Kurzbeschreibung
(Englisch)
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The objective of this project is to elucidate the complex of causal agents involved in Apple Replant Disease (ARD) in Swiss orchards. The etiology of this disease is complex and causal agents vary among regions and sites. Identified causal agents cover a broad spectrum including fungi (Cylindrocarpon destructans and Rhizoctonia solani), oomycetes (Phytophthora cactorum and Pythium spp.), actinomycetes and nematodes. We will investigate the presence of pathogens possibly associated with ARD in selected orchards using conventional and molecular techniques. Soil and rhizosphere samples will be taken from orchards with trees showing repressed growth on previous tree rows compared to trees growing on previous grass lanes (resulting from replanting with altered row distance). Such sites are known from a survey performed by ACW in Switzerland. Diagnostic PCR assays have been developed for environmental samples for organisms involved in the ARD complex. These assays will be established in our lab and used to assess the presence of these organisms in soil and root samples. Plant parasitic nematodes will be extracted from the soils and/or roots and identified to elucidate their contribution to the ARD complex. In addition, we will use molecular fingerprinting techniques to address differences in microbial soil and rhizosphere communities associated with trees showing poor and good growth, respectively.
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Partner und Internationale Organisationen
(Englisch)
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AT, BE, BG, CH, CZ, DE, DK, ES, FI, FR, HU, IE, IL, IT, LT, LU, LV, NL, NO, PL, SE, SI, TR, UK
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Abstract
(Englisch)
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Apple replant disease (ARD) is a soil borne disease-complex that affects tree growth and reduces yields when orchards are replanted. The generally observed improvement of apple tree growth after soil pasteurization or fumigation demonstrates the biotic nature of the phenomenon but the etiology of this disease is complex and causal agents vary among regions and sites. Identified causal agents cover a broad spectrum including fungi (Cylindrocarpon destructans and Rhizoctonia solani), oomycetes (Phytophthora cactorum and Pythium spp.), actinobacteria and nematodes. In the first part of the project, investigations focused on the presence of pathogens reported to be associated with ARD in other regions. Soil and rhizosphere samples were taken from three Swiss orchards with trees planted on soil with different replant history, e.g. first vs. thrid generation of apple plantation. In addition soil samples were taken from ARD field trials performed in Germany and Austria. Apple seedlings grown on soil from the selected orchards showed growth enhancement through chemical soil sterilization confirming the presence of biotic replant problems. In non-sterile soil, differences in seedling growth were observed between soil from replanted and first time planted plots, but not between soil sampled in previous tree rows and previous driving lanes, respectively. The fungus C. destructans was detected with a specific PCR assay in all orchards. Its high recovery on apple seedling roots in bioassays confirms a pathogenic potential but detection frequency was not increased in replanted soil. The other organims tested with specific PCR assays, beeing R. solani, P. cactorum and P. ultimum, were found occationally without an obvious correlation to replant history. Nematode populations were investigated in two replanted orchards. At both sites, plant parasitic nematodes were present at too low number to play an important role in ARD. In summary, our investigations could not confirm the importance of causal agents of ARD known from other sites. In the second part of the project, ribosomal internal spacer analysis (RISA) was used to address differences in soil fungal communities. Cluster analysis and redundancy analysis of RISA profiles from different soil samples showed significant differentiation between sampling in spring and autumn and between plots with first and repeated apple plantation. No consistent separation was possible between soils from previous tree rows and previous driving lanes, confirming the observations in apple seedling bioassays, but questioning the usefulness of replanting in previous driving lanes. Comparison of RISA profiles identified fragments present in replanted soil only. The identification of the organisms behind these fragment is part of a current follow-up project, possibly leading to identification of new organisms involved into ARD complex.
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