## 1 Introduction  
Viroids are non-encapsulated, covalently closed, non-coding circular RNA molecules consisting of 246 to 399 nucleotides. Viroid species are phylogenetically classified into two families: _Pospiviroidae_ and _Atsunviroidae_. Viroids of the family _Pospiviroidae_ are replicated through an asymmetric rolling-circle mechanism in the nucleus using host DNA-dependent RNA polymerase II [1]. Hop (_Humulus lupulus_ L.) plants are hosts to several viroids from the family _Pospiviroidae_. Originally, hop stunt vivid (HSVd), of the genus _Hostwirid_, and hop latent vivid (HLVd), of the genus _Cocaviroid_, have occurred in hop [2]. Recently, apple fruit crinkle vivid (AFCvd), of the genus Apscarviroid [3], and citrus bark cracking vivid (CBCVd), of the genus _Cocaviroid_[4], have been found in hop plants. AFCvd and HSVd infection induces symptoms, which include stunting, leaf curling, small cone formation, and a considerable reduction of alpha bitter acid content in cones [3, 5]. Bitter acids, which are essential compounds of beer bitterness, are prenylated polyketides synthesized in lupulin glands and divided to alpha and beta with one more prenylation step in synthesis [6]. CBCVd infection causes similar, more severe symptoms, which often lead to plant death [7; 8]. The spreading of these viroids has been limited to local outbreaks but HLVd infection has been reported worldwide in hop-growing regions [2; 7; 9; 10]. Although HLVd-infected hop plants are symptomless, infection leads to a significant reduction in cone yield and bitter acids content in hop cones [11]. The yield was lower, from 8% to 35%, for infected plants of the cultivars Wye Challenger and Omega. The content of alpha bitter acids was reduced by 15% and 30%, while the content of beta bitter acids was slightly higher. Follow-up experiments with Wye Challenger showed an 11% reduction of the yield, 11% reduction of the content of alpha bitter acids, and 8% increase of the content of beta bitter acids [12]. The reduction of alpha bitter acids' content due to HLVd infection ranged from 20% to 50% within English hop cultivars and was genotype-dependent [13]. Similar results were found for hop cultivars Saaz, Premiant (40% reduction) [9]; Aurora (18% reduction) [8]; and Sybillla, Marynka, Pulawski, and Magnat (from 11% to 23% reduction) [14]. The content of beta bitter acids was slightly higher (0-5%) for all the cultivars. The loss of yield due to HLVd infection reached from 15% to 37.5% for Slovenian hop cultivars [8] and from 6.4% to 15.3% for Polish hop cultivars [14]. Viroid infection also influences the essential oils composition in hop cone, which contributes to the aroma flavor of beer. The increase of myrcene content by 38% for Wye Challenger-infected plants was first reported [12]. Similar results were found for hop cultivars Saaz and Premiant when the content of myrcene was increased by 29% together with monoterpene pinene isomers (about a 40% increase) for infected plants. On the contrary, all sesquiterpenes were reduced by 4.4% to 29% in cones of infected plants. From other compounds, terpene alcohols (linalool, geraniol, and methylgeranate) and epoxides were increased and ketones were decreased for infected plants [9]. Therefore, the composition of essential oils in hop cones is genotype-dependent and specific [15], and these changes cannot be general. Trends for the content of sesquiterpenes and monoterrenes (myrcene and \(\beta\)-pinene) were similar within Polish hop cultivars, with the exception of myrcene for cultivar Sybilla [14]. The content of linalool was higher for cultivars Sybillla, Lubelski, and Pulawski, but lower for cultivars Marynka and Magnat in cones of infected plants. The content of methylgeranate was lower for infected plants of all cultivars. Hop secondary metabolites are biosynthesized and accumulated in glandular trichomes, lupulin glands, and on the inner side of cone bracteoles and bracts.