Screening of hop breeding material for field resistance to downy mildew (Pseudoperonospora humuli Wilson) in the early stages of plant development

Aim. To screen the Ukrainian collection of hop breeding material for the level of field resistance to Pseudoperonospora humuli at the early stages of the hop plant development and growth, and to identify valuable genotypes for further breeding. Methods. The research was conducted in years 2019–2022 in small experimental plots in a field where natural infection of P. humuli was present (Zhytomyr district, the Polissia zone, Ukraine). Sixty-five newly created hop genotypes and four standard Ukrainian varieties, Alta, Slоvyanka, Zahrava, and Ruslan (as controls), were evaluated for the field resistance against P. humuli on base of disease index using a 9-point scale during the initial growth phase of the plant at two stages: in stage I, emergence of sprouts and stage II, shoot growth. Results. For the first time a descriptive analysis of field resistance to P. humuli of hop plants in two stages of its development and growth was performed for 65 new genotypes from the Ukrainian collection of genetic hop resources and the most resistant ones were selected. The disease development was less intense in stage I than during stage II, which was possibly associated with increased humidity and temperature during the latter stage. No genotype showed immunity (full resistance) to the disease. The four standard varieties used showed moderate susceptibility. Eight genotypes (7667, 8156, 8195, 8367, 8382, 8388, 8604, and 8555) demonstrated high resistance during stage I, while a slightly different set of eight genotypes (7667, 7886, 8156, 8195, 8367, 8438, 8605, and 8555) showed high resistance in stage II. The differences observed across evaluation stages and genotypes were statistically significant (F=13.4; р < 0.001). Conclusions. Evaluation of the Ukrainian collection of hop breeding material for field resistance to Pseudoperonospora humuli during early stages of plant development and growth showed that 25% of the genotypes were classified as resistant, 39% as moderate resistant, 32% as moderate susceptible, and 4% as susceptible. Six genotypes (7667, 8156, 8195, 8367, 8382, and 8555) with the highest total field resistance to P. humuli were advised for inclusion into further breeding crossings and investigations for the presence of useful genetic resistance markers.