Genetic diversity in Plasmopara viticola in South Africa

[摘要] ENGLISH ABSTRACT: Downy mildew, caused by the obligate pathogen Plasmopara viticola, is a verydestructive grapevine disease. The asexual phase (sporangia) of the pathogen has longbeen viewed as the life cycle stage that is most important in causing expansion ofepidemics. Contrarily, the role of the sexual phase (oospores) has primarily been viewedas only providing the initial primary inoculum of the epidemic at the start of the season.However, population genetic studies in Europe have challenged these long standingepidemiological views.Downy mildew is mainly controlled through the application of fungicides, sinceno commercially acceptable resistant cultivars are available. The use of reliable highthroughput in vitro resistance screening methods is very important for identifying newsources of resistance, as well as for mapping of quantitative trait loci (QTLs) involved indowny mildew resistance. Resistance screenings also require the use of effective longtermpathogen storage methods, since it allows the continual use of the same wellcharacterised P. viticola isolates in different resistance screenings over seasons.The first main aim of this study was to investigate the population genetic structureof P. viticola populations in South Africa in two vineyards. The second aim was todetermine whether an in vitro leaf disk method is a reliable and reproducible resistancescreening method for determining downy mildew resistance of grapevine seedlings. Thethird aim was to evaluate long-term storage techniques for P. viticola isolates.The population genetic structure of P. viticola was investigated m twoconsecutive grape growmg seasons in an organically managed and a conventionalfungicide sprayed vineyard. The study showed that population differentiation betweenthe two vineyards was low (0.004 and 0.016) in both growing seasons, suggesting onemetapopulation. New genotypes (12% to 74%) contributed to the epidemic throughoutthe growing seasons in both years and vineyards. The epidemic in both years andvineyards were dominated by one or two genotypes, which contributed between 14% and 67% through asexual reproduction to the epidemic. The remaining genotypes showedlow levels of asexual reproduction, with most genotypes never being able to reproduceasexually. Ten genotypes were able to survive asexually from one season to the next.Moreover, the predominant genotype in the organically grown vineyard during 2004/05survived asexually to the next season, where it also dominated the epidemic.Evaluation of an in vitro leaf disk method showed that the method was a reliableand reproducible method for screening the downy mildew resistance of the progeny of aRegent x Red Globe cross. Spearman correlation analyses revealed a moderate to high(0.64 to 0.82) correlation between three screening trails that were conducted over twogrowing seasons. However, the percentage seedlings that belonged to the different OIV452 rating classes differed between the third (2005/06) and the first two (2004/05)resistance screening trials. This difference was statistically supported by one-wayanalysis of variance of rank means of these screenings, as well as Chi-square test of thescreening x rating scale contingency table. This discrepancy indicates the importance ofthe inclusion of tolerant and sensitive reference seedlings, as well as the parents of thecross in each screening trial.Evaluation of different long-term storage methods for P. viticola showed that thepathogen was best stored as lesions. Successful storage methods included the storage ofwhole leaves with sporulating lesions in sealed Petri dishes, or the storage of small leaflesion fragments within 2 ml centrifuge tubes at -20 and -80°C. Viability testing of thesestorage methods after a period of 6 (leaves within Petri plates) and 1 7 months (lesionswithin centrifuge tubes) showed that the pathogen remained viable for these periods,although the viability of sporangia were reduced.

[发布机构] Stellenbosch University