Tree Decline Research Articles

Greater risk of hydraulic failure due to increased drought threatens pine plantations in Horqin Sandy Land of northern China

Widespread decline and mortality of shelterbelt forests in water-limited regions of northern China have been occurring over the past few decades and may become more severe under the influence of climate change; however, our understanding on how droughts affect tree performance in a long-term context and the underlying water-related physiological mechanisms are still limited. Here, using a combination of tree-ring analysis and measurement of water-related functional traits, we investigated patterns of long-term radial growth and the underlying physiology in Pinus sylvestris var. mongolica (Mongolian Pine) and Pinus tabuliformis (Chinese Pine) in relation to variation in environmental water availability in Horqin Sandy Land (HSL), i.e. a key area of land desertification control in northern China where the two species are commonly used for afforestation. Particularly, we studied the association between tree-ring growth responses to drought and xylem hydraulics of the two species at the two sites of HSL that differ in climate regimes, examining the potential impacts of climate warming-related aridification on tree performances using a space-for-time substitution approach. Tree-ring analyses revealed greater degrees of recent growth decline, lower resilience to drought, and more severe tree hydraulic limitations in both pine species at the semiarid site compared to the semi-humid site. Tree-ring width indices of both species in the drier site showed significant positive correlations with May-July total precipitation of the current year and negative correlations with mean temperature of the same period, whereas such correlations were not significant at the wetter site. Lower stem hydraulic conductivity and higher degrees of xylem embolism in the drier site suggest that both pine species suffer from stronger hydraulic limitation when the environment gets drier. Moreover, Mongolian pine with a significantly higher hydraulic conductivity consistently showed higher basal area increments than Chinese pine at both sites. Our results suggest that Mongolian pine, the currently most widely used trees to create shelterbelt plantations in desertified areas of northern China, is more susceptible to decline than Chinese pine as the climate becomes hotter and drier, although the former species has a higher growth rate than the latter one. Our investigation provides a mechanistic explanation for the commonly observed decline and dieback of pine trees in water-limited environments of northern China and suggests that Chinese pine may be a better choice to replace Mongolian pine as the main tree species in plantations of the study area with a changed climate regime in the future.

Neighbourhood diversity mitigates drought impacts on tree growth

Abstract Biodiversity is considered to mitigate detrimental impacts of climate change on the functioning of forest ecosystems, such as drought‐induced decline in forest productivity. However, previous studies produced controversial results and experimental evidence is rare. Specifically, the biological mechanisms underlying mitigation effects remain unclear, as existing work focuses on biodiversity effects related to the community scale. Using trait‐based neighbourhood models, we quantified changes in above‐ground wood productivity of 3,397 trees that were planted in a large‐scale tree diversity experiment in subtropical China across gradients of neighbourhood diversity and climatic conditions over a 6‐year period. This approach allowed us to simultaneously assess to what extent functional traits of a focal tree and biodiversity at the local neighbourhood scale mediate the growth response of individual trees to drought events. We found that neighbourhood tree species richness can mitigate for drought‐induced growth decline of young trees. Overall, positive net biodiversity effects were strongest during drought and increased with increasing taxonomic diversity of neighbours. In particular, drought‐sensitive species (i.e. those with a low cavitation resistance) benefitted the most from growing in diverse neighbourhoods, suggesting that soil water partitioning among local neighbours during drought particularly facilitated most vulnerable individuals. Thus, diverse neighbourhoods may enhance ecosystem resistance to drought by locally supporting drought‐sensitive species in the community. Synthesis. Our findings demonstrate that mechanisms operating at the local neighbourhood scale are a key component for regulating forests responses to drought and improve insights into how local species interactions vary along stress gradients in highly diverse tree communities.

Spectral differentiation of oak wilt from foliar fungal disease and drought is correlated with physiological changes.

