Variation between and within grapevine families in reaction to leaf inoculation with downy mildew sporangia under controlled conditions

Vine reaction to downy mildew [Plasmopara viticola (Berk. & Curt.) Berl. & de Toni] inoculation was investigated between and within seven full-sib grapevine families under controlled culture conditions. Families were generated by crossing resistant x susceptible and susceptible x susceptible genotypes. Leaf infection following downy mildew inoculation was assessed using cultured leafed single node cuttings under controlled conditions. The severity of disease infection 7 days after inoculation was based on the expression of leaf chlorosis and sporulation symptoms using a 1 to 9 scale where a score of 1 meant there was no visible sign of infection and 9 meant > 80 % of the leaf area was infected. A measure of the hypersensitive response (discrete necrotic spots) was also used to rank vine reaction to inoculation. Hybrids within families varied widely in their reaction to inoculation. Any hybrid that received a mean symptom expression score ≤ 3, which was equivalent to or less than that assigned to 'Chambourcin', was classified as downy mildew resistant. The proportion of vines within families scored as resistant ranged from 4.6 to 22.5 % and from 4.6 to 47.4 % for leaf chlorosis and sporulation, respectively, between crosses. There was a strong correlation between leaf chlorosis and sporulation expression within each family (r2 ranged from 0.6 to 0.8). The number of resistant hybrids selected within families by the combined symptoms of leaf chlorosis and sporulation ranged from 3 to 7. Depending on family, segregation of resistant to susceptible phenotypes based on symptoms of leaf chlorosis and sporulation fitted 1:15, 1:7, 1:3 or 1:1 ratios. Segregation for hypersensitive reaction (HR) to non-HR fitted 1:1, 1:2 or 1:3 ratios within families. Hybrids that displayed the HR had mean scores for leaf chlorosis and sporulation less than those not displaying the necrotic spots characteristic of the reaction in four of the families investigated. The results are discussed in terms of the inheritance of resistance and the development of a strategy for future breeding.