New insights into host specialization and mycoparasitic interaction between powdery mildew fungi and Ampelomyces quisqualis

Pycnidial fungi belonging to the genus Ampelomyces quisqualis are the most widespread natural antagonists of Erysiphales. A. quisqualis is a specific mycoparasite of many species of Erysiphales and the most studied biocontrol agent of powdery mildews. Genetically different A. quisqualis strains are available from culture collections and one strain has been already commercialized under the trade name of AQ10. Report data on the aggressiveness and morphology and cultural patterns of A. quisqualis strains found in the literature are controversial and incomplete. Screening is a crucial step in the selection of strains capable of providing highly effective biocontrol. There is a need for further investigations aiming at the identification of phenotypic markers that can be used to differentiate genetically distinct groups within A. quisqualis in order to find more effective strains within A. quisqualis species, which differ considerably with respect to their biocontrol effectiveness. The first objective of the present work was to verify the presence of natural strains of A. quisqualis in a wide viticulture area (Trentino Alto Adige region). We aimed to isolate and select new strains better adapted to the local environmental conditions than commercial strain AQ10 and highly aggressive against Erysiphaceae for a potential development as biocontrol agents. During a three-year survey, a limited amount of natural parasitism of E. necator by Ampelomyces spp. (0.17−3.51 %) was observed. Pycnidia and conidia of Ampelomyces spp. parasitizing E. necator chasmothecia were found both in conventional, organically grown on and untreated vineyards. Some of the isolated A. quisqualis strains have conidia that are shaped differently than those of the commercial A. quisqualis strain (AQ10) and are phylogenetically different from AQ10. Second objective of the thesis was to characterize several A. quisqualis strains from different hosts and geographic regions and possessing different ITS rDNA sequences and investigate whether the host or site of origin of the strains or their cultural, morphological and/or growth characteristics are related to their phylogenetic group which would indicate an adaptation to the host or geographic area. Strains were molecularly characterized by sequencing the ITS rDNA and sequence polymorphisms were used to classify and group the strains. The results revealed some significant variation among the selected strains, which provides evidence for the existence of different physiological forms within the A. quisqualis species. Phylogenetic analysis revealed that these A. quisqualis strains can be classified into five different genetic groups, which generally correlate with the fungal host of origin and morphological and growth characteristics. Finally, the pathogenicity, virulence and host range of a group of A. quisqualis strains was assessed on different powdery mildew agents. Strains were screened both for their ability to colonize different powdery mildews (mycoparasitic activity) and for in vitro production of cell wall degrading enzymes (CWDEs). This study showed a positive correlation between mycoparasitic activity and production of chitobiases and proteases. A. quisqualis strains with similar levels of mycoparasitic activity originate from the same host species and share an identical ITS rDNA sequence. In the 5th chapter it was investigated whether it is possible to enhance the efficiency of this fungus in the biological control of powdery mildews by increasing the conidial germination rate of the fungus. The obtained results revealed that some natural extracts can stimulate the germination of A. quisqualis conidia and enhance its biocontrol ability of the powdery mildew. This part of the thesis demonstrates that the conidial germination efficacy of A. quisqualis strains is positively related to virulence against powdery mildew and can, therefore, be considered as relevant factor in the selection of biocontrol agents. The results obtained in this thesis provide a deeper understanding of the process of mycoparasitism and a sound basis for developing new screening strategies for detecting highly effective A. quisqualis strains for the biocontrol of powdery mildews. Moreover, we demonstrated the existence of A. quisqualis strains well adapted to local environmental conditions. Their discovery may be the starting point for their development as biocontrol agents to control powdery mildew under the environmental conditions found in Northern countries