Hyperspectral reflectance tools have been used to detect multiple pathogens in agricultural settings and single sources of infection or broad declines in forest stands. However, differentiation of any one disease from other sources of tree stress is integral for stand and landscape-level applications in mixed species systems. We tested the ability of spectral models to differentiate oak wilt, a fatal disease in oaks caused by Bretziella fagacearum ``Bretz'', from among other mechanisms of decline. We subjected greenhouse-grown oak seedlings (Quercus ellipsoidalis ``E.J. Hill'' and Quercus macrocarpa ``Michx.'') to chronic drought or inoculation with the oak wilt fungus or bur oak blight fungus (Tubakia iowensis ``T.C. Harr. & D. McNew''). We measured leaf and canopy spectroscopic reflectance (400-2400nm) and instantaneous photosynthetic and stomatal conductance rates, then used partial least-squares discriminant analysis to predict treatment from hyperspectral data. We detected oak wilt before symptom appearance, and classified the disease with high accuracy in symptomatic leaves. Classification accuracy from spectra increased with declines in photosynthetic function in oak wilt-inoculated plants. Wavelengths diagnostic of oak wilt were only found in non-visible spectral regions and are associated with water status, non-structural carbohydrates and photosynthetic mechanisms. We show that hyperspectral models can differentiate oak wilt from other causes of tree decline and that detection is correlated with biological mechanisms of oak wilt infection and disease progression. We also show that within the canopy, symptom heterogeneity can reduce detection, but that symptomatic leaves and tree canopies are suitable for highly accurate diagnosis. Remote application of hyperspectral tools can be used for specific detection of disease across a multi-species forest stand exhibiting multiple stress symptoms.

Foliar endophytic fungi: diversity in species and functions in forest ecosystems

As mycorrhizal fungi, endophytic fungi are ubiquitous in natural forest ecosystems. A great number of foliar endophytic fungi (FEFs) have been isolated from leaves of forest trees, and some metabolites are also isolated and characterized from the FEFs. These metabolites show strong potential to inhibit or kill insects and pathogenic fungi, especially those causing decline or damage of forest trees. In addition, FEFs can inhabit in leaf litter for a short or long period depending on endophytic species, thus they affect decomposition rate of leaf litter. Some FEFs can colonize in plant roots and promote plant growth. In the review, we provide an overview of the ecological roles of FEFs and hope the resources of FEFs can be developed and be used in forestry and agriculture to reduce use of insectcides and fungicides and in pharmacology to maintain human health.

Macrophomina Crown and Root Rot of Pistachio in California.

In this study, declining pistachio rootstocks were detected in newly planted commercial pistachio orchards in Kern County, California. Symptoms were characterized by wilted foliage combined with crown rot in the rootstock. From diseased trees, 42 isolates were obtained, and all had similar cultural and morphological characteristics of Macrophomina phaseolina. Analyses of nucleotide sequences of three gene fragments, the internal transcribed spacer region (ITS1–5.8S–ITS2), partial sequences of β-tubulin, and translation elongation factor 1-α (TEF1) confirmed this identification, and 20 representative isolates are presented in the phylogenetic study. Testing of Koch’s postulates showed that M. phaseolina, when inoculated to stems and roots of the pistachio rootstocks using mycelial plugs or a microsclerotial suspension, is indeed pathogenic to this host. The widely used clonal University of California Berkeley I (UCBI) rootstock appeared highly susceptible to M. phaseolina, suggesting that this pathogen is an emerging threat to the production of pistachio in California. This study confirmed the association of M. phaseolina with the decline of pistachio trees and represents the first description of this fungus as a crown rot-causing agent of pistachio in California.

Pathogenicity and Distribution of Fusarium solani Isolates Associated with Erythrina Decline in Japan.

Erythrina spp. trees have been declining since the 2000s worldwide, and fungi belonging to Fusarium solani species complex (FSSC) have been suggested to be a causal factor of decline and mortality of Erythrina variegata trees in Okinawa Island, Japan. In addition to the FSSC isolate grouped as "it-1" based on ITS sequence data (previously called strain A), we conducted an inoculation experiment with two isolates grouped as "it-2" (previously strain B), which is genetically close to it-1. Two it-2 isolates originating from two islands showed pathogenicity to E. variegata with the same symptoms as those caused by it-1 isolate. We also found the isolates of it-1 and it-2 were widely distributed, including on Ishigaki Island, ∼400 km south of Okinawa Island across the ocean. All isolates of it-1 and it-2 belong to the ambrosia Fusarium clade of the FSSC, a group of symbionts of ambrosia beetles, including the pathogens of Fusarium dieback in avocados and teas. The detection of ambrosia beetles Euwallacea spp. from our specimens provided information on the vectors of the pathogens. Our present results suggest the fungi of the FSSC could be responsible for the Erythrina decline in other areas with damage.

