The effects of pine mistletoe (Viscum album subsp. austriacum) on the growth of Scots pine and Crimean pine in Turkey

In this study, the effect of pine mistletoe (Viscum album subsp. austriacum) on basal area increment of Crimean pine and Scots pine was investigated. Dendrochronological data were collected from 223 (71 uninfected and 152 infected) Crimean pines and 195 (77 uninfected and 118 infected) Scots pines located in Kastamonu province of Turkey in 2014. Infected sample trees were classified as light, moderate or severe infection levels. Growth trends and basal area increment loses were compared between uninfected and infected trees for the periods of the last 10, 20 and 30 years. In addition, infection status of forest stands was investigated using temporary sample plots; 27 plots in Crimean pine stands and 26 plots in Scots pine. Results demonstrated that basal area increments were negatively affected by pine mistletoe for both species. Mean basal area increment losses of infected trees for the last decade were determined as 24% for Scots pine and 26% for Crimean pine. Basal area increment losses varied by infection levels (light, moderate and severe) as follows: 25%, 20% and 28% for Scots pines and 20%, 32% and 9% for Crimean pines. Scots pine stands were more severely infected by pine mistletoe than Crimean pine stands. There were negative correlations between number of infected trees and stand density for both species, while positive correlation was detected between the number of infected trees and mean diameter for Scots pine. The results of this study indicate that the pine mistletoe infection has negative effect on radial growth of Scots pine and Crimean pine trees. The results can be an important contribution to the forest management and protection activities in mistletoe-infected stands.     

Fungi in leaves,twigs and stem bark of Populus tremula from northern Spain

Fungal species were recorded from Populus tremula leaves, twigs and bark (trunk) at eight sites in the northern region of Spain, with the aim of evaluating the effect of sampling site, sampling tissue and isolation method on the frequency and species distribution of the fungi recovered from necrotic and healthy tissues. Two different isolation methods were used: the first consisted of finding fruitbodies on plant tissues after incubating them in wet and warm conditions, and the second included growing mycelia from plant fragments plated on potato dextrose agar (PDA). A total of 48 fungal species were isolated from 960 plant fragments. Cytospora chrysosperma, previously reported to be a pathogen of poplar, was recovered very frequently and was recorded as much in healthy as in dead or dying tissues. Cladosporium maculicola, Elsinoe veneta, Alternaria alternata, Aureobasidium pullulans and Pollaccia radiosa were also frequently isolated. The isolation frequencies from necrotic and healthy plant tissues were significantly different between sites and isolation methods. The incubation in moist chambers revealed significantly more fungal isolates and a higher species richness (38 species) than isolations on PDA. However, 16% of the fungal species were exclusively recorded from cultures. Therefore, a combination of several isolation methods is recommended for surveys of fungal communities associated with trees.     

Occurrence and pathogenicity of fungi in necrotic and non-symptomatic shoots of declining common ash (<Emphasis Type="Italic">Fraxinus excelsior</Emphasis>) in Sweden

Currently, massive dieback of Fraxinus excelsior is observed in countries of eastern, northern and central Europe, and the reasons for it are unclear. The aims of the present work were (a) to study fungal communities in declining F. excelsior crowns; (b) to clarify role of fungi in the decline. Shoots from symptomatic crowns were collected in four localities in central Sweden, and distributed into the following categories: (a) visually healthy; (b) initial necroses; (c) advanced necroses; (c) dead tops. The most frequently isolated fungi were Gibberella avenacea, Alternaria alternata, Epicoccum nigrum, Botryosphaeria stevensii, Valsa sp., Lewia sp., Aureobasidium pullulans and Phomopsis sp., and these taxa were consistently found in shoots of all four symptomatic categories. Forty-eight taxa of other fungi were isolated, and fungal diversity was not exhausted by the sampling effort. The same taxa of fungi were dominant in F. excelsior shoots of different symptomatic categories, and moderate to high similarity of fungal communities was observed in shoots despite the symptoms. Forty-four isolates from 24 fungal taxa were used for artificial inoculations of 277 1-year-old F. excelsior seedlings in bare root nursery. After 2 years, only four fungi caused symptomatic necroses of bark and cambium: A. alternata, E. nigrum, Chalara fraxinea and Phomopsis sp. The most pathogenic was C. fraxinea, inducing symptoms on 50% of inoculated trees, while three other fungi caused necroses on 3–17% of inoculated trees. Infection biology of C. fraxinea and environmental factors determining susceptibility of F. excelsior to decline deserve future investigations.     

