A method using leafed single-node cuttings to evaluate downy mildew resistance in grapevine

Abstract

A method using leafed single-node cuttings, incubated at 25 ± 2 °C and 100 % humidity, was examined to screen grapevine genotypes for resistance to downy mildew (Plasmopara viticola (Bert. & Curt.) Berl. & de Toni). Cuttings were taken at the 4^th, 5^th and 6^th nodes back from apices of actively growing shoots. Disease symptoms, namely chlorosis, sporulation and necrosis, were observed on the leaves of cuttings within 6 days of incubation after inoculation. Based on chlorosis and sporulation, genotypes were ranked from highly susceptible to resistant in the order of Sultana, M46-32 (Bicane x Villard blanc), Joannes Seyve 23.416, Marroo Seedless and Chambourcin. The expressions of both symptoms increased with the concentration of inocula applied to leaves (1 x 10⁵ and 5 x 10⁴ sporangia per ml), but the overall genotypic ranking was unaltered. The third symptom of leaf necrosis occurred on infected leaves either as progressively enlarging dead areas or as smaller localised necrotic spots. The latter has been call ed the ‘necrotic response’ symptom and its expression depended on genotype, although its value for separating or ranking genotypes was unreliable. Leaf chlorosis was associated with leaf sporulation (r²=0.41-0.47). Genotypes with necrotic response exhibited reduced leaf chlorosis and sporulation. A comparison between this new leafed singlenode cutting method and a previously published leaf disc method indicated it was more reliable for separating genotypes for downy mildew resistance.