Biodiversität im Wirt-Pathogen-System Apfel/ Apfelmehltau (<em>Malus</em> spp./ <em>Podosphaera leucotricha</em> (Ell. & Ev.) E.S. Salmon)

Variabilität des Apfelmehltaus auf molekularer Ebene, in der Virulenz auf <em>Malus</em>-Genotypen mit pyramidisierten Resistenzen sowie in Cytochrom b-bedingter Strobilurinresistenz

Abstract

The objective of this work was to gain new information about the variability of apple powdery mildew relevant for their usage in apple resistance breeding. A selection of isolates has been tested for their virulence on a collection of Malus-genotypes and greater numbers of samples of the pathogen have been screened with molecular markers for their variability. A third part of this work was concentrated on the question whether the strobilurin-fungicide-resistance G143A, which can be found in other pathogens, is also present in apple powdery mildew. Twelve isolates of Podosphaera leucotricha have been tested on a collection of 41 Malus genotypes. The collection included, besides susceptible controls, mostly genoytpes which are known to carry well described mildew resistance genes but also wild species and cultivars which are considered to be resistant or moderately susceptible. The tests have shown that all isolates differ in their virulence and thus proved the variability to be quite high. Also isolates differed clearly although they originated from a single bud. On the plant side the experiments have confirmed the effectiveness of the known resistances in the genotypes D12 (Pld), MIS (Plmis) and 'White Angel' (Plw). No infection has been observed on these genotypes with the tested isolates. The results for the resistances from M. x robusta (Pl1) and M. zumi (Pl2) were different. The reference genotypes for these major resistances (A142/5 and M. zumi 274) could be infected by most of the isolates, the resistances were clearly overcome. Similar findings of virulent isolates on Pl1- and Pl2-genotypes in the field have been published. Apart from the reference genotypes also progeny of a cross between Pl1- and Pl2-genotypes have been tested, which partly carry the resistance genes in combination. The results showed that these plants reacted differently than the reference genotypes: plants with the single genes also showed infection but were less susceptible. Genoytpes which carried the two genes in combination were in contrast mainly resistant. These results suggest that the Pl1- and Pl2-resistances are probably not monogenic but the result of several interacting genes. Pyramiding of resistances appears to be an effective method to protect plants from virulent isolates even if the single resistances have been overcome. Possibly additive effects from the different genes offer the additional protection against the pathogen. Molecular AFLP-markers have been developed and applied on a collection of 27 isolates from Europe, China and India. Pedigrees have been produced based on the marker data which separate the isolates into two distinct groups. There has been no clear correlation between origin of the isolates, primary or secondary infection type or degree of aggressiveness. It is unclear by what the determination into the two groups is caused. It was noticeable that isolates in group 1 showed much more homogeneity than isolates in group 2. Moreover, field samples from different locations in Europe, China and India have been analysed with RAPD- and SCAR-markers and showed the same division into two separate groups, of which group 2 appeared subdivided into two subgroups. Subgroup 2-2 included only field samples and no isolates. Similarities between these results and findings for grape powdery mildew have been discussed. Regarding the fungicide resistance the objective of this work was to find out whether a mutation in the mitochondrial cytochrome b-gene of P. leucotricha leads to resistance against strobilurin fungicides, as has been shown for other pathogens. Molecular analyses showed a high variability in the corresponding region of the gene. All isolates tested were heteroplasmatic for the G143A-mutation. This has been shown both for isolates sampled from trees recently treated with strobilurin fungicides and for isolates which have been cultivated without fungicide contact for several years. The proportion of resistant and wildtype forms within a single isolate varied between the isolates regardless to their origin. Sensitivity tests and germination tests with three isolates also showed differences in the reaction to the fungicide. One isolate has been able to grow under much higher fungicide concentrations than the two others. At a concentration ten times higher than the recommended field concentration still 18% of the spores were able to germinate. The results suggest that the G143A-mutation is prevalent in apple powdery mildew but maybe independent from a previous application of strobilurin fungicides. Normally the frequent application of fungicides leads to development or selection of resistant forms of the pathogens. To prevent the selection of highly resistant strains the guidelines for fungicide application should be followed strictly: strobilurins should be used only in combination with contact fungicides and repeated application should be avoided.