The assessment of partial resistance to powdery mildew disease in hops

Progeny comprising 1643 hop seedlings from crosses between eight partially resistant parents were screened under glasshouse conditions for resistance to a single pathotype of Sphaerotheca humuli. Results indicated polygenic inheritance of resistance. Genotypes representative of seedlings with 'light', 'moderate' and 'severe' infection from each family were transplanted to a hop garden where a natural epidemic of an introduced pathotype of powdery mildew developed. There was a significant positive association between amounts of disease on seedlings and on leaves of adult male plants, but no corresponding agreement between amounts of disease on female plants. A weak positive association was found between infection of seedlings in the glasshouse and subsequent cone infection in the field. Estimates of infection on leaves of adult plants and on cones were very closely associated. Partial resistance to powdery mildew was not associated with any deleterious agronomic or brewing quality characters.