Individual resistance of Fraxinus angustifolia clones to ash dieback

The presumed resistance of individual ash trees to ash dieback caused by invasive pathogen Hymenoscyphus fraxineus is an important issue for the maintenance of ash in European forests. All known studies regarding the resistance of ash trees to ash dieback were conducted in plantations and stands of F. excelsior; however, no such data exist for F. angustifolia. Crown damage assessments were performed over four consecutive years between 2009 and 2012 at a F. angustifolia clonal plantation in Hra??ica, Slovenia. Inoculation of H. fraxineus into the branches of the most and least damaged clones of F. angustifolia and leaf phenology assessments was performed to verify the presence of defence mechanisms that limit fungal growth or promote disease escape. Additionally, root collars of selected clones were inspected for fungal infections. The crown damage assessments showed considerable differences among F. angustifolia clones, indicating genetic variability in susceptibility to ash dieback. Crown dieback progressed significantly over the 4‐year time period; the mean crown damage of individual clones in 2012 varied between 16.7% and 83.8%. Significant differences among F. angustifolia clones were found in the inoculation trials and leaf phenology assessments. However, defence mechanisms such as early leaf flushing, early leaf shedding and the ability to inhibit pathogen growth in host tissues were not confirmed. High frequency of Armillaria spp. and H. fraxineus root collar infection demonstrated the need for whole tree inspection to determine causal agent of damages on individual ash trees. Armillaria spp. may be highly associated with ash decline epidemiology.     

Genetic variation in dieback resistance in Fraxinus excelsior confirmed by progeny inoculation assay

Ash dieback caused by the pathogenic fungus Hymenoscyphus fraxineus [previously known as H. pseudoalbidus (sexual stage) and Chalara fraxinea (asexual stage)] is a widespread problem in Europe. Here, we assess crown damage from natural infection and necrosis development following artificial controlled inoculations on full‐sib and half‐sib progeny from Danish Fraxinus excelsior clones with contrasting and well‐characterized levels of susceptibility to the disease. The inoculation assay was performed on a total of 123 offspring, and necrosis development monitored over two years. The offspring from low susceptible mother clones developed smaller necroses when compared to offspring from susceptible clones. Their crown damage due to natural infections was also significantly less. The correlation coefficient between average crown damages of mother clones and the average of their progeny was 0.85 (natural infections), while the correlation between crown damage of mother clones and the average necrosis development in their progeny after controlled inoculation was 0.73. The correlation between resistance of parent trees and crown damage/necrosis development on their offspring confirms the presence of heritable resistance and indicates that a bioassay based on controlled inoculations has the potential of becoming a fast and cost‐effective tool for estimation of dieback susceptibility in breeding programmes for resistance in ash trees.     

Temporal development of ash dieback symptoms and spatial distribution of collar rots in a provenance trial of Fraxinus excelsior

European ash (Fraxinus excelsior) is threatened by Hymenoscyphus pseudoalbidus, the agent of ash dieback disease. Beside ordinary ash dieback symptoms, collar rots have been reported on declining ashes as an additional problem of increasing severity. Since 2009, ash dieback has been surveyed annually in a provenance trial on four different sites in southwest Germany (Metzler et al. in Ger J For Res 183:168–180, 2012). The trial was established in 2005. Data of tree growth, ash dieback symptoms and collar rot prevalence were collected by surveying the trial in summer 2012 and compared with previously published data of the trial. Evaluations revealed a continuous and considerable increase in dieback severity since 2009. The results suggest that the infection process has not come to a standstill yet. Up to 2012, 6 % of the trees remained symptomless, whereas mortality added up to 9 %. There were significant differences in ash dieback severity between the investigated provenances. Collar rot prevalence ranged from 19 to 59 % between study sites. Moreover, high spatial dependency of collar rot prevalence could be detected within sites. Collar rots were more abundant on trees of severe ash dieback intensity, but could also be detected on 15 % of otherwise healthy trees. Mycelium from collar rots could be identified by means of RFLP analyses and sequencing of the ITS region as most likely belonging to Armillaria gallica. The possible roles of Armillaria spp. and H. pseudoalbidus in collar rot formation are discussed.     

