An unknown and aggressive disease was observed in 2007 in hop gardens in Slovenia. The disease spread extremely rapidly and caused severe stunting and death of affected plants. Diagnostic analysis, which was based on screening for a range of all know hop pathogens, revealed the presence of Hop stunt viroid - HSVd (https://gd.eppo.int/reporting/article-1882) the causal agent of hop stunt disease. The new disease in Slovenia corresponds to descriptions of HSVd infections, although some characteristics, such as a shorter incubation period, higher aggressiveness and unreliable RT-PCR detection limited to hop cone tissue, were unusual and atypical for HSVd. Further diagnostic research of symptomatic plants using next generation sequencing analysis revealed also the presence of Citrus bark cracking viroid (CBCVd) (Jakse et al., 2014) which, until this finding, had been described as a minor pathogen of citrus plants. Infection tests using a biolistic inoculation technique proved the high aggressiveness and infectivity of CBCVd on hop and, on the basis of this and the results of NGS and RT-PCR testing of samples from all infected hop gardens, CBCVd was recognised as the causal agent of a new viroid disease on hop, named ‘severe hop stunt disease’ (Jakse et al., 2014). Hop is a new and highly susceptible host for CBCVd. The location of outbreaks outside citrus growing areas defines a new and unexpected region for this viroid.
The main host plants are Citrus spp., Poncirus trifoliata and, since its discovery in Slovenia, also hop (Humulus lupulus). The main pathway are host plants for planting and other plant material, including fruits, and contaminated machinery.
Jakše J. Radisek S., Pokorn T., Moatoušek J., Javornik B. (2014) Deep-sequencing revealed a CBCVd viroid as a new and highly aggressive pathogen on hop. Plant Pathology; doi: 10.1111/ppa.12325
Radisek S, Majer A, Jakse J, Javornik B, Matoušek J, (2012) First report of Hop stunt viroid infecting hop in Slovenia. Plant Disease, 96(4), 592.