and beech trees has notbeen observed.     

Induced response of the Siebold's beech (Fagus crenata Blume) to manual defoliation

Siebold's beech (Fagus crenata) was manually defoliated for two successive years. The beech caterpillar (Quadricalcarifera punctatella) was used in a bioassay to determine insect performance. Survival and body size were low on foliage from defoliated trees. Reduced foliar nitrogen and increased tannin content were probably the main causes of the low insect performance. Leaves were less tough on defoliated trees than in controls. Two sucessive years of manual defoliation caused stronger induced resistance than one year defoliation. The quality, as well as the quality of the foliage, decreased the year following manual defoliation; total weight of leaves on a tree was less than one half of that before treatment. Severe defoliation may cause a decrease of leaves the following year and starvation may limit populations. Delayed induced resistance of beech trees is proposed as a possible cause of the cyclical population dynamics ofQ. punctatella. The delayed induced response also affected folivorous insects other thanQ. punctatella.     

Nutrient concentrations in roots, leaves and wood of seedling and mature sugar maple and American beech at two contrasting sites

Differences in sensitivity to soil conditions across tree species and developmental stage are important to predicting forest response to environmental change. This study was conducted to compare elemental concentrations in leaves, stems, and roots of (1) sugar maple (Acer saccharum Marsh.) seedlings vs. mature trees and (2) mature sugar maple vs. mature American beech (Fagus grandifolia Ehrh.) in two sites that differ in soil base saturation and pH. Both sites are located in Huntington Forest, NY, USA; one site (hereafter ‘H’) has higher soil pH and Ca, Mg, and Mn concentrations than the other site (hereafter ‘L’). Sugar maple growth at H (14.8 cm² year⁻¹ per tree) was much greater than at L (8.6 cm² year⁻¹ per tree), but the growth of beech was not different between the two sites. Leaves, roots, and stem wood of mature beech trees and sugar maple seedlings and mature trees were sampled for nutrient analysis. Foliar Ca, K, and Al concentrations were positively correlated with soil elements, but Mn concentrations were negatively correlated. Sugar maple differed more than beech between sites in foliar K and Mn concentrations. Root Mg and P concentrations reflected soil chemistry differences, in contrast to foliar concentrations of Mg and P, which were indistinguishable between the sites. In sugar maple, seedlings differed more than in mature trees in nutrient concentrations in roots, especially for Mg and Mn. Although beech was not as responsive to nutrient availability as sugar maple in foliar and root nutrient concentrations, Ca and Mg concentrations in beech wood were higher in H (52% higher for Ca and 68% higher for Mg), while sugar maple did not differ between sites. Sugar maple regeneration failure on acidic soils in the same region is consistent with our finding that sugar maple seedlings were very sensitive to nutrient availability. This sensitivity could ultimately contribute to the replacement of sugar maple by American beech in regions of low pH and base cations if base cation leaching by anthropogenic deposition and tree harvesting continues.     

Notes on beech (Fagus crenata Blume) seed and seedling mortality due to rodent herbivory in a northernmost beech forest, Utasai, Hokkaido

We investigated the mortality of post-dispersal seeds and current-year seedlings of a beech species (Fagus crenata Blume) at the northernmost extent of its geographical range at Utasai (Kuromatsunai), Hokkaido, Japan. Introduced seeds that were protected from rodent predation by wire cages had a higher survival rate from winter to the following spring (mean ± SD: 84 ± 16.7%,n = 10) than controls without cages (mean ± SD: 12 ± 17.9%,n = 10). Unprotected transplanted current-year seedlings with almost fully opened primary leaves never survived more than three days (0%,n = 30), while more than 80% (n = 30) of seedlings within cages survived to the end of the growing season. These results indicate that rodent herbivory is a major mortality factor that strongly prevents the establishment of beech seedlings. In a natural population, we observed that 90% of seedlings (n = 197) disappeared within 10 days after their emergence and rodent herbivory caused this heavy mortality. A herbivorous rodent, the gray-backed vole,Clethrionomys rufocanus bedfordiae, was remarkably abundant in late June (101.5 voles/ha), suggesting that this species strongly affects the formation of the seedling bank. The presence ofC. rufocanus bedfordiae may be one of the reasons for the scarcity of beech seedlings and saplings and the rarity of recruitment in this northernmost beech forest.     

Growth–climate relationship of European beech at its northern distribution limit

In general, trees growing at or near their limit of distribution are more sensitive to climate than those growing at their ecological core. Here we examined the growth–climate relationship of European beech (Fagus sylvatica L.) close to its northern distribution limit in southern Sweden. Tree-ring width chronologies were developed from four well-separated sites where the species was dominant (Djupeåsen, DJ; Baldringe, BLD; Komperskulla, KSK and Ryssberget Nature Reserve, RYSS). The chronologies extended from 52 years (BLD) to 150 years (RYSS). Significant negative relationships were found between tree growth and previous summer (July and August) temperatures at three sites. July temperature of the year of growth had a negative relationship with beech growth at BLD and DJ. In contrast, current summer (July and August) precipitation was positively correlated with beech growth at DJ and KSK. This sensitivity of European beech to drought at its northern limit is in line with the previous research. However, following the exceptionally dry summer in southern Sweden in 1970, a marked growth decrease was noted as well as a shift in the relationship between beech growth and current growing-season temperature. Our results show that that the radial growth of European beech has become more sensitive to drought and precipitation than temperature at its northern distribution limit in the last several decades.     

Endophytische Pilze in Blättern und Ästen gesunder und geschädigter Buchen (Fagus sylvatica L.)

Endophytic fungi in leaves and twigs of healthy and diseased beech trees (Fagus sylvatica L.). Endophytic fungi were isolated from surface-sterilized leaves and twigs of healthy and diseased trees. Three fungal species could be isolated frequently: Apiognomonia errabunda, Diaporthe eres and Bisporella sp. D. eres occurred in leaves of healthy trees more frequently, than in those of diseased trees. It has been hypothesized, that less capacity of water and nutrient uptake of diseased trees is responsible for this effect. A mutualistic symbiosis of A. errabunda and D. eres with beech trees is supposed and discussed.     

Effects of enhanced nitrogen deposition on foliar chemistry and physiological processes of forest trees at the Bear Brook Watershed in Maine

Effects of enhanced nitrogen deposition on nutrient foliar concentrations and net photosynthesis of sugar maple (Acer saccharum Marsh.), American beech (Fagus grandifolia Ehrh) and red spruce (Picea rubens Sarg.) were evaluated at the Bear Brook Watershed in Maine (BBWM). The BBWM is a paired-watershed forest ecosystem study with one watershed treated since 1989 with bimonthly dry ammonium sulfate ((NH₄)₂SO₄) additions at a rate of 25.2 kg N ha⁻¹ year⁻¹, while the other watershed serves as a reference. The (NH₄)₂SO₄ treatment resulted in significant increases in foliar N concentrations for all three species and significant reductions in foliar Ca, Mg and Zn concentrations for American beech and red spruce. Treatment effects on foliar concentrations of other nutrients were not significant in any species. Despite higher N concentrations in all species, only treated sugar maple showed significantly higher photosynthetic rates. The non-response in net photosynthesis to higher foliar N in American beech and red spruce might be attributed to their low foliar Ca and/or Mg concentrations. Higher net photosynthetic rates in sugar maple might be explained by the higher foliar N and by the ability of this species to maintain an adequate Ca and Mg supply. Results suggested that nutrient imbalances due to inadequate supply of Ca and Mg might have counteracted a potential increase in net photosynthesis induced by higher N concentrations in American beech and red spruce.