Evaluation of resistance to powdery mildew (Erysiphe necator) in twelve Croatian grapevine cultivars (Vitis vinifera L.)

Abstract

The fungus Erysiphe necator causes powdery mildew (PM), one of the most significant diseases of grapevine. Observing visual disease symptoms is essential in virtually all pathosystems. Consistent phenotypic evaluations that accurately reflect the biology are of paramount importance for understanding disease resistance, even at the genotypic level.

Croatian grapevine germplasm represents an important portion of Mediterranean grapevine diversity, shaped by centuries of interaction between eastern and western gene pools. Despite its historical and genetic significance, it remains poorly studied in relation to PM resistance. In Croatia, only wild grapevine (Vitis vinifera subsp. sylvestris) germplasm has previously been evaluated for PM resistance, confirming partial resistance associated with Ren1-linked SSR markers and a notable presence of R-alleles. In this study, twelve cultivars were initially assessed for PM resistance through visual evaluation of spontaneous infection in vivo and by in vitro inoculation using the leaf disc method, applying the OIV 455 descriptor (inverted scale). Selected genotypes were further screened using three SSR markers linked to the Ren1 gene (UDV124, SC8-0071-014, and SC47-18). Two white cultivars, ‘Malvazija istarska’ and ‘Kujundžuša’, demonstrated elevated partial resistance under both greenhouse (open-style) and laboratory conditions over two consecutive seasons. ‘Malvazija istarska’ did not carry R-alleles at the analyzed SSR loci, whereas ‘Kujundžuša’ harbored R-alleles at two markers (UDV124/208 and SC47-18/246–246). ‘Malvazija istarska’ exhibited higher average phenotypic resistance than ‘Kujundžuša’ in the leaf disc assay (3.9 vs. 4.9), despite the absence of detected R-alleles. The leaf disc assay provided a more accurate assessment of the susceptibility of sensitive genotypes than observations based on spontaneous PM infection. This likely reflects differences in the source, quantity, and distribution of natural inoculum, as well as variation in infection dynamics and environmental conditions. Furthermore, it is essential to investigate the early responses of both resistant and susceptible genotypes to PM, particularly at stages before visible symptoms develop.