Hosts of Viscum album

The reported hosts of Viscum album include more taxa, than any other mistletoe, including Dendrophthoe falcata. From a review of the literature, we identified hosts among 452 species, subspecies, varieties, and hybrids in 96 genera of 44 families. Rosaceae has the most hosts with 128 taxa parasitized. Commonly infested genera include Salix, Populus, Acer, Malus, Crataegus, Prunus, Sorbus, Abies, and Pinus. The host range of V. album increased greatly with introduction of susceptible taxa into its native range and with spread of the mistletoe to new regions with rich floras, such as California.     

Pine mistletoe (Viscum album ssp. austriacum) contributes to Scots pine (Pinus sylvestris) mortality in the Rhone valley of Switzerland

In recent years unusual high mortality of Scots pine (Pinus sylvestris) has been observed in the Swiss Rhone Valley. The exact causes, however, are not known. At a 2‐ha monitoring plot, tree mortality and crown condition have been monitored since 1996. Between 1996 and 2004, 59% of the Scots pines died, most of them following the drought periods 1996–1998 and 2003–2004, while only 15% of the deciduous trees died. Crown transparency, needle discolouration, dead branch percentage, mistletoe (Viscum album ssp. austriacum) rating, Tomicus sp. shoot feeding, male flowering effect, tree stem diameter, crown shading and social tree class assessed in 1998 were used in a logistic regression model to predict tree mortality. Crown transparency, mistletoe rating and percentage of dead branches were found significant in the model and the probability of tree mortality increased with increasing rankings of these parameters. Needle discolouration could be used to substitute ‘dead branch percentage’ as predictor. While crown transparency increased with mistletoe rating, for trees in the same transparency class, trees with medium and heavy mistletoe infection were two to four times more likely to die than trees with no or only low mistletoe infection. For the surviving trees we found that trees with mistletoes showed a significantly higher increase in transparency in the year following a drought than trees without, while in a drought year the opposite was true. At the beginning of the observations no significant differences in transparency had been found between the trees with and without mistletoe. However, by the end of the observation period trees with mistletoe had significantly higher crown transparency. We conclude that mistletoe infection can be considered as both a predisposing factor for tree death, by increasing needle loss following drought and a contributing factor by increasing water stress during drought.     

Host specificity of co‐infecting Botryosphaeriaceae on ornamental and forest trees in the Western Balkans

The Botryosphaeriaceae is a diverse family of endophytes and fungal pathogens of mainly woody plants. We considered the host range and distribution of these fungi by sampling diseased ornamental and forest trees and shrubs in Serbia, Montenegro, Bosnia and Herzegovina, spanning a Mediterranean and a Continental climatic region. In total, ten Botryosphaeriaceae species were identified in the Western Balkans and with the exception of Sphaeropsis visci and Phaeobotryon cupressi, which occurred on one host, all the species had a broader host range. Phaeobotryon cupressi was found only in the Mediterranean region and S. visci, Dothiorella sp., Dothiorella sarmentorum and Diplodia seriata were present only in the Continental region. Pathogenicity tests were conducted on a variety of hosts from which the Botryosphaeriaceae species were isolated. These included leaves and/or stems of seedlings of 21 hosts, and cut leaves and/or branches of six hosts. Moreover, stems of seedlings of Chamaecyparis lawsoniana, Cedrus deodara, Picea omorika, Pinus patula and Eucalyptus grandis were inoculated as hosts from which some or all of the Botryosphaeriaceae species used for inoculation were not isolated. Inoculations showed that the majority of these fungi could also co‐infect hosts other than those from which they were isolated. The results suggest that most of the species have broad host ranges and can potentially cause disease on a broad range of tree species under certain conditions.     