Temporal evolution of collar lesions associated with ash dieback and the occurrence of Armillaria in Belgian forests

Ash dieback, caused by the fungus Hymenoscyphus fraxineus, has been observed in Europe for several years. In Belgium, the disease was first reported in 2010. Besides crown defoliation and dieback, collar lesions have sometimes been reported. To evaluate the prevalence and the progression of collar lesions and crown defoliation in ash dieback‐affected stands of various ages, a survey was conducted in 2013 and 2014 on 268 ash trees (Fraxinus excelsior) originating from 17 Walloon forest stands. The results showed that the proportion of trees with collar lesions greatly increased between June 2013 and September 2014 and that there appeared to be no significant link between a tree's diameter‐at‐breast height (DBH) and collar lesion occurrence. The mean percentage of defoliation increased in each forest stand across time, with observations conducted in September 2013 and 2014 showing a positive correlation with the mean percentage of trees with collar lesions. Molecular tests were carried out on 103 additional trees originating from 12 of the 17 stands to evaluate the occurrence of H. fraxineus and Armillaria spp. at the collar level. Most of the trees (98%) were infected by H. fraxineus. In contrast, only 41% of the samples were infected with Armillaria spp., most commonly A. gallica and A. cepistipes. This study discusses the role of Armillaria spp. and the rapid increase in the number of trees with collar lesions within the context of the evolution of ash dieback in Europe.     

Differences in susceptibility to ash dieback in Czech provenances of Fraxinus excelsior

The impact of ash dieback caused by Hymenoscyphus fraxineus on 17 provenances of Fraxinus excelsior and one provenance of Fraxinus angustifolia was studied in an extensive field trial established in the Czech Republic prior to the H. fraxineus invasion in 1999. A difference in the level of resistance to ash dieback between the species was found: F. angustifolia was significantly less affected by the disease than F. excelsior. Moreover, particular provenances of F. excelsior showed important differences in the level of resistance to H. fraxineus. A relationship between the impact of ash dieback and altitude was also discovered – the provenances from altitudes above 600 m a.s.l. were less affected by the pathogen than were the provenances from lower areas. No difference in the impact of the disease among provenances of F. excelsior from different ecotopes (ravine, calcareous ravine and alluvial) was found. Substantial among‐tree variability in resistance to H. fraxineus was observed throughout the trial – promising genotypes (with crown defoliation up to 5%) were identified in all 18 tested provenances. In regard to this finding, it appears that the main source of resistance to the pathogen is probably at the individual genotype level in the trial. A secondary but massive attack by Hylesinus fraxini was identified in the trees that had been greatly damaged by ash dieback, and the beetle caused their health to deteriorate significantly. A significant negative effect of the presence of collar necroses caused by H. fraxineus and browse damage was also identified.     

Vegetation and weather explain variation in crown damage within a large mixed-severity wildfire

The 2002 Biscuit Fire burned through more than 200,000 ha of mixed-conifer/evergreen hardwood forests in southwestern Oregon and northwestern California. The size of the fire and the diversity of conditions through which it burned provided an opportunity to analyze relationships between crown damage and vegetation type, recent fire history, geology, topography, and regional weather conditions on the day of burning. We measured pre- and post-fire vegetation cover and crown damage on 761 digital aerial photo-plots (6.25 ha) within the unmanaged portion of the burn and used random forest and regression tree models to relate patterns of damage to a suite of 20 predictor variables. Ninety-eight percent of plots experienced some level of crown damage, but only 10% experienced complete crown damage. The median level of total crown damage was 74%; median damage to conifer crowns was 52%. The most important predictors of total crown damage were the percentage of pre-fire shrub-stratum vegetation cover and average daily temperature. The most important predictors of conifer damage were average daily temperature and “burn period,” an index of fire weather and fire suppression effort. The median level of damage was 32% within large conifer cover and 62% within small conifer cover. Open tree canopies with high levels of shrub-stratum cover were associated with the highest levels of tree crown damage, while closed canopy forests with high levels of large conifer cover were associated with the lowest levels of tree crown damage. Patterns of damage were similar within the area that burned previously in the 1987 Silver Fire and edaphically similar areas without a recent history of fire. Low-productivity sites on ultramafic soils had 92% median crown damage compared to 59% on non-ultramafic sites; the proportion of conifer cover damaged was also higher on ultramafic sites. We conclude that weather and vegetation conditions — not topography — were the primary determinants of Biscuit Fire crown damage.     

