The hop stunt viroid (HSVd) is one of a group of the smallest self-replicating RNAs pathogenic to plants. The viroid is a covalently closed, circular, single-stranded RNA consisting of 297 nucleotides. Infect the common hop plant, citrus plants and grapevines, among others. It is a member of the Pospiviroidae family and the Hostuviroid genus. Causes stunt, a serious disease of hop (Humulus lupulus) recognized by dwarfing of the plant, shortening of the internodes of the main and lateral bines, yellowing and curling of the upper leaves. The main bines of diseased hops reach only 60% of the height of healthy ones. The cones of diseased hop are smaller, and the number per bine fewer, than those of healthy plants, resulting in about 50% loss in cone yield. This viroid has been detected in Europe, Asia, North America, Australia and the Middle East. Methods for the identification of this viroid in host plants should help to control the spread of this agriculturally important plant pathogen.