Epidemiology of Heterobasidion abietinum and Viscum album on silver fir (Abies alba) stands of the Pyrenees

In the last two decades, stand decline and increased mortality has affected silver fir (Abies alba) forests in the Spanish Pyrenees. Simultaneously severe occurrences of the root rot fungus Heterobasidion annosum s.l. and of the mistletoe Viscum album have been reported. We aimed to improve the understanding of the epidemiology of both pathogens in our region. All H. annosum isolates found on silver fir were typed as H. abietinum. H. abietinum was more frequently observed where cuttings had targeted fir trees rather than other species. H. abietinum fruiting bodies were observed in the most recently cut stumps. V. album was more abundant on more dominant fir trees, and in southern aspect stands. The number of V. album colonies in the stand correlated (R² = 0.40) with silver fir mortality. Stands with a high level of V. album infection tended to have a smaller percentage of basal area in species other than silver fir, and they tended to be located on more south‐facing slopes. H. abietinum was widespread in silver fir forests of the Pyrenees. Our data suggest that, in the Pyrenees, the observed H. abietinum incidence may represent a combination of both primary and secondary spread of the pathogen. Favouring mixed forests should be tested as a potential control method for V. album. The correlation between silver fir mortality and V. album infection warrants further study, as the observed tree mortality could have occurred due to other factors than V. album, such as drought damage.     

Effects of flooding on growth of seedlings of woody riparian species

The effects of flooding on growth of seedlings were compared over a 7-month period (April–November) among six different woody species: Aesculus turbinata, Cercidiphyllum japonicum, Fraxinus platypoda, Pterocarya rhoifolia, Pterostyrax hispida, and Quercus mongolica var. grosseserrata. Flooding reduced the shoot length of F. platypoda, P. rhoifolia, C. japonicum, P. hispida, and Q. mongolica var. grosseserrata seedlings but did not affect that of A. turbinata seedlings. Among control seedlings, shoot elongation occurred once in A. turbinata and twice in F. platypoda and Q. mongolica var. grosseserrata; the other species continued to grow from April to August. Among the flooded plants of all species, shoot elongation occurred only once at the beginning of the growing season. On August 25, flooding significantly reduced the number of developed leaves as compared with control plants except for A. turbinata. In the flooded plants except for F. platypoda, leaf fall began on June 30; in controls, by contrast, the number of developed leaves increased until August 25. Flooding reduced the total dry weight increment in all species. The survival ratio of flooded plants after the experiment differed with species. All of the F. platypoda and A. turbinata seedlings survived the flooding treatment, while only 20% of P. hispida and 30% of Q. mongolica var. grosseserrata survived. Flooding seriously affected the growth of riparian pioneer species including P. rhoifolia, C. japonicum, P. hispida, and Q. mongolica var. grosseserrata. The effects of flooding on growth of the seedlings differed with the tree species because of differences in leaf-emergence pattern and physiological flood tolerance. The responses of tree seedlings to flooding reflected species habitats and growth patterns.     

Mycobiota in needles of Abies alba with and without symptoms of Herpotrichia needle browning

The ascomycete Nematostoma parasiticum (syn.: Herpotrichia parasitica) is commonly perceived as the causal agent of the so‐called Herpotrichia needle browning in silver fir (Abies alba). However, its fruitbodies are rarely present on symptomatic needles, which are also colonized by many presumably saprotrophic fungi. We compared the internal colonization of healthy and symptomatic needles on two sites in Poland. In addition, the endophytic mycobiota in needles of various age was recorded on two other sites without disease symptoms. Fungi were isolated from 95.6% of the dead needles and from 62.9% of the living needles on symptomatic trees, whereas on healthy trees, only 45.0% of the needles were colonized internally. Colonization frequency increased with needle age. From a total of 2017 isolates, 116 fungal taxa were identified. Frequency of many species was influenced by needle type. Anthostomella formosa, Gloeosporidiella sp., Hypoxylon fragiforme, Xylaria hypoxylon and X. polymorpha were the most common fungi isolated from living needles. In symptomless living needles, fungi occurred significantly more often in the basal than in the apical parts. In dead needles, the most common fungi were Alternaria alternata, Paraconiothyrium sporulosum, Fusarium sp., Mollisia cinerea, Rhizoctonia sp., Rhizosphaera oudemansii, Thysanophora penicillioides, Xylaria hypoxylon and X. polymorpha. Rhizoctonia sp. was the most frequently isolated fungus in dead needles (23.4%) but occurred rarely also in living needles (0.3–1.1%). The supposed pathogen N. parasiticum was detected only sporadically (at most in 0.6% of the needles). Our findings demonstrate the need for understanding the role of Rhizoctonia sp. in Herpotrichia needle browning disease aetiology.     

