Evaluation of Apple Scab and Occurrence of Venturia Inaequalis Races on Differential Malus Genotypes in Latvia

Abstract

Apple scab caused by the heterothallic ascomycetous fungus Venturia inaequalis is one of the most severe diseases in apple-growing regions worldwide. Annual sexual reproduction of the pathogen ensures the formation of populations with high variability and better adaptability to resistant cultivars. The present study aimed to assess the virulence of the V. inaequalis population in the experimental orchard where introduction, evaluation and breeding of apple cultivars are carried out, and the apple germplasm collection of Latvia is located. Fifteen differential Malus genotypes, obtained within the international Vinquest initiative, were planted in 2016 in the germplasm collection sector of the orchard in five blocks, each containing one tree of each geno-type. In 2017–2021, apple scab symptoms on the leaves were scored according to the methodology used in the Vinquest network. The disease severity increased with each vegetation season, and more genotypes were affected, reaching the maximum in 2021 and clearly indicating the infection source build-up and pathogen’s adaptation on particular genotypes due to the absence of scab control. At least four races of V. inaequalis were found to be established in Latvia, and the resistance genes Rvi1, Rvi3, Rvi4, and Rvi8 have been overcome. On h(9) and h(13), the first symptoms appeared in 2021, indicating the first signs of Rvi9 and Rvi13 gene breakdown.

Genetic Diversity of Venturia inaequalis in Latvia Revealed by Microsatellite Markers

“…The clustering of the strains was possibly related to the virulence, and this is supported by our previous findings. In Zemgale, the V. inaequalis races (1), (4), and (8) are established and capable of overcoming the Malus resistances genes Rvi1 , Rvi4 , and Rvi8 [ 45 ]. This finding is not surprising since races (1) and (8) occur widely in several European countries, whereas race (4) is only established in some countries in Eastern Europe [ 11 , 46 ], where several of the apple cultivars grown in Latvia originate.…”

“…The segregation of the V. inaequalis population in the orchard, depending on the virulence, has been demonstrated in Germany, where the studied population was split into two subpopulations based on the strain sampling from host genotypes with or without the Rvi6 gene [ 47 ]. The ability of the pathogen to overcome the Malus resistance genes Rvi2 , Rvi5, and Rvi6 had not yet been observed in closely located orchard with V. inaequalis races differential hosts in Dobele; however, the first signs of overcoming the Rvi6 resistance gene were noticed in organic farms in other regions of Latvia [ 45 , 48 ]. In commercial orchards, especially in organic farms, and in home gardens, locally bred cultivars with resistance genes, predominantly Rvi6 , have been grown in small quantities since 2011 [ 49 , 50 ].…”

Screening of Venturia inaequalis virulence and resistance of Malus genotypes to apple scab using in vitro methodology

“…Caffier, personal communication for strain 333), were also included. The strain maintenance and inoculum preparation was performed as described previously (Sokolova & Moročko-Bičevska, 2022).…”

“…However, climatic conditions are unstable and uncontrollable, making it challenging to interpret cultivar resistance in the field. In addition, the infection background of the pathogen is difficult to quantify, and the pathogen's virulence can differ among the different regions (Sandskär & Liljeroth, 2005;Patocchi et al, 2020;Sokolova & Moročko-Bičevska, 2022). Tests in the greenhouse using an artificial inoculation of plants with the selected V. inaequalis strains are the standard technique for investigating the pathogen's virulence and host resistance (Chevalier et al, 2004;Chevalier et al, 2008;Lê Van et al, 2011;Caffier et al, 2015).…”

“…Although the establishment of inoculation experiments for the evaluation of cultivar resistance and pathogen virulence in a greenhouse is laborious and requires a large space and a quantity of inoculum, it provides the necessary favourable conditions for the disease development and allows the quantification of the pathogen's inoculum (Yepes & Aldwinckle, 1993a, b;Chevalier et al, 2004;Chevalier et al, 2008). Along with greenhouse experiments, in vitro experiments have been carried out using detached leaves, leaf discs and detached fruits to study the interaction of the Venturia species with their hosts (Yepes & Aldwinckle, 1993b;Bénaouf & Parisi, 1998;Stehmann, Pennycook, & Plummer, 2001;Caffier et al, 2012;Sokolova & Moročko-Bičevska, 2022). In this study, apple cultivar resistance and the virulence of V. inaequalis strains were evaluated by inoculating detached leaves and immature fruits with V. inaequalis strains originating from different geographic regions and host genotypes.…”