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A method for evaluating the potential threat of selection for resistance to organically-based fungicides in populations of P. viticola is needed to screen a large panel of products alternative to copper in organic viticulture. Populations from an unexposed plot were compared throughout one season with a population sprayed with azoxystrobin (Quadris), reported as engendering selection pressure and resistance, and a population sprayed with an organically-based fungicide (Mycosan). The evolution of the three populations was followed with neutral specific SSR markers and with the specific marker for strobilurin resistance, as control of selection for resistant mutants. A reduction in genetic diversity of the P. viticola population was observed in the population sprayed with azoxystrobin, consistent with directional selection toward higher resistance, confirmed by an enhanced frequency of resistant mutants with respect to the unexposed population. In contrast, a higher diversity and a reduced frequency of resistant mutants were observed in the population sprayed with the organically-based fungicide. Assessing a reduction of genotypic diversity allows the detection of selection for resistance and constitutes a valid instrument for screening a large panel of products with non-specific, different and possibly indirect modes of action

The genetic structure of a Plasmopara viticola population was characterized on five single vines, one for each cultivar Regent, Merlot, Isabella, Müller-Thurgau and Solaris, using four neutral specific polymorphic microsatellite markers. Five-hundred and seventy samples were collected at four dates in the period between the 10th of July and the 23rd of August 2006. On average over all five cultivars, 67% of the genotypes present on the single selected vines derived from primary infections and caused 37% of the lesions genotyped. Fifty-three percent of these genotypes occurred only once on the vine throughout the survey period, while 14% were able to asexually reproduce on the selected single vine throughout the survey period, causing 23% of the lesions. Thirty-three percent of the genotypes on the single vine derived from other vines, 28% from vines of other cultivars in the other rows, and 5% from vines of the same cultivar in the same row. New primary infections appear all along the sampling dates. The overwhelmingly quantitative role of primary infections at vineyard scale was known, however here we observed the phenomenon also at the single vine scale and the reduced contribution of secondary lesions to the populations present on more resistant cultivars compared to the susceptible cultivars. As the sampling extended almost to defoliation, the results are judged to be representative of a typical P. viticola epidemic

This study aimed to evaluate the impact of genetically modified (GM) wheat with introduced pm3b mildew resistance transgene, on two types of root-colonizing microorganisms, namely pseudomonads and arbuscular mycorrhizal fungi (AMF). Our investigations were carried out in field trials over three field seasons and at two locations. Serial dilution in selective King's B medium and microscopy were used to assess the abundance of cultivable pseudomonads and AMF, respectively. We developed a denaturing gradient gel electrophoresis (DGGE) method to characterize the diversity of the pqqC gene, which is involved in Pseudomonas phosphate solubilization. A major result was that in the first field season Pseudomonas abundances and diversity on roots of GM pm3b lines, but also on non-GM sister lines were different from those of the parental lines and conventional wheat cultivars. This indicates a strong effect of the procedures by which these plants were created, as GM and sister lines were generated via tissue cultures and propagated in the greenhouse. Moreover, Pseudomonas population sizes and DGGE profiles varied considerably between individual GM lines with different genomic locations of the pm3b transgene. At individual time points, differences in Pseudomonas and AMF accumulation between GM and control lines were detected, but they were not consistent and much less pronounced than differences detected between young and old plants, different conventional wheat cultivars or at different locations and field seasons. Thus, we conclude that impacts of GM wheat on plant-beneficial root-colonizing microorganisms are minor and not of ecological importance. The cultivation-independent pqqC-DGGE approach proved to be a useful tool for monitoring the dynamics of Pseudomonas populations in a wheat field and even sensitive enough for detecting population responses to altered plant physiology.

The genetic structure of a Plasmopara viticola population was characterized on five single vines, one for each cultivar Regent, Merlot, Isabella, Müller-Thurgau and Solaris, using four neutral specific polymorphic microsatellite markers. Five-hundred and seventy samples were collected at four dates in the period between the 10th of July and the 23rd of August 2006. On average over all five cultivars, 67% of the genotypes present on the single selected vines derived from primary infections and caused 37% of the lesions genotyped. Fifty-three percent of these genotypes occurred only once on the vine throughout the survey period, while 14% were able to asexually reproduce on the selected single vine throughout the survey period, causing 23% of the lesions. Thirty-three percent of the genotypes on the single vine derived from other vines, 28% from vines of other cultivars in the other rows, and 5% from vines of the same cultivar in the same row. New primary infections appear all along the sampling dates. The overwhelmingly quantitative role of primary infections at vineyard scale was known, however here we observed the phenomenon also at the single vine scale and the reduced contribution of secondary lesions to the populations present on more resistant cultivars compared to the susceptible cultivars. As the sampling extended almost to defoliation, the results are judged to be representative of a typical P. viticola epidemic

A method for evaluating the potential threat of selection for resistance to organically-based fungicides in populations of P. viticola is needed to screen a large panel of products alternative to copper in organic viticulture. Populations from an unexposed plot were compared throughout one season with a population sprayed with azoxystrobin (Quadris), reported as engendering selection pressure and resistance, and a population sprayed with an organically-based fungicide (Mycosan). The evolution of the three populations was followed with neutral specific SSR markers and with the specific marker for strobilurin resistance, as control of selection for resistant mutants. A reduction in genetic diversity of the P. viticola population was observed in the population sprayed with azoxystrobin, consistent with directional selection toward higher resistance, confirmed by an enhanced frequency of resistant mutants with respect to the unexposed population. In contrast, a higher diversity and a reduced frequency of resistant mutants were observed in the population sprayed with the organically-based fungicide. Assessing a reduction of genotypic diversity allows the detection of selection for resistance and constitutes a valid instrument for screening a large panel of products with non-specific, different and possibly indirect modes of action