Breeding habitat, nest-site characteristics and productivity of the little eagle (Hieraaetus morphnoides) near Armidale, New South Wales

We aimed to elucidate nesting requirements and nest success of the threatened little eagle (Hieraaetus morphnoides). Nest sites (n=12 active and 2–5 recent historical nests) near Armidale, New South Wales, were measured in 2017 at three scales: the nest tree, the nest woodland (≤25m from the nest tree), and (using GIS) the landscape scale (within 200-m and 2-km radii of the nest). The eagles typically nested ≥14m above ground in the canopy of emergent (>20m tall) living eucalypts in sheltered positions (midslope, with a north-easterly to southerly aspect), in woodland patches >5ha (mean 76ha), <200m (mean 78m) from the woodland edge, though ≥11m (mean 190m) from an agricultural edge, ≥38m (mean 485m) from the nearest rural dwelling, >1km from suburbia, and farther from sealed roads (mean 832m) than gravel roads (mean 490m) than minor tracks (mean 291m). Breeding productivity in 2017–18 (n=15 and 18 territories, respectively) was 0.91 young fledged per attempt (clutch laid) and 0.67 young fledged per occupied territory per year. Nest sites were used annually for at least 3–7 years. Nest abandonments or site shifts were associated with human disturbance (e.g. clearing, earthmoving, subdivision and construction in or beside the nest patch), death of the nest tree or nest stand (‘eucalypt dieback’ or rural tree decline), pindone baiting for rabbits (Oryctolagus cuniculus), and displacement by wedge-tailed eagles (Aquila audax) and ravens (Corvus sp.). As most little eagle nests were located on private land, we recommend, inter alia, greater protection of breeding habitat, nest sites and foraging habitat, woodland regeneration (especially riparian), and a buffer around established nests of ≥1km from major developments such as urbanisation.

Effects of complex cocoa-based agroforests on citrus tree decline

Citrus are important fruit crops of high economic and ecological importance in southern Cameroon. Smallholders usually plant citrus in complex cocoa-based agroforests (CBAS). Health and productivity of citrus are generally jeopardized by a wide range of pests and diseases for which environmental conditions can be critical drivers. An assessment of the effects of two structural characteristics of CBAS on citrus tree decline (CTD) intensity and six pests and diseases that induce it was conducted in 26 farms. The study was carried out in four citrus-growing basins in the Centre Region of Cameroon between June and September 2016. Observations were made by visual recognition of CTD, as well as disease symptoms and pest damage. Their intensity or abundance was assessed using a scale ranging from 1 to 4. Depending on the situation of citrus trees in the CBAS and with regard to the upper strata, three shade level (dense, moderate and no shade), and three spatial structural patterns (aggregate, random and regular) were defined. Results showed that, CTD intensity under dense shade conditions was significantly low (P < 0.001), as compared to those in moderate and no shade conditions. Plots with aggregated or random spatial structure of citrus trees showed significantly higher CTD intensities (P < 0.001) compared to those with a regular spatial structure of citrus trees. The involvement of CBAS structural characteristics in CTD regulation has been demonstrated and could contribute to the development of an integrated management tool for citrus pests and diseases in an associated crop context.

