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Assessment of phenotypic diversity of Plasmopara viticola on Vitis genotypes with different resistance

Gómez-Zeledón, Javier, Zipper, Reinhard, Spring, Otmar
Crop protection 2013 v.54 pp. 221-228
Plasmopara viticola, Vitis riparia, Vitis rupestris, Vitis vinifera, clones, cultivars, downy mildew, fungicide resistance, fungicides, leaves, pathogens, pathotypes, races, sporulation, virulence
The lack of characterized isolates of Plasmopara viticola is a very limiting issue in the management of downy mildew of grapevine. Although molecular studies have previously confirmed a high diversity of this pathogen, there are still no phenotypically characterized pathotype strains or races available which could be used to study the mechanisms of interaction with host genotypes of different resistance. A leaf disk inoculation technique was used to assess the variability of reactions of six different Vitis genotypes infected with single sporangial clones of five field isolates of P. viticola from different geographical origins. The virulence of thirty P. viticola clones was characterized on grapevine cultivars (Müller-Thurgau, Regent and Cabernet Cortis) and wild species (Vitis vinifera ssp. sylvestris, Vitis rupestris and Vitis riparia) with different susceptibility to downy mildew. Five categories ranging from full susceptibility with profuse and not clearly restricted sporulation (type A) to complete resistance with no sporulation or necrotic reaction (type E) were defined to evaluate the pathogen phenotype. A high variation of pathotypes was found amongst the samples from different field accessions and an unexpected variability was observed even between the cloned strains from the same field. This also accounted for the fungicide tolerance against the two most common fungicides, metalaxyl-M (Phenyl Amide group) and dimethomorph (Carboxylic Acid Amide group), where clones showing high sensitivity and high tolerance to the one or both fungicides were found within the same field. The virulence patterns found within clones of single field isolates supports previous reports that numerous genotypes may be involved in the infection of a single plant or even a single leaf. Moreover, the study provides a methodology to identify and select specific host–pathogen combination suitable for future studies in mechanisms of grapevine downy mildew interaction.