Identifying Fraxinus excelsior tolerant to ash dieback: Visual field monitoring versus a molecular marker

Ash dieback (ADB) caused by the pathogen Hymenoscyphus fraxineus is the cause of massive mortality of Fraxinus spp. in Europe. The aim of this work was to check for the presence of the molecular marker for ADB tolerance in mapped healthy‐looking F. excelsior trees, and to compare its occurrence in trees exhibiting severe ADB symptoms. Monitoring of 135 healthy‐looking F. excelsior on the island of Gotland, Sweden, showed that after 3–4 years 99.3% of these trees had 0%–10% crown damage, thus remaining in a similar health condition as when first mapped. After 5–6 years, 94.7% of these trees had 0%–10% crown damage. Molecular analysis of leaf tissues from 40 of those showed the presence of the molecular marker in 34 (85.0%) trees, while it was absent in 6 (15.0%) trees. Analysis of leaf tissues from 40 severely ADB‐diseased trees showed the presence of the molecular marker in 17 (42.5%) trees, but its absence in 23 (57.5%) trees (p < .0001). The results demonstrated that monitoring of healthy‐looking F. excelsior is a simple and straightforward approach for the selection of presumably ADB‐tolerant ash for future breeding. The cDNA‐based molecular marker revealed moderate capacity on its own to discriminate between presumably ADB‐tolerant and susceptible F. excelsior genotypes.     

Seed Biology,Stand Characteristics,and Regeneration of Fraxinus rhynchophylla Hance

Fruit size and seed moisture content were measured and seed production and dispersal were monitored to understand the seed biology of the Korean ash (Fraxinus rhynchophylla Hance) in two Korean ash forests. A combination of four ground treatments and four crown closure levels were created in a natural forest to understand the site conditions needed for seedling emergence in natural stands and to determine practical regeneration methods for direct seeding. The seed size of Korean ash increased until late May and its moisture content decreased rapidly in early November. Prolific seed bearing occurred every 3 yr. The distance of seed dispersal by wind was about 30 m, but more than 90% of the seeds dispersed within 10 m from the seed trees. Twenty-five seed trees per hectare provided for successful natural regeneration. The seedling emergence in the natural stands was best with scarification treatment and 25～50% of crown closure.     

Status of thousand cankers disease on eastern black walnut in the eastern United States at two locations over 3 years

Thousand cankers disease (TCD), a lethal fungal dieback of eastern black walnut (Juglans nigra), caused by Geosmithia morbida, and spread by the walnut twig beetle, Pityophthorus juglandis, was documented in 2009 to be very destructive in the western United States and was identified in the native range of J. nigra at Knoxville, Tennessee, in 2010, and in 2011 at Richmond, Virginia. Beginning late 2010, we studied branch dieback levels (per cent live crown) and new TCD symptom development at these two quarantined locations monthly for 3 years. Of the 106 trees studied (53 at each location), 31 trees had low live crown ratings of 70 to 0% with little change over the 3 years of the study. One per cent of the trees developed new symptoms on a per‐year basis. Thus, a moderate level of TCD (mean = 76% live crown) was present in these two locations, and most trees were in a quiescent or dormant TCD condition for 3 years, an important finding not previously reported. We found new TCD symptoms developed in Richmond in 2011 and 2012 when precipitation from January 1 to the end of August was low (60–64 cm), and not when the precipitation in Richmond was higher (99 cm). In late 2012, in Richmond, soil water potential assays indicated that some black walnut trees were under severe physiological stress (?15 bars). In contrast, in 2013, high precipitation levels (99 and 130 cm) and high soil water potentials (?0.1 to ?3 bars) at both locations were associated with extensive new foliage and stem growth and recovery from TCD. Further research is needed on water relationships in regard to TCD and black walnut health.     

