Previous phytopathological studies of the fungal pathogen Nectria cinnabarina have been focused on its distribution and host diversity but little is known about the spread of this pathogen and the defence responses of forest trees to an infection inside host tissues. Histopathological alterations of bark, periderm, phloem and woody tissues were investigated in sycamore maple (Acer pseudoplatanus) branches following their natural attack by the advanced anamorph and teleomorph developmental stages of the fungus. Light, fluorescence, confocal laser scanning and scanning electron microscopy techniques supplemented by X-ray micro-computed tomography imaging were used to distinguish between healthy and disintegrated plant tissues. The intercellular spread of fungal hyphae was found primarily in the phelloderm. Expanding hyphae aggregations produced ruptures in the phellem and the disintegration of both phellogen and phellodermal parenchyma cells in close proximity to the expanding fruiting bodies of the fungus. Thicker hyphae of the teleomorph fungal stage heavily disintegrated the phelloderm tissues and also induced enhanced sclerification of the nearby phloem tissues that limited the spread of the infection into the sieve tubes. Both the intercellular and intracellular spread of hyphae inside the peripheral parts of sclereid clusters led to the disintegration of the compound middle lamellae but the hyphae were only rarely able to pass through these structural phloem barriers. The massive fungal colonization of both lumens and disintegrated tangential cell walls of ray parenchyma cells resulted in severe cambial necroses. Although the hyphae penetrated into the outermost annual growth rings of the xylem, no cell wall disintegration of the parenchyma cells, vessels and fibres was revealed. Despite the local cambial necroses and severe phloem ray disintegration, the bark remained attached to the examined branches and no bark cankers were formed. Full article

As an important nonwood bioresource, fishscale bamboo (Phyllostachys heteroclada Oliver) is widely distributed in the subtropical region of China. Rhombic-spot disease, caused by Neostagonosporella sichuanensis, is one of the most serious diseases that threatens fishscale bamboo health. However, there is limited knowledge about how rhombic-spot disease influences the diversity and structures of phyllosphere fungal communities. In this study, we investigated the phyllosphere fungal communities from stems, branches, and leaves of fishscale bamboo during a rhombic-spot disease outbreak using 18S rRNA sequencing. We found that only the phyllosphere fungal community from stems was significantly affected by pathogen invasion in terms of community richness, diversity, and structure. FUNGuild analysis revealed that the major classifications of phyllosphere fungi based on trophic modes in stems, branches, and leaves changed from symbiotroph-pathotroph, no obvious dominant trophic mode, and symbiotroph to saprotroph, saprotroph–pathotroph–symbiotroph, and saprotroph–symbiotroph, respectively, after pathogen invasion. The fungal community composition of the three tissues displayed significant differences at the genus level between healthy and diseased plants. The associations among fungal species in diseased samples showed more complex co-occurrence network structures than those of healthy samples. Taken together, our results highlight the importance of plant pathological conditions for the assembly of phyllosphere fungal communities in different tissues. Full article

Scots pine (Pinus sylvestris L.) is one of the most economically important species to the Swedish forest industry, and cost-efficient planting methods are needed to ensure successful reestablishment after harvesting forest stands. While the majority of clear-cuts are replanted with pre-grown seedlings, direct seeding can be a viable option on poorer sites. Organic fertilizer has been shown to improve planted seedling establishment, but the effect on direct seeding is less well known. Therefore, at a scarified (disc trencher harrowed) clear-cut site in northern Sweden, we evaluated the effect of early, small-scale nitrogen addition on establishment and early recruitment of fungi from the disturbed soil community by site-planted Scots pine seeds. Individual seeds were planted using a moisture retaining germination matrix containing 10 mg nitrogen in the form of either arginine phosphate or ammonium nitrate. After one growing season, we collected seedlings and assessed the fungal community of seedling roots and the surrounding soil. Our results demonstrate that early, small-scale N addition increases seedling survival and needle carbon content, that there is rapid recruitment of ectomycorrhizal fungi to the roots and rhizosphere of the young seedlings and that this rapid recruitment was modified but not prevented by N addition. Full article

