SummaryAn analysis of incidence of Phytophthora spp. in 732 European nurseries producing forest transplants, larger specimen trees, landscape plants and ornamentals, plus 2525 areas in which trees and shrubs were planted, is presented based on work conducted by 38 research groups in 23 European countries between 1972 and 2013. Forty-nine Phytophthora taxa were recorded in 670 nurseries (91.5%); within these nurseries, 1614 of 1992 nursery stands (81.0%) were infested, although most affected plants appeared healthy. In forest and landscape plantings, 56 Phytophthora taxa were recovered from 1667 of 2525 tested sites (66.0%). Affected plants frequently showed symptoms such as crown thinning, chlorosis and dieback caused by extensive fine root losses and/or collar rot. Many well-known highly damaging host-Phytophthora combinations were frequently detected but 297 and 407 new Phytophthora-host associations were also observed in nurseries and plantings, respectively. On average, 1.3 Phytophthora species/taxa per infested nursery stand and planting site were isolated. At least 47 of the 68 Phytophthora species/taxa detected in nurseries and plantings were exotic species several of which are considered well established in both nurseries and plantings in Europe. Seven known Phytophthora species/taxa were found for the first For. Path. 46 (2016) 134-163 doi: 10.1111/efp.12239 © 2015 http://wileyonlinelibrary.com/ time in Europe, while 10 taxa had not been previously recorded from nurseries or plantings; in addition, 5 taxa were first detections on woody plant species. Seven Phytophthora taxa were previously unknown to science. The reasons for these failures of plant biosecurity in Europe, implications for forest and semi-natural ecosystems and possible ways to improve biosecurity are discussed.
Summary Chestnut blight, caused by Cryphonectria parasitica, is a devastating disease infecting American and European chestnut trees. The pathogen is native to East Asia and was spread to other continents via infected chestnut plants. This review summarizes the current state of research on this pathogen with a special emphasis on its interaction with a hyperparasitic mycovirus that acts as a biological control agent of chestnut blight. Taxonomy Cryphonectria parasitica (Murr.) Barr. is a Sordariomycete (ascomycete) fungus in the family Cryphonectriaceae (Order Diaporthales). Closely related species that can also be found on chestnut include Cryphonectria radicalis, Cryphonectria naterciae and Cryphonectria japonica. Host range Major hosts are species in the genus Castanea (Family Fagaceae), particularly the American chestnut (C. dentata), the European chestnut (C. sativa), the Chinese chestnut (C. mollissima) and the Japanese chestnut (C. crenata). Minor incidental hosts include oaks (Quercus spp.), maples (Acer spp.), European hornbeam (Carpinus betulus) and American chinkapin (Castanea pumila). Disease symptoms Cryphonectria parasitica causes perennial necrotic lesions (so‐called cankers) on the bark of stems and branches of susceptible host trees, eventually leading to wilting of the plant part distal to the infection. Chestnut blight cankers are characterized by the presence of mycelial fans and fruiting bodies of the pathogen. Below the canker the tree may react by producing epicormic shoots. Non‐lethal, superficial or callusing cankers on susceptible host trees are usually associated with mycovirus‐induced hypovirulence. Disease control After the introduction of C. parasitica into a new area, eradication efforts by cutting and burning the infected plants/trees have mostly failed. In Europe, the mycovirus Cryphonectria hypovirus 1 (CHV‐1) acts as a successful biological control agent of chestnut blight by causing so‐called hypovirulence. CHV‐1 infects C. parasitica and reduces its parasitic growth and sporulation capacity. Individual cankers can be therapeutically treated with hypovirus‐infected C. parasitica strains. The hypovirus may subsequently spread to untreated cankers and become established in the C. parasitica population. Hypovirulence is present in many chestnut‐growing regions of Europe, either resulting naturally or after biological control treatments. In North America, disease management of chestnut blight is mainly focused on breeding with the goal to backcross the Chinese chestnut's blight resistance into the American chestnut genome.
Ink disease. caused by Phytophthora cambivora (Petri) Buism .. attacks the roots of chestnuts, leads to branch mortality. symptoms of general decline. and finally to the death of the plants.
