Short Report Nettlehead and Related Hop Diseases Associated with the Hop Strain of Arabis Mosaic Virus and Its Vector Xiphinema Diversicaudatum

Abstract

The hop strain of arabis mosaic virus (AMV-H) differs considerably from the ‘type’ strains (AMV-T) found in most other hosts (e.g. strawberry, raspberry & many wild hosts) both in its host range and the symptoms it produces on herbaceous indicator plants. Like AMV-T, it is transmitted very effectively by X. diversicaudatum (2). The epidemiology of AMV on hops (Humulus lupulus) which are large, long—lived perennials, differs from that on other, smaller and shorter—lived crops and poses special problems to plant pathologists and hop growers (1). Each individual hop root system occupies at least 2 m³ of soil so that a site containing only one X. diversicaudatum/200 ml (i.e. the smallest population detectable in the usual soil sampling unit) exposes each hop plant to c 10,000 vector nematodes, each of which is capable of infecting it. The population of X. diversicaudatum which can be tolerated in a hop field when AMV-H is also present is therefore very low. Any foci of AMV-H infection will, eventually, extend to all adjacent soil containing X. diversicaudatum the virus moving faster within the plant (2m per plant) than via migrating nematodes, which need to move little to transmit the virus from plant to plant wherever the root systems interlace.