Antioxidant activity of Viscum album ssp. album

Methanolic extracts of Viscum album ssp. album (mistletoe) grown on different host trees were investigated for their potential antioxidant activity. Scavenging activity was tested by 1,1­diphenyl­2­picrylhydrazyl (DPPH) method and the inhibitory effect on lipid peroxidation was examined by ferric thiocyanate and thiobarbituric acid methods. The extract from mistletoe grown on lime tree in summer showed the highest activity. It was found that antioxidant capacity of the plant differed according to the harvesting time as well as the host tree.     

Effects of Mexican dwarf mistletoe (Arceuthobium vaginatum subsp. vaginatum) on the growth of Pinus cooperi in Durango,Mexico—A case study

Mexican dwarf mistletoe (Arceuthobium vaginatum subsp. vaginatum, Viscaceae) is the most widespread and damaging parasitic plant in Mexico. It parasitizes 10 species of pines (Pinus spp., Pinaceae) as principal hosts, including Pinus cooperi, one of the economically most important pines in the state of Durango. As a case study, we used stem analysis to estimate the effects of Mexican dwarf mistletoe on volume and height growth of dwarf mistletoe‐infected P. cooperi in western Durango (Ejido El Brillante, Municipality Pueblo Nuevo). This case study sampled a total of 48 trees, 12 in each of four infection classes estimated using the 6‐class dwarf mistletoe rating system (DMR): uninfected (DMR 0), lightly infected (DMR 1–2), moderately infected (DMR 3–4) and severely infected (DMR 5–6). Significant reductions in both volume and height growth were found for moderately and severely infected trees when compared to uninfected trees. On average, reductions in volume growth and height growth were as high as 50% and 17%, respectively. The largest growth reductions were for moderately infected trees, but large growth reductions also occurred for severely infected trees. Because of the reduced growth associated with moderate to severe infection, Mexican dwarf mistletoe‐infested pine forests in Durango, where timber production is a high priority, should be managed using harvesting practices that reduce dwarf mistletoe infection, and thereby, increase forest productivity.     

Fungi associated with the fir bark beetle Cryphalus piceae in Poland

The aim of this study was to identify fungi associated with Cryphalus piceae on European silver fir (Abies alba) in Poland and to test the pathogenicity of selected isolates. Fungi were isolated from five populations of overwintered adults and their galleries. A great diversity of taxa was associated with C. piceae. In total, 2487 isolates, including 58 species distributed in 25 genera, were obtained. The two most frequently isolated fungi, an undescribed species of the genus Geosmithia and Ophiostoma piceae, appeared to be specifically associated with C. piceae, whereas Pesotum fragrans, Pesotum sp. and Sporothrix sp. were sporadically associated. Two‐year‐old seedlings of silver fir were wound‐inoculated with three species of fungi (Geosmithia sp., O. piceae and Pesotum sp.) recovered from C. piceae. Only Pesotum sp. showed pathogenic ability, but we do not consider it to be an important pathogen of A. alba.     