Susceptibility of Corymbia species and hybrids to arthropod herbivory inAustralian subtropical hardwood plantations

Corymbia hybrids are becoming significant plantation varieties in subtropical and tropical Australian plantation forestry. Although primarily developed for disease resistance and amenability to clonal propagation, they have also proven to have good growth rates and site plasticity. Here we examined the susceptibility of pure Corymbia species and hybrids to pest attack. Three trial sites containing C. citriodora subsp. variegata, C. torelliana, and the hybrids C. torelliana × C. citriodora subsp. variegata, C. torelliana × C. citriodora subsp. citriodora and C. torelliana × C. henryi were assessed for pest identity, incidence and severity. Pests caused about three-quarters of the visible crown damage to trees in these trials. At the site that had the most arthropod damage, hybrid trees had higher damage scores and higher growth scores (height, diameter at breast height over bark, and volume) than pure species. Site was more important than taxon in explaining damage scores, and taxa performed differently for most traits between sites. Tree growth was negatively correlated with general crown damage, while arthropod damage alone showed no significant relationship with growth. Our results highlight the importance of establishing taxa trials across a range of sites when selecting for pest resistance.     

Pruning enhances the susceptibility of Picea abies to infection by the bark beetle‐transmitted blue‐stain fungus,Ophiostoma polonicum

Defoliation of conifers occasionally precedes bark beetle attacks, suggesting that a severe loss of foliage and ensuing reductions in carbohydrate availability may enhance host tree susceptibility. To shed light on this question, different degrees of defoliation on young Picea abies were simulated by removing whole whorls of branches from below, the trees retaining 100, 50, or 25% of their original crown biomass. After one week or one year, the trees were inoculated with Ophiostoma polonkum, a tree‐killing fungus transmitted by Ips typographus.

Fungal proliferation and tree mortality increased with increasing levels of pruning. Pruning reduced stem diameter growth, but not carbohydrate reserves in foliage and bark. Foliar N, P, and Ca increased with increasing pruning. The results lend support to the hypothesis that a reduction in the photosynthesis capacity increases host tree susceptibility to a beetle‐fungus attack, and that induced defence against infection depends on efficient translocation of assimilates to the sites of infection.     

Comparative susceptibility of pine,spruce and larch to sapstain

Summary This study compared the susceptibility of five UK‐grown conifer species to colonization by sapstain fungi in two trials carried out in consecutive years. The conifers consisted of Sitka spruce (Picea sitchensis), Japanese larch (Larix kaempferi), Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and lodgepole pine (Pinus contorta). Freshly cut 1‐m logs were exposed to the available inoculum of sapstain fungi from April to August in a woodland environment in the south east of England. Logs of each species were removed after 1‐, 2‐ and 4‐month exposure and sampled destructively to assess the amount of sapstain. In the second trial, per cent moisture content and concentrations of nitrogen, carbohydrate and phenolic compounds in the sapwood were also measured at the start and end of the trial. After 2 months, only the sapwood of both pine species had significant levels of sapstain; mean values of 37% and 19% for lodgepole pine (year 1 and year 2 respectively) and 12% and 1% for Scots pine. After 4 months, the levels of sapstain in both pine species exceeded 60% in both years. By contrast, very little sapstain developed in the other conifer species with maximum mean values of 10% for Norway spruce, 3.5% for larch and less than 1% for Sitka spruce. Overall, the moisture content of the logs decreased progressively in all species over the length of the trial. However, pine logs tended to retain higher levels of moisture throughout the trial compared with spruce or larch. The relatively higher moisture content of pine sapwood may be closer to the optimal moisture content that sapstain fungi require for infection and colonization, thereby contributing to the increased susceptibility of pine compared with the other conifer species. The pine logs also suffered from some colonization by bark beetles (Ips sexdentatus), which increased the inoculum potential and the opportunity for colonization by sapstain fungi. In addition, particular phenolic compounds in conifer sapwood may play a role in determining the resistance of some species to sapstain. Notably the most resistant species, Sitka spruce, was the only softwood that still retained detectable levels of phenolics in the sapwood to the end of the trial.     

