Studies on Hop Stunt Disease

Abstract

Hop stunt disease (HSD) is prevalent in hop gardens in Japan since 1952. The cone yield of the diseased hop plants (Humulus lupulus cv. Kirin II) was reduced to at most a half of that of HSD-free hop plants. The alpha-acid content of the diseased cones was also decreased from one-third to one-half of that of HSD-free cones. HSD has been long suspected to be a virus disease according to graft and sap transmission, although no virus particle was detected. The only known hosts of HSD are H. lupulus and H. japonicus which have long incubation periods of 2-3 years and more than 3 months, respectively after inoculation. Thus studies on the properties of HSD agent have been extremely limited by lack of suitable indicator plants.Cucumis sativus, C. melo, C. melo var. conomon, Lagenaria siceraria var. clavata, gourda and microcarpa, Luffa cylindrica and Lycopersicon esculentum were confirmed as the host plants of HSD agent. Except L. esculentum plant which was recognized as a symptomless carrier, all the other infected plants showed mainly stunting one month after inoculation. Twenty-one cultivars of cucumber plants inoculated were all infected. The symptoms of cucumber plants appeared earlier at about 33C than at about 21C. HSD agent was not transmitted by Myzus persicae. Virus particles were not detected from diseased cucumber plants by dip preparations and ultra-thin sections.It is interesting to note that the symptoms on infected cucumber plants and the host range of HSD agent are quite similar to those of cucumber pale fruit disease in Europe.