Infection of hop rootstocks by Pseudoperonospora humuli has been shown to occur as a result of inoculation of the tips of shoots 15 cm. or less in length. No rootstock infection developed when shoots were more than 17.5 cm. long at the time of inoculation. Infection via the base of shoots can occur at any time during the growing season and is thought to be the main source of rootstock disease under natural conditions. The importance of direct infection of hop rootstocks by zoospores of P. humuli, which occurs readily under laboratory conditions, is not known.

Application of Bordeaux (6.5% Cu) and captan (15% a.i.) dusts to the crowns and shoot bases of field plants retarded development of rootstock disease and shoot infection (basal spikes) in originally disease-free plantings of Golding hops.