Blue‐stain infections in roots,stems and branches of declining Pinus sylvestris trees in a dry inner alpine valley in Switzerland

Increased mortality rates in Scots pine (Pinus sylvestris) forests have recently been observed in the inner alpine Swiss Rhone valley. Drought, in combination with stand competition, mistletoe infections as well as nematode and insect infestations, appears to be the main factor for the decline. In focus of this study was the occurrence and role of fungal pathogens in the decline dynamics. Branches, stems and roots of 208 trees in five different crown transparency classes were collected and inspected for blue stain and fungal infections. Neither Armillaria species nor Heterobasidon annosum s. str. were detected, but blue stain was commonly observed. Visible blue stain increased with increasing crown transparency. Among the recently dead trees, 80% showed visible blue stain in the branches, 90% in the roots and 100% in the stems. In the crown transparency classes 2 and 3 (25–60% crown transparency), five of the 103 trees showed visible blue stain in the roots, one of 130 trees in the stem but none in the branches. Blue‐stain fungi were isolated from all parts of the trees and from all crown transparency classes. Overall incidence of blue stain was highest in the roots and lowest in the branches. In class 2, roots of 60% of the trees were visibly blue‐stained or developed blue stain in culture, but stems of only 24% and branches of 14% of the trees. In the roots Leptographium species, mostly L. serpens, dominated. From stems and branches, mainly Ophiostoma species were isolated. The positive relationship between the incidence of blue stain and crown transparency, in combination with the high infection levels of roots of fairly vigorous Scots pines, indicates the pathogenic potential of the blue‐stain fungi. Hence, these fungi together with their insect vectors may well act as an important contributing factor involved in pine decline.     

Performance of planted indigenous trees,Acacia galpinii and Faidherbia albida,in semi-arid Botswana

Two popular indigenous browse, shade and poles trees, Acacia galpinii and Faidherbia albida, were evaluated for survival and growth on-farm in Malotwana, Botswana, with a mean annual rainfall of 450 mm. The trial was a two species × three spacing factorial experiment in a randomised complete block design replicated five times. The three spacings were 5 m × 5 m, 6.3 m × 5 m and 8.3 m × 5 m. The study was conducted over 6.5 years. The results showed that A. galpinii is a promising poles, browse and shade species, while F. albida exhibited high mortality (67%) at 4.5 years (after below-average rainfall). The crown width of A. galpinii ranged from 5.86 ± 0.24 m at high density to 6.56 ± 0.25 m in low-density plots. The difference in crown width among densities was significant (p = 0.0406). Diameter at breast height ranged between 10.4 ± 0.68 cm at high density to 12.5 ± 0.64 cm in low-density plantings and was significantly different among spacings (p = 0.0003).     

Rapid living crown reduction caused by Gremmeniella abietina affects foliar nutrient concentrations of Scots pine

The foliar chemistry of diseased and healthy trees was studied one growing season after severe reduction in living crown caused by Gremmeniella abietina in four young Scots pine (Pinus sylvestris) stands. Sample trees were chosen pairwise on the basis of the living crown length: a diseased tree with about 50% live crown reduction and a healthy tree in each pair. Fifteen elements were determined in the youngest healthy needles on the lateral top shoots of each sample tree. Diseased trees had higher foliar boron, manganese and sodium concentrations and lower magnesium, iron, zinc, copper, potassium, nitrogen and sulphur concentrations compared to the healthy trees. Foliar calcium, aluminium, phosphorus, carbon and hydrogen concentrations did not differ between the diseased and healthy trees, except for P and A1 in two of the stands when the stands were analysed separately. Significant correlations between the needle element concentrations and crown ratio (length of the living crown/tree height) were found especially for B (increasing B with decreasing crown ratio) and for Mg, Fe and Zn (decreasing concentrations with decreasing crown ratio). The effect of G. abietina-induced living crown reduction on tree nutrient status and the role of these mineral nutrients in the susceptibility are discussed.     

Effects of silicon-rich soil amendments on growth,mortality and bark feeding damage of Sitka spruce (Picea sitchensis) seedlings under field conditions

Alternative methods of protection are required against feeding by the large pine weevil (Hylobius abietis) on the bark of conifer seedlings. Silicon (Si) has been shown to enhance the resistance of plants to insect herbivores. This study investigated the effects of low doses of Si-rich soil amendments on growth, mortality and bark feeding damage of Sitka spruce (Picea sitchensis) seedlings. Two-year old seedlings were grown, individually, in soil taken from a tree nursery treated with coal ash, peat ash, rice husk ash, slag, sodium metasilicate or a commercially available Si fertiliser (Pro-Tekt) and planted out on two reforestation sites in Ireland. Seedlings grew well (about 20% growth in terms of height, 66% in root collar diameter, after two growing seasons), and Si-rich amendments did not have a significant effect on growth or mortality. Bark feeding damage on Si-treated seedlings did not vary significantly from control seedlings. Bark Si concentrations were not significantly larger in treated seedlings than in control seedlings, but control seedlings already had comparatively high bark Si concentrations (560?mg?kg^?1 dry tissue). In conclusion, Sitka spruce seedlings grown in the presence of Si-rich soil amendments prior to planting did not show greater resistance to weevil feeding under the present conditions.     

Remote sensing analysis of forest damage by selection logging in the Kabaung Reserved Forest, Bago Mountains, Myanmar

Selection logging is a principal management scheme in natural teak-bearing forests in Myanmar. Monitoring the spatial extent and intensity of selection logging is important for sustainable forest management. This study applied the normalized difference vegetation index (NDVI) image differencing method using two SPOT-5 pan-sharpened images (2.5 m spatial resolution) taken in October 2007 and January 2009 to analyze canopy changes associated with damage from forest harvesting. According to the pixel-based analysis, NDVI changes were larger in most logging road/log landing points whereas smaller NDVI changes were seen in most unlogged points. NDVI changes in teak stump areas were related to distance from a logging road and the number of stumps within the estimated crown area (a circular area with a 10 m radius from the center of each stump). A Fisher’s exact test showed that one of the main factors causing the high NDVI change in teak stump areas was the effect of road construction. The distribution pattern of teak stumps indicated that teak stumps with estimated crown areas that contained more than one stump had high mean change in NDVI. The spectral difference between before and after logging revealed that logging roads had a greater effect on canopy changes than teak stumps.     

Infestation Density of Eucalyptus Gall Wasp,Leptocybe invasa Fisher and La Salle (Hymenoptera: Eulophidae) on Five Commercially Grown Eucalyptus Species in Tanzania

This study explores infestation density of Leptocybe invasa on five commercially grown Eucalyptus species in Coastal, Plateaux, and Southern Highlands agroecological zones of Tanzania. Infestation density between agroecological zones, Eucalyptus species, age classes and tree crown parts, relationship between stand altitudes and the magnitude of infestation, damage index, species age, and abundance of L. invasa on different Eucalyptus species were examined. There were significant differences in infestation between zones and Eucalyptus species. Eucalyptus tereticornis was more affected, followed by E. camaldulensis, and E. saligna was the least while E. grandis and E. citriodora were not affected. No significant differences in damage between different crown parts were observed. Trees with age of 1–3 yr were damaged more than those of age 4–6 yr. Pest infestation increased with an increase of L. invasa abundance but decreased with an increase of altitudes. Control efforts needs to focus on controlling the spread of the pest, using silvicultural methods and planting resistant Eucalyptus species.     

Impacts of Teratosphaeria leaf disease on plantation Eucalyptus globulus productivity

The effects of Teratosphaeria leaf disease (TLD) on Eucalyptus globulus are varied, and it is currently poorly understood whether infection by TLD can cause long‐term growth effects. Such information would greatly assist disease management and pruning regimes on E. globulus plantation sites, resulting in both financial and ecological benefits. Two trials were established to quantify the effects of TLD on long‐term growth. The first was a 2‐year fungicide exclusion trial that aimed to determine initial growth losses between trees treated with fungicide and untreated trees. It was found that tree growth was not affected until a threshold value of 20% damage was reached. Volume was reduced by 17% between treated and untreated trees over the course of the 2‐year trial. The second trial, a 5‐year growth study, used differentially affected adjacent stands (one infected and the other unaffected) to look at the longer term effects of more severe defoliation (44–60%) caused by an epidemic of TLD. Results recorded 5 years after the epidemic showed that trees recovered to regain normal growth trajectories after the epidemic, but growth was retarded by ca. 1.2 years for both height and diameter compared with that of the adjoining unaffected stand. As the growth of trees was not permanently reduced by the epidemic, it is concluded that the financial impacts of TLD are more likely to be associated with the loss of income resulting from extensive branch death in the lower crown after leaf and stem infection, which makes the affected stands not suitable for pruning and hence prevents them from being managed as a higher value solid wood crop.