Severe hop stunt disease was observed in 2007 on hops Humulus lupulus, grown as perennial industrial crop in Slovenia. On the site of an outbreak the disease spread extremely rapidly and caused severe stunting and death of affected hop plants. Screening studies on all known pathogens of hops, revealed the presence of Hop stunt viroid (Pospiviroidae, HSVd) known to cause hop stunt disease in Japan. Emergency measures were first adopted against HSVd (surveillance, delimitation of infected areas, phytosanitary measures, prohibitions, restrictions and the hygienic measures). Viroids are sap-transmissible and can be transmitted by vegetative propagation, grafting, foliar contact between neighboring plants, contaminated tools and machinery, clothing and human hands.
However, the new disease observed in Slovenia presented some unusual characteristics for HSVd, such as shorter incubation period, higher aggressiveness, and unreