Surveys on Hop stunt viroid on hops in Slovenia and detection of an unexpected pathogen: Citrus bark cracking viroid
In 2007, symptoms resembling those of Hop stunt viroid (Pospiviroidae, HSVd) were observed for the first time in Slovenia in hop (Humulus lupulus) gardens in the Savinja valley and Koroška region. Molecular tests (RT-PCR, sequencing, hybridization) confirmed the presence of HSVd in symptomatic plants in 2011 (see EPPO RS 2012/055). Affected hop plants showed shortened internodes of the main and lateral branches resulting in a general plant stunting, leaf yellowing and downward curling, and a reduced cone production.
Since 2011, official measures have been adopted in Slovenia to prevent any further spread of the disease. Systematic surveys have been carried out on a yearly basis and included visual inspections of hop gardens, sampling, laboratory analysis and expert support to growers. Despite the identification of HSVd in symptomatic plants, the disease observed in Slovenia on hop presented some unusual characteristics. The incubation period of HSVd which is observed in Japan before the expression of symptoms ranges from 3 to 5 years, whereas in Slovenia, severe symptoms already appear during the first year of contamination. In addition, RT-PCR detection of HSVd was found to be unreliable (except when using cones) which was considered unusual for a systemic pathogen causing such severe symptoms. Further investigations using next generation sequencing analysis (NGS) revealed the presence of another viroid, Citrus bark cracking viroid (Pospiviroidae, CBCVd), in symptomatic hop plants. Until this finding, CBCVd had only 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. On the basis of the latter and on the results of NGS and RT-PCR testing of samples from all infected hop gardens, CBCVd was recognized as the causal agent of this new viroid disease of hop, called ‘severe hop stunt disease’.
Results of the systematic surveys revealed the presence of HSVd in 1 sample out of a total of 59 samples in 2012. In 2013 and 2014, HSVd was not detected in any sample. In contrast, the analysis of all samples taken from symptomatic plants (regardless of the sampling year) revealed the presence of CBCVd. Although it became clear that HSVd was not the causal agent of the disease observed on hop in Slovenia, its presence in symptomatic plants remains to be explained. Systematic surveys showed that the majority of CBCVd outbreaks occurred near the initial outbreak site in Šempeter (Savinja valley) which is located in the main hop-growing region in Slovenia. CBCVd infections were also detected in 2 hop gardens in Koroška region near Slovenj Gradec, and in Polskava near Slovenska Bistrica. Although HSVd is not the causal agent of the ‘severe hop stunt disease’, the Slovenian NPPO considered that its presence on hop and its potential pathogenicity still required phytosanitary measures. Therefore, strict official measures are currently implemented to prevent any further spread and to eradicate both HSVd and CBCVd.
The pest status of Hop stunt viroid in Slovenia is officially declared as: Present, under eradication.
The pest status of Citrus bark cracking viroid in Slovenia is officially declared as: Present: only in some areas where hop is grown and under eradication.
NPPO of Slovenia (2015-06).
Phytosanitary Administration of the Republic of Slovenia.
Archive on HSVd. http://www.arhiv.fu.gov.si/en/services_and_measures/regulated_organisms/hop_stunt_viroid/index.html
Hop stunt viroid – HSVD and Citrus bark cracking viroid – CBCVd (in Slovenian)