INOCULATION EXPERIMENTS WITH THE DOWNY MILDEWS OF THE HOP AND NETTLE (PSEUDOPERONOSPORA HUMULI (MIY. ET TAKA.) WILS. AND P. URTICAE (LIB.) SALMON ET WARE)

Abstract

Summary. The evidence now available indicates that the Downy Mildew of the hop, Pseudoperonospora Humuli (Miyabe and Tak.) Wils, is a distinct species coming from Japan, or possibly North America, into Europe about 1920. The views advanced by us in 1924 that this species might be co‐specific with P. Urticae and that it is endemic to England, are abandoned. Inoculation experiments have shown that P. Humuli is capable of infecting and producing spores on Urtica urens. No evidence was obtained, however, as to whether it can establish itself permanently on this host‐plant. Spores produced on U. urens as the result of inoculation with P. humuli can infect fully the hop. Zoospores of P. Humuli placed on leaves of Urtica dioica and Parietaria ramiflora produced phenomena approaching sub‐infection. Since P. Humuli in its native countries has been observed only on species of Humulus, the fact that in Europe a species of Urtica can be artificially infected, may indicate a widening of its host range. Spores of P. Humuli, taken from cultivated hops, failed to infect fully Humulus japonicus. Spores of P. Urticae (Lib.) Salmon et Ware, taken from Urtica dioica, were found to infect fully Urtica urens.