Mechanical transmission of Apple mosaic virus in Australian hop (Humulus lupulus) gardens

The transmission of Apple mosaic virus (ApMV; hop, H and intermediate, I serotypes) in Australian hop cultivars was assessed in glasshouse and field trials. Under field conditions, the rate of ApMV transmission was halved when contact between neighboring plants was prevented by early season applications of paraquat to restrict basal shoot growth. However, in a separate field trial the presence of root grafts between hop plants, which may contribute to virus transmission, was also suggested. In glasshouse trials, ApMV was transmitted successfully to hop by the mechanical inoculation of infective sap, simulated pruning, foliar contact, and root grafting, but not by root contact. The rate of mechanical transmission of ApMV to the hop cultivar 'Victoria' was greater than to other hop cultivars commonly grown in Australia. However, success of mechanical transmission of ApMV also appeared to be influenced by the cultivar from which inoculum was obtained. ApMV was detected throughout the year in all tissues, in chronically infected field grown plants of cultivar 'Victoria', suggesting a uniform virus distribution. The reliability of ApMV detection by serology did not decline in 'Victoria' plants later in the growing season as occurred in other cultivars.