Quantifying the effect of pine mistletoe on the growth of Scots pine

Mistletoe infection results in substantial growth losses in mistletoe‐infected forests. This study reports and evaluates the results of retrospective analyses of radial growth of Scots pine (Pinus sylvestris) in relation to the level of infection of pine mistletoe (Viscum album ssp. austriacum). A total of 43 Scots pine trees were destructively sampled from different sites. Of these trees, 14 were uninfected and 29 were infected. Infection classes were determined using six‐class dwarf mistletoe rating system (DMRS). All needle and mistletoe biomass were removed completely and weighed for each sampled tree. Subsamples from needles and all mistletoe biomass were taken to the laboratory for oven‐dried weight determinations. Five‐cm‐thick wood discs were cut from the stem at the breast height (1.3 m) to determine annual basal area increment for the last 25 years. In addition to DMRS, new infection classes were created using mistletoe‐to‐needle biomass (MB/NB) ratio. The results showed that the radial growth losses could be as much as 41% to 64% at different infection levels. The rate of growth loss in relation to DMRS and MB/NB ratio was similar, but with a larger variability in DMRS values. The results showed that both DMRS rating and MB/NB ratio seem to be important for quantifying growth loss on Scots pine trees infected with mistletoe. The results of this study can also be invaluable in modelling the effects of mistletoe on the growth of Scots pine trees.     

Root‐colonizing ophiostomatoid fungi associated with dying and dead young Scots pine in Poland

Ophiostomatoid fungi are carried by various bark beetles. However, very little is known about the role of these fungi in conifer roots. We studied ophiostomatoid fungi in roots of dying and dead Pinus sylvestris trees and tested the potential phytotoxicity of some isolates using a sensitive bioassay with Lepidium sativum in Poland. Fungi were identified based on their morphology and DNA sequencing. Three ophiostomatoid fungi, Leptographium procerum, Sporothrix inflata and Ophiostoma pallidulum, were isolated from the roots. The most abundant soil‐borne fungus, S. inflata, and relatively rare O. pallidulum were isolated for the first time from roots of dying and dead pine trees. The frequency of S. inflata and O. pallidulum correlated with tree decline. The fungi were isolated more frequently from roots of dead than dying trees. Sporothrix inflata and O. pallidulum slightly reduced the stem and root growth of L. sativum. Leptographium procerum reduced more significantly root than stem growth. This species reduced root elongation 32–54% after 10–17 days of incubation.     

Susceptibility of Healthy and Ozone-injured Needles of Pinus strobus to Invasion by Lophodermium pinastri and Aureobasidium pullulans1

Lophodermium pinastri and Aureobasidium pullulans were the fungi most often associated with foliar symptoms, including those caused by ozone, on Pinus strobus in the field. Following artificial inoculation of ozone-fumigated and untreated foliage on pine seedlings, L. pinastri was Isolated without relation to visible symptoms of ozone injury. A, pullulans was isolated only from tissues killed by ozone. No evidence of additive or interacting effects of ozone and the fungi was found.     

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.     

Fungi associated with Ips typographus on Picea abies in southern Poland and their succession into the phloem and sapwood of beetle‐infested trees and logs

This study dealt with the species distribution and frequency of fungi associated with the bark beetle Ips typographus (Scolytidae) on spruce trees of various states of health in southern Poland. The spruce trees were assessed by their degree of defoliation and damage of their crowns and trunks after attack by I. typographus. The state of health of trees from which samples were obtained was related to varying stages of brood development of I. typographus. Fungi were isolated from phloem taken from and around insect galleries and the sapwood underneath brood systems. Samples were taken from ‘healthy‐looking’, weakened, wind‐fallen and wind‐broken trees as well as from trap trees. The mycobiota associated with I. typographus was quite diverse in respect of the number of detected species, 65 fungal taxa were obtained from the phloem of trees infested by I. typographus, and 36 taxa occurred in the sapwood underneath insect galleries. The spectrum of fungi mainly consisted of ascomycetes and anamorphic fungi. The ophiostomatoid fungi were represented by 14 species and were the most numerously represented group in all niches examined. The most frequent ophiostomatoid species were Ceratocystis polonica, Ophiostoma ainoae, O. bicolor, O. penicillatum, O. piceae and O. piceaperdum. The frequency of occurrence of ophiostomatoid fungi differed significantly between the phloem and sapwood as well as in relation to the varying states of health of the spruce trees. These quantitative differences in the mycobiota of I. typographus between spruce trees belonging to different health categories can be explained by successional patterns of fungal colonization of host tissues following attack by I. typographus. The pathogenic species C. polonica was the primary invader, occurring most frequently in the sapwood of ‘healthy‐looking’ trees. Ophiostoma bicolor, O. penicillatum and O. piceaperdum also occurred during the early stages of brood development of I. typographus on ‘healthy‐looking’ trees, but they mainly colonized the phloem. In contrast, O. ainoae, O. minuta and O. piceae likely follow the aforementioned species as secondary and tertiary invaders into the phloem and the sapwood of spruce trees.     