The major threats to the sustainable supply of forest tree products are adverse climate, pests and diseases. Climate change, exemplified by increased drought, poses a unique threat to global forest health. This is attributed to the unpredictable behavior of forest pathosystems, which can favor fungal pathogens over the host under persistent drought stress conditions in the future. Currently, the effects of drought on tree resistance against pathogens are hypothetical, thus research is needed to identify these correlations. Norway spruce (Picea abies (L.) H. Karst.) is one of the most economically important tree species in Europe and is considered highly vulnerable to changes in climate. Dedicated experiments to investigate how disturbances will affect the Norway spruce—Heterobasidion sp. pathosystem are important, in order to develop different strategies to limit the spread of H. annosum s.l. under the predicted climate change. Here, we report a transcriptional study to compare Norway spruce gene expressions to evaluate the effects of water availability and the infection of Heterobasidion parviporum. We performed inoculation studies of three-year-old saplings in a greenhouse (purchased from a nursery). Norway spruce saplings were treated in either high (+) or low (−) water groups: high water group received double the water amount than the low water group. RNA was extracted and sequenced. Similarly, we quantified gene expression levels of candidate genes in biotic stress and jasmonic acid (JA) signaling pathways using qRT-PCR, through which we discovered a unique preferential defense response of H. parviporum-infected Norway spruce under drought stress at the molecular level. Disturbances related to water availability, especially low water conditions can have negative effects on the tree host and benefit the infection ability of the pathogens in the host. From our RNA-seq analysis, 114 differentially expressed gene regions were identified between high (+) and low (−) water groups under pathogen attack. None of these gene pathways were identified to be differentially expressed from both non-treated and mock-control treatments between high (+) and low (−) water groups. Finally, only four genes were found to be associated with drought in all treatments. Full article

This study provides new information on the infection biology and pathogenicity of an important root-rot fungus, Heterobasidion annosum sensu stricto (Fr.) Bref., through a detailed examination of the vegetative spread of clonal individuals and their capacity to produce fruiting bodies on young pine seedlings. The seedlings were planted in a clear-cutting area (c. 1.2 ha in size) after a pine generation that showed slight external symptoms of Heterobasidion root rot. The first dead seedlings were found five years after planting and during a nine-year monitoring period; nearly 600 seedlings were killed by H. annosum s.s. in 48 individual disease centers. Based on pairing tests of 482 isolates, 117 different H. annosum s.s. genotypes were identified. On average, 2.9 genotypes occurred in a single disease center. The extensive secondary spread of genotypes within root systems (up to 48 pine seedlings infected by the same genotype) resulted in annually expanding disease centers. In addition, more than half of the seedlings killed by H. annosum s.s. produced perennial fruiting bodies thus providing air-borne inoculum. The risk of spore infection should be taken into account in any type of cutting operation in young pine stands. Moreover, new control measures directed towards the secondary spread of H. annosum s.s. in pine regeneration are urgently needed in order to maintain the productivity of the pine forest on infested sites. Full article

The European spruce bark beetle (Ips typographus) has become a major forest pest in Finland in recent years. The beetle is a well-known vector of mainly ophiostomatoid fungi causing blue-stain of timber and pathogens that have the ability to amplify the insect damage. It also vectors other associated organisms, such as phoretic mites. The ecology of these mites remains poorly understood, including their associations with fungi. In this study, we considered filamentous fungi and yeasts associated with mites phoretic on I. typographus. Fungal identifications were based on DNA sequences and phylogenetic analyses of the ITS and/or partial β-tubulin gene regions. Fifteen fungal species were detected, including eight yeasts and seven filamentous fungi. Eleven percent of the beetles carried mites and of these 74% carried at least one fungal species. An average of two fungal species were carried per mite. The most commonly found filamentous fungi were Grosmannia penicillata (25%), Ophiostoma bicolor (19%), O. ainoae (12%) and O. brunneolum (12%). Of the yeast species, the most commonly found was Wickerhamomyces bisporus (47%). This study is the first to report yeasts associated with I. typographus and its phoretic mites in Finland. Majority of the filamentous fungal species found are those previously reported in association with I. typographus. The results also confirmed that many of the fungal species commonly found on I. typographus are also associated with its phoretic mites. However, the nature of the symbiosis between the mites, beetles and fungal associates remains to be understood. Full article