The experimental transmission of hop latent viroid and its elimination by low temperature treatment and meristem culture

Summary Two aspects of hop latent viroid (HLVd) relevant to control were examined: the production of viroid-free plants from infected material and transmission of HLVd in the field. Plants free from HLVd were obtained by a combination of storing infected source plants at low temperature (2–4oC in the dark) for several months followed by meristem culture using small explants. A total of 77 plants of six cultivars and male pollinator clones were grown from meristems and 28 of these were free from HLVd. Tests showed that the cutting of stems (mimicking the use of tools) was more effective than abrasion (mimicking natural plant to plant contact) for the mechanical transmission of HLVd between hop plants. When field-grown test plants were inoculated, infection occurred more commonly in May before plants had grown large enough for significant contact between neighbouring plants than later in the season. The aphid Phorodon humuli could not be shown to transmit HLVd. These results indicate that all hop varieties and pollinator clones can be made available to the industry free from HLVd and that the chances of infection can be reduced by avoiding early-season cultural operations that cut into hop shoots.