T he genus Armillaria causes root rot disease in both gymnoand angiosperms, in forests, parks, and even vineyards in more than 500 host plant species 1 across the world. Most Armillaria species are facultative necrotrophs, which, after colonizing and killing the root cambium, transition to a saprobic phase, decomposing dead woody tissues of the host. As saprotrophs, Armillaria spp. are white rot (WR) fungi, which can efficiently decompose all components of plant cell walls, including lignin, (hemi-)cellulose and pectin 2 . They produce fleshy fruiting bodies (honey mushrooms) that appear in large clumps around infected plants and produce sexual spores. The vegetative phase of Armillaria is predominantly diploid rather than dikaryotic like most basidiomycetes.Individuals of Armillaria can reach immense sizes and include the 'humongous fungus' , one of the largest terrestrial organisms on Earth 3 , measuring up to 965 hectares and 600 tons 4 , and can display a mutation rate ≅ 3 orders of magnitude lower than most filamentous fungi 5 . Individuals reach this immense size via growing rhizomorphs, dark mycelial strings 1-4 mm wide that allow the fungus to bridge gaps between food sources or host plants 1,6 (hence the name shoestring root rot). Rhizomorphs develop through the aggregation and coordinated parallel growth of hyphae, similar to some fruiting body tissues 7,8 . As migratory and exploratory organs, rhizomorphs can grow approximately 1 m yr −1 and cross several metres underground in search for new hosts, although roles in uptake and longrange translocation of nutrients have also been proposed 1,9,10 . Root contact by rhizomorphs is the main mode of infection by the fungus, which makes the prevention of recurrent infection in Armillariacontaminated areas particularly difficult 1 . Despite their huge impact on forestry, horticulture and agriculture, the genetics of the pathogenicity of Armillaria species is poorly understood. The only -omics data published so far have highlighted a substantial repertoire of plant cell wall degrading enzymes (PCWDE) and secreted proteins, among others, in A. mellea and A. solidipes 11,12 , while analyses of the genomes of other pathogenic basidiomycetes (such as Moniliophthora 13,14 , Heterobasidion 15 and Rhizoctonia 16 ) identified genes coding for PCWDEs, secreted and effector proteins or secondary metabolism (SM) as putative pathogenicity factors. However, the lifecycle and unique dispersal strategy of Armillaria prefigure other evolutionary routes to pathogenicity, which, along with other potential genomic factors (such as transposable elements 17 ) are not yet known.Here, we investigate genome evolution and the origin of pathogenicity in Armillaria using comparative genomics, transcriptomics
The Cryphonectria parasitica populations in two 6-year-old European chestnut (Castanea sativa) coppices were investigated in southern Switzerland over a period of 4 years. Occurrence of white isolates indicating an infection with Cryphonectria hypovirus, vegetative compatibility groups (VCGs), hypovirulence conversion capacity, and mating types were used to characterize the populations. Sampling of randomly chosen cankers in the first year yielded 59% white isolates in one and 40% in the other population. The distribution of the VCGs and mating types was similar among white and orange isolates, indicating a homogeneous infection of the two populations by the hypovirus. Fourteen VCGs were found in the first population, 16 VCGs in the second. Altogether, 21 VCGs were determined. The same three VCGs dominated in both populations, comprising more than 60% of all isolates. Several VCGs were represented only by white isolates. Five of the six most common VCGs were clustered in two hypovirulence conversion groups, with almost 100% hypovirus transmission within each cluster. Repeated sampling of the same cankers in 1990, 1992, and 1994 did not reveal an increase of white isolates. The portion of blighted stems rose from 37% to about 60% in both plots within 4 years. In this time, chestnut blight killed 15% and competition an additional 21% of the sprouts. Predominantly, sprouts with low diameters at breast height were killed. The growth rate of new cankers was high in their first year and decreased gradually in the following years. A role of hypovirulence in the decline of disease severity was evident since (i) cankers yielding white isolates grew slower and killed considerably fewer sprouts than cankers with orange isolates; and (ii) the majority of the cankers yielded white isolates at least once during the 4-year observation period.
A total of 72 hypovirus-infected isolates of the chestnut blight fungus Cryphonectria parasitica were sampled from nine European countries between 1975 and 1997. The double-stranded RNA of the Cryphonectria hypoviruses (CHV1) was isolated and reverse transcription (RT)-PCR products were obtained for two different regions of the viral genome (ORF A and ORF B) using primer sequences of the type species CHV1-EP713. Both PCR products of each viral isolate were digested with four restriction endonucleases recognizing sequences of four nucleotides. The restriction fragment length polymorphism (RFLP) analysis revealed 41 genetically distinct RFLP types of CHV1 with 10 types occurring more than once. Identical RFLP types were detected nine times among viruses collected in the same location. Cluster analysis based on the RFLP banding patterns separated the viral isolates into five CHV1 clusters or subtypes. Most viral isolates (64 out of 72) grouped into one large cluster which comprised all viruses from Italy (including CHV1-EP747), Switzerland, Crotia, Bosnia, Hungary, Greece, and the French island Corsica, as well as five out of 11 isolates from continental France. Two additional subtypes of CHV1 were found in France (one related to CHV1-EP713) and one each in Spain and Germany. The Swiss samples collected over a period of 20 years showed that very little RFLP variation has evolved during this time. The results of this study are consistent with the hypothesis of multiple introductions of CHV1 into Europe.
Summary1 This paper assesses whether tree-ring patterns found in recently dead mountain pines ( Pinus mugo Turra) infected by Armillaria spp. differ from those infected by Heterobasidion annosum , and determines whether and to what extent tree rings may be used as indicators of tree-decline history (i.e. tree health conditions in relation to disease history) prior to death. 2 Dendroecological and phytopathological analyses were undertaken in the Swiss National Park. The calendar year of death of the standing dead trees was determined by cross-dating ring-width patterns of dead trees to reference chronologies from living trees. This procedure is not, however, exact as there may be multiple intermittent missing rings. 3 A remarkable discrepancy (up to 31 years) was found between the tree-death year estimated through crown condition assessment (i.e. the presence or lack of green needles) and the date of the outermost tree ring (when tree-ring production ceased). New needles may form and existing ones remain green for some years after the cambium at different heights along the stem has ceased activity and no new wood cells are being formed. 4 Ring-patterns in trees infected by Armillaria differ from those in trees infected by H. annosum . All dead trees infected by Armillaria had a slow growth decrease indicating suppression for several decades, and suggesting that Armillaria attacked trees that were already weakened by competition. In contrast, trees infected by H. annosum died over a very short period of time, although they may have been infected a long time previously. Nevertheless H. annosum seems to infect and kill trees directly , whereas Armillaria , at this site, is a secondary pathogen. 5 This study demonstrates that tree rings may be used as indicators of the history of tree decline prior to tree death. However, the history of tree disease is difficult to reconstruct fully, e.g. tree rings do not enable the onset of infection to be dated.
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