Airborne LiDAR and high resolution multispectral data integration in Eucalyptus tree species mapping in an Australian farmscape

Rapid decline and death of rural Eucalypts trees of all ages and species have been reported in the farmscapes of regional Australia due to various environmental and farming management related factors. The identification of existing farm tree species is important for long term management strategies to provide ecosystem stability in the region. This study explored the feasibility of structural attributes of LiDAR and spectral and spatial characteristics of high resolution remote sensing data to identify and map Eucalyptus tree species. An object based image segmentation and rule-based classification algorithm were developed to delineate tree boundaries and species classification. The integration of two datasets improved the classification accuracy (65%) against their separate classification (52% and 41%, respectively). The identification of tree species will help in getting first-hand information on existing farm trees, which may be used in assessing tree condition in time series related to management practices and complex dieback problem.

The Crumbling Backbone of the Savannah Woman: Adjusting to the Declining Shea and Locust Tree Densities in Northern Ghana

Rural women in Northern Ghana depend largely on indigenous forest resources, particularly Shea and locust bean trees as a major source of livelihood support. The fruits of these trees, are so essential to rural livelihoods that their collection and processing, are subjects of intense competition among rural women. Recent studies, however suggest a significant degradation of agroforestry parklands, and the population densities of Shea and locust bean trees. Using a case study approach, and employing participatory tools such as focus group discussions, interviews and structured observations, the study sought to unpack the experiences of rural women on the decline in these tree species, and examine how this new reality affects their major livelihood assets. The study found that, the decline in Shea and locust bean trees, affects the social, human and economic capital of rural women. Exploitation of other environmental resources, the use of improved Shea processing methods, and vegetable farming, were identified as major adaptation strategies among rural women. The study also found evidence of deep-seated historical and socio-economic factors militating against the successful adaptation of rural women. Consequently, the study highlighted the need for micro credit accessibility programmes, and gendered land tenure arrangements to support rural women in Agriculture. Keywords: Agroforestry parklands, Shea trees, Locust trees, Livelihoods, Rural women, Response mechanisms. DOI : 10.7176/JCSD/54-05 Publication date: December 31 st 2019

Decline of large-diameter trees in a bamboo-dominated forest following anthropogenic disturbances in southwestern Amazonia

Reduction in the aboveground biomass of larger trees is the main consequence of disturbances in open forests dominated by bamboo. Because these trees are of central importance both for ecosystem function and for the economic value of the forest for management, the impact on these trees due to the increase of bamboo abundance following anthropogenic disturbances is both an environmental and a commercial concern. Bamboo-dominated forests in southwestern Amazonia are increasingly exposed to the combined impacts of fire and selective logging. Although the climbing bamboos (Guadua spp.) are considered native species of these forests, disturbances contribute to a discontinuous canopy, which is an ideal scenario for an increase in abundance of these opportunistic plants. The regeneration of tree biomass is limited in these cases, decreasing the carbon stock of the forest. This study compares changes in bamboo abundance and forest structure in the face of forest fire and post-burn logging in the municipality (county) of Rio Branco, Acre state, Brazil. The study was conducted on the Transacreana Highway (AC 090) in the eastern portion of Acre state, Brazil. We compared changes in bamboo abundance and aboveground biomass (AGB) of an area of forest with no known recent disturbances to areas in the same forest that had been disturbed by fire and post-burn logging, which are frequent sources of disturbance in this region. The live and dead AGB values were estimated by field inventory in 2016, which was 11 years after a fire and 9 years after selective logging. The AGB values for trees and palms were estimated by allometric equations in three 2-ha areas. Bamboo abundance was expressed both by bamboo biomass (Mg ha−1) and culm density (culms ha−1), sampled by direct measurements in three 200-m2 areas. Bamboo abundance was over 20% higher in forest that had been burned but not logged, as compared to undisturbed forest. Compared to undisturbed plots, aboveground forest biomass (live + dead) was 34% lower in plots exposed to fire but not logging, and 36% lower if exposed to both. Live trees and palms represented 77% of the total forest biomass (live + dead), and almost a half of this contribution (41%) was in individuals in the largest diameter class (diameter at breast height > 50 cm). Increase in the level of impact led to a reduction in aboveground carbon stock of the forest. One of the main consequences of disturbances in open forests dominated by bamboo is reduction of live trees and palms, especially in the diameter at breast height (DBH) class over 50 cm.

