Abstract
Downy mildew, caused by Plasmopara viticola, is the main disease affecting vineyards in subtropical Brazil. Here, we collected 94 P. viticola isolates from four organic and conventional vineyards in the two main grape-growing states of Brazil to evaluate the sensitivity to the quinone outside inhibitor (QoI) azoxystrobin by pheno- and genotyping assays. The impact of location, production system and sensitivity to QoI fungicides on the population genetics and structure of P. viticola was determined using 10 microsatellite markers. Cytochrome b sequencing revealed that 28 and 100% of the isolates from vineyards under organic and conventional management carried the G143A mutation, respectively. The G143A mutation was associated with high levels of azoxystrobin resistance. Three out of the 94 isolates analyzed carried the M125I alteration, not previously described in P. viticola, which was associated with a five-fold reduction in azoxystrobin sensitivity compared to wild-type isolates. Haplotype network analysis based on cytochrome b gene sequences suggested that the Brazilian populations are more closely related to the European than the North American population. A total of six haplotypes were identified, with two of them carrying the G143A mutation. Microsatellite analysis revealed high allelic and genotypic variation among the four populations. Population differentiation analyses indicated that state of origin directly influences the population biology of P. viticola, while production system and QoI sensitivity have little effect. Great genetic diversity, sexual reproduction and high levels of admixture were observed in Rio Grande do Sul State. In contrast, populations in São Paulo State were dominated by a few clonal genotypes, and no admixed genotype was detected between the two genetic pools identified in the state. This study raises the hypothesis that winter weather conditions influence the overwinter survival strategy with profound effects in the population biology of P. viticola.
Highlights
Downy mildew, caused by Plasmopara viticola, is considered one of the most destructive diseases of grapevine worldwide
The haplotype network analysis revealed the existence of six haplotypes in Brazilian populations, three of them previously reported in the European population (IR, IS, and IIS)
The results of the present study showed that the population genetics of P. viticola is climate-driven, while production system and quinone outside inhibitor (QoI) sensitivity have little or no effect
Summary
Downy mildew, caused by Plasmopara viticola, is considered one of the most destructive diseases of grapevine worldwide. Under favorable weather conditions and absence of control measures, the pathogen can infect all green parts of the grapevine causing losses up to 100% (Caffi et al, 2011). After the introduction in Brazil, P. viticola successfully spread and sustained itself across Southern and South-eastern regions, where the subtropical climate is characterized by frequent rain and average daily temperatures between 20 and 27◦C during the grape-growing season (Amorim et al, 2016). After 5 to 6 days at 18 to 26◦C, the pathogen produces new sporangia on the lesions that cause secondary infections (Gobbin et al, 2006; Kassemeyer et al, 2015). In Southern Brazil, the temperatures during the winter are low enough to induce grapevine bud dormancy and complete leaf fall that may promote mating and oospore formation, serving as a survival strategy. The double cropping system increases yield significantly, but may serve as a green bridge for pathogens, where P. viticola appears to survive predominantly asexually between seasons (Camargo et al, 2019)
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