Relationship between salt tolerance and proline accumulation in Australian acacia species

 The effects of salinity on the seedlings of five acacia species, Acacia ampliceps, A. salicina, A. ligulata, A. holosericea, and A. mangium were studied with respect to mortality, growth, and proline accumulation. There was marked variation among the species in their response to salinity. A. ampliceps showed the highest level of salt tolerance among the five acacia species. In contrast, A. holosericea and A. mangium were classified as the least tolerant. The concentration which caused 100% mortality in 2-week-old seedlings during 1 week of treatment was 2.5% (w/v) and 1.5% NaCl for A. ampliceps and A. holosericea, respectively. The dry weights of whole plant of two acacia species decreased markedly under the above salt stress conditions. Proline accumulation was found in leaves and roots after NaCl treatment. The maximum level of proline accumulation in A. holosericea was about twice as great as that in A. ampliceps throughout the stress period. Thus, the level of proline accumulation was not related to the degree of salt tolerance in these acacia species, although a positive correlation between the extent of salt stress and the accumulation of proline was found. Received: July 18, 2002 / Accepted: October 10, 2002 Acknowledgments The author thanks Dr. Kiyoshi Tazaki for his technical advice on the quantitative analysis of proline and Toshiro Sumizono for use of the phytotron. He is also grateful to Dr. Kenji Shinohara for reviewing the language. This work was supported by a grant from the Science and Technology Agency of Japan to the Forestry and Forest Products Research Institute (Encouragement of Basic Biology).     

Effects of flooding stress on the photosynthetic apparatus of leaves of two <Emphasis Type="Italic">Physocarpus</Emphasis> cultivars

We applied under pot-culture conditions and the double-casing pot method to study the characteristics of photosynthetic gas exchange and chlorophyll fluorescence in the leaves of Physocarpus amurensis Maxim (PA) and Physocarpus opulifolius under flooding stress. Our results indicate a significantly higher flooding tolerance of P. opulifolius compared to P. amurensis. Especially in P. amurensis, the limitation of non-stomatal factors played a major role in the advanced stages of flooding stress, observed as a rapid increase of the intercellular CO₂ concentration (C _i) and a decrease of the stomatal limitation value (L _s). The maximal PSII photochemical efficiencies (F _v/F _m) and actual photochemical efficiency (Ф _PSII) in the leaves of P. opulifolius were significantly higher, and the extent of decrease during the flooding process was smaller than in P. amurensis. In addition, the non-chemical quenching (NPQ) in the leaves of P. opulifolius significantly increased from the 10th day under flooding stress, while the variation of NPQ in the leaves of P. amurensis was much smaller. This indicates that the leaves of P. opulifolius had not only higher PSII photochemical activity, but also improved tolerance to flooding stress, which may be caused by its ability to dissipate excess excitation energy by starting NPQ. At the 16th day under flooding stress, the P _IABS significantly decreased with greater extent of decrease than F _v/F _m in the leaves of both Physocarpus, but the decreasing extent of P _IABS in P. opulifolius was significantly smaller than in P. amurensis. In the 16th day under flooding stress, the fluorescence at J and I point (V _J and V _I) in P. amurensis were significantly higher, and the extent of increase in V _J was greater than V _I. However, the variations of V _J and V _I in the leaves of P. opulifolius were smaller, suggesting that the damage sites of flooding stress to PSII in the leaves of P. amurensis were mainly located in the electron transport process from Q_A at the PSII receptor side to Q_B. Flooding stress reduced the proportion (φE _o ) of luminous energy absorbed by the PSII reaction center for the electron transport following Q _A ^? , while the maximum quantum yield (φD _o) of non-photochemical quenching increased. However, the TR_o/RC and ET_o/RC in the leaves of P. amurensis decreased accompanied by a dramatic increase of energy (DI_o/RC) from the dissipation in the reaction center. This further indicated that the function of the PSII reaction center in the leaves of P. amurensis was significantly lower than in P. opulifolius.     

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.