2019 Julie S Denslow &amp; Peter Ashton Prizes for the Outstanding Articles Published in Biotropica

2019 Julie S Denslow &amp; Peter Ashton Prizes for the Outstanding Articles Published in <i>Biotropica</i>

Impact of Common Mistletoe (Viscum album L.) on Scots Pine Forests—A Call for Action

Common mistletoe is increasingly mentioned as contributing not only to the decline of deciduous trees at roadside and in city parks, but to conifers in stands. The presence of Viscum in fir stands has been known for many years, but since 2015 has also been the cause of damage to pine. In 2019, mistletoe was observed on 77.5 thousand hectares of Scots pine stands in southern and central Poland. Drought resulting from global climate change is implicated as an important factor conducive to weakening trees and making them more susceptible to the spread of mistletoe and other pests. This paper presents an overview of the latest information on the development of this semi-parasitic plant in Poland, its impact on tree breeding traits and raw material losses, as well as current options for its prevention and eradication.

Distribution of Pseudocercospora fruit and leaf spot, Phytophthora foot rot and scab diseases and their effect on Citrus tree decline prevalence in the humid zones of Cameroon

Introduction – Declining Citrus trees (CTD) is a major constraint causing Citrus production depletion in Cameroon. It has been established that some pests and diseases are the major sources of that phenomenon. Improving knowledge of the contributing factors to the occurrence of CTD is key to the control of this. Materials and methods – A study based on the presence of fungal diseases caused by Pseudocercospora angolensis, Elsinoe spp., and Phytophthora spp., was carried out in 39 sites of Citrus producing basins of Cameroon. The aim of the study was to determine environmental factors favouring CTD occurrence in an effort to reducing their devastating effects on three common species of Citrus. Disease symptoms were noticed by examining tree's organs (leaves, fruits, branches, trunk). A regression analysis was applied with the General Linear Model to select the main factors involved in CTD development. The prevalence of CTD was treated as dependent variable. Independent variables were Citrus species, soil texture, vegetation, rainfall, temperature, relative humidity, altitude, observation year, prevalence of PLFSD, Citrus scab and Phytophthora diseases. Results and discussion – CTD was found in all the sites, with a mean prevalence of 25%. Citrus species, soil texture, vegetation, rainfall, Pseudocercospora fruit spot disease, scab, and Phytophthora foot rot disease were the main factors associated with CTD prevalence. Conclusion – The importance of CTD was confirmed. It has been shown that PLFSD and Phytophthora diseases were much more likely to contribute to tree decline contrary to Citrus scab. However, further analysis could pave the way for the development of a risk assessment model for CTD.

Coexistence of three specialist predators of the hemlock woolly adelgid in the Pacific Northwest USA.

The hemlock woolly adelgid (Hemiptera: Adelgidae: Adelges tsugae Annand) is an invasive insect, introduced from Japan to eastern North America, where it causes decline and death of hemlock trees. There is a closely related lineage of A. tsugae native to western North America. To inform classical biological control of A. tsugae in the eastern USA, the density and phenology of three native western adelgid specialist predators, Leucopis argenticollis (Zetterstedt), Le. piniperda (Malloch) (Diptera: Chamaemyiidae), and Laricobius nigrinus Fender (Coleoptera: Derodontidae), were quantified in the Pacific Northwest. Infested branches were collected from western hemlock (Pinaceae: Tsuga heterophylla (Raf.) Sarg.) at four sites around the Puget Sound, Washington and three sites in Oregon. Immature Leucopis were identified to species using DNA barcodes. Leucopis argenticollis was roughly twice as abundant as Le. piniperda. Laricobius nigrinus larvae were more abundant than the two species of Leucopis during the egg stage of the first adelgid generation, but Leucopis were present as feeding larvae during the second adelgid generation when La. nigrinus was aestivating in the soil, resulting in Leucopis being more abundant than La. nigrinus across the entire sampling period. Adelges tsugae and La. nigrinus densities were not correlated, while A. tsugae and Leucopis spp. densities were positively correlated. Leucopis spp. and La. nigrinus densities were negatively correlated. These results support the complementary use of La. nigrinus and the two Leucopis species for biological control of A. tsugae in the eastern USA, and point to the need for further investigation of spatial and temporal niche partitioning among the three predator species.

