Preliminary field studies of resistance of Georgian grapevine germplasm to powdery mildew (Erysiphe necator)

Abstract

Context and purpose of the study – Erysiphe necator Schwein is a fungus that causes grapevine powdery mildew. It is one of the most problematic pathogens attacking Vitis vinifera L. The pathogen infects all green parts of the plant and reduces grape yield and quality. The suppression on mildew-susceptible cultivars requires intensive use of fungicides against pathogen, which has negative impact on the environment and human health. The identification, quantification, and use of host resistance is therefore a research priority favored by grape growers, genetics, breeders and winemakers, governments, regulatory and grant agencies.

At the end of 19th century, the breeding programs were carried out by crossing V. vinifera with resistant species, and in particular with North American Vitis that co-evolved with the pathogen. However, obtained hybrids were mostly unsuitable for the production of high-quality wines, due to their unpleasant foxy aromas.  Nearly all major cultivars of V. vinifera exhibit little evidence of strong genetic resistance against E. necator, with the exception of ontogenic resistance that appears to be a universal trait. However, recent studies have indicated that certain V. vinifera cultivars of Caucasus and Central Asian origin exhibit an unusual degree of field resistance to powdery mildew. Some Georgian V. vinifera accessions from the South Caucasus have been reported to be resistant to E. necator and to have no known genetic relationship to non-vinifera species. The aim of our preliminary research was to quantitatively measure the field resistance of local Georgian varieties to powdery mildew, as a necessary first step to more in-depth phenotyping and genetic studies.

Material and methods – Three Georgian grapevine collections (Shumi, Mukhrani, and Jighaura) were monitored three times per year in order to reveal resistance to powdery mildew of Georgian V. vinifera germplasm (both cultivated and wildly-growing grapevines) in the field condition. Resistance to powdery mildew was recorded as field ratings of disease incidence and severity following natural infection at different phenological stages in June, August, and September based on the OIV 455 descriptor for the leaf and OIV 456 for cluster resistance.

The evaluation was conducted on the cultivated vines treated with commonly-used fungicides for disease management in the collections.

Results – We assessed the resistance of 346 Georgian grapevine varieties under field conditions in 2022. Twenty seven of the foregoing varieties revealed very high resistance to powdery mildew, 62 had high resistance and 221 medium resistance, and 36 low or very low resistance. Our results suggest that there is substantial diversity in resistance to powdery mildew among the foregoing local varieties of V. vinifera.