Changes in rhizosphere bacterial communities associated with tree decline: grapevine esca syndrome case study.

An investigation was carried out on rhizosphere bacteria to determine if they may be associated with perennial crops affected by nonspecific decline, a phenomenon that is difficult to diagnose and prevent. Esca disease of grapevine was chosen for this case study because of its easy foliar symptom identification. Ribosomal DNA fingerprint analysis by polymerase chain reaction - denaturing gradient gel electrophoresis (PCR-DGGE), quantitative PCR (qPCR), and rDNA amplicon sequencing by next-generation sequencing (NGS) were adopted to investigate the bacterial communities associated with grapevines, which were selected for the presence and absence of external foliar symptoms in 11 vineyards. According to PCR-DGGE and qPCR, bacterial communities differed in site of origin (vineyards), but not between symptomatic and asymptomatic plants, whereas qPCR gave a significantly higher presence of total bacteria and Pseudomonas spp. in asymptomatic plants. NGS confirmed no difference between symptomatic and asymptomatic plants, apart from a few minor genera (<0.5%) such as Salinibacterium, Flavobacterium, Nocardia, and Janthinobacterium, which were, in all cases, higher in asymptomatic plants and whose functional role should be the object of further investigation. The fact that total bacteria and Pseudomonas were more abundant in the rhizosphere of asymptomatic grapevines and that some bacterial genera were associated with the latter, represents a new element when investigating the multiple-origin phenomenon such as esca disease of grapevine.

Road mediated spatio-temporal tree decline in traditional agroforests in an African biosphere reserve

Understanding the role of protected areas in conserving biodiversity is a central goal in conservation biology. Anthropogenic activities around and inside these protected areas and, in particular, roads can alter the spatiotemporal dynamics of biological diversity in protected areas. However, our understanding of how the presence and position of roads affect human attitude, subsequent agricultural practices and biodiversity conservation is limited. In this study, we tested the effects of the proximity of traditional agroforestry parklands to main roads used by park rangers for surveillance on the diversity and abundance of woody species in the Pendjari Biosphere reserve over 16 years (2000–2016). Tree density in agroforestry parklands decreased over time from an average of 20 trees/ha in 2000 to 7 trees/ha in 2016. Species such as Vitellaria paradoxa and Parkia biglobosa, which are economically important, experienced the largest density reduction Trees density was also significantly higher in farms close to the monitoring roads used by park rangers to patrol the park. Farms that are far from the roads were monitored less frequently given that the number of park rangers declined over time. However, there was no significant variation in species richness and diversity over time, perhaps because of the low tree diversity in these systems. This masks evidences of species local extinctions. For example, species such as Pterocarpus erinaceus, Anogeissus leiocarpa, and Burkea africana which were present in the traditional agroforestry parklands in 2000 disappeared by 2016. This is associated with important land-use changes including the conversion of gallery forests into cropland and wooded savannas indicating that human pressure not only affects species occurrence but also their habitats. Our study suggests that where land demand for agriculture is high, it is challenging for local people to maintain sustainable management practices in the absence of collective action.

First report of Candidatus Phytoplasma prunorum, the European stone fruit yellows phytoplasma on peach trees on the territory of Canton of Geneva, Switzerland

In recent years, ‘Ca. P. prunorum’, the agent of ESFY was reported from several apricot orchards of Canton of Wallis, the main apricot production region in Switzerland (Genini and Ramel, 2004). The psyllid vector Cacopsylla pruni was also found in several locations in the Lake Geneva area (Ackermann et al., 2006). The presence of the disease and of its proven vector C. pruni at the eastern part of the Lake Geneva area, as well as the existing risk of dissemination of ESFY to other stone fruit orchards along the Lake Geneva are the reasons for which ESFY needs to be studied further. ESFY and its possible dissemination through C. pruni on the territory of Canton of Geneva, bordering France, has never been yet studied. In 2016, visual observations were conducted in several stone fruit orchards near Geneva. Plant material was obtained from peach trees, displaying some of the typical symptoms such premature leaf colouration, leaf-roll, tree decline (Sabaté et al., 2015), in the autumn, when the concentration of the phytoplasma in the upper parts of the trees is the highest. Phloem was prepared from branches and was extracted with a CTAB-based adapted protocol (Lefort and Douglas, 1999). PCR amplification of phytoplasma DNA was achieved with the universal primers: fP1/rP7 (Deng and Hiruki, 1991; Schneider et al., 1995). All positive samples were tested with the ESFY-specific non-ribosomal primers ECA1/ECA2 (Jarausch et al., 1998). Typical symptoms of ESFY, like premature leaf colouration, leaf yellowing with reddish edges, leaf-roll, severe chlorosis, die-back of top branches and partial or, complete decline of the trees (Figure 1) were found in a peach orchard (GPS coordinates: 46°15'17.4"N 6°12'40.7"E) located in the area of Collonge-Bellerive, close to Geneva city. Ten trees were sampled in this orchard. The presence of ‘Ca. P. prunorum’ was confirmed in two of them (Figure 2 and Figure 3). At the beginning of spring 2017, the monitoring of the ESFY symptoms in the infested orchard was resumed. Early bud break was found on the two infected trees and the infection was confirmed again by PCR (Figure 4 and Figure 5). The same symptoms were observed in many trees of this orchard, which correlated with the previous autumn observations. Trapping confirmed the presence of the insect vector Cacopsylla pruni (Bodnár et al., 2018) in the infected area. In order to understand more about the origin and the diversity of ESFY phytoplasma in this area, additional plant and insect samples will be analyzed. The correlation between symptoms and infected plants will be studied further. We report here for the first time on the occurrence of ‘Candidatus Phytoplasma prunorum’ (‘Ca. P. prunorum’) the agent of ESFY on the territory of Canton of Geneva.

A GIS approach to identify the Cork Oak Decline in M’Sila Forest (North-West of Algeria)

The decline of cork oak trees in North-West of Algeria was supposed to be caused by a combination of primary or predisposing factors, and secondary or opportunistic factors. There is a broad consensus about the role of water scarcity and nutrients in the primary causes, given the similarity of symptoms of cork oak decline and of chronic water deficits. To identify and characterize the cork oak decline in M’sila forest, a cork oak mortality index was determined for each transience area using a GIS approach and a linear regression model formulated by analyzing the interactions between the attributes of station (competition of superior and inferior floor); and allowing a justification of cork oaks mortality. The mortality index is significantly high in the invasive shrub plots and where the Aleppo pine density is very high; in which competition for soil water between the trees and the inferior floor increases. The lowest mortality value characterized by a lower density of Aleppo pine and herbaceous inferior floor.

Identification of Key Residues Required for RNA Silencing Suppressor Activity of p23 Protein from a Mild Strain of Citrus Tristeza Virus.

The severe strain of citrus tristeza virus (CTV) causes quick decline of citrus trees. However, the CTV mild strain causes no symptoms and commonly presents in citrus trees. Viral suppressor of RNA silencing (VSR) plays an important role in the successful invasion of viruses into plants. For CTV, VSR has mostly been studied in severe strains. In this study, the N4 mild strain in China was sequenced and found to have high sequence identity with the T30 strain. Furthermore, we verified the functions of three VSRs in the N4 strain, and p23 was found to be the most effective in terms of local silencing suppressor activity among the three CTV VSRs and localized to both nucleus and plasmodesmata, which is similar to CTV T36 strain. Several conserved amino acids were identified in p23. Mutation of E95A/V96A and M99A/L100AA impaired p23 protein stability. Consequently, these two mutants lost most of its suppressor activity and their protein levels could not be rescued by co-expressing p19. Q93A and R143A/E144A abolished p23 suppressor activity only and their protein levels increased to wild type level when co-expressed with p19. This work may facilitate a better understanding of the pathogenic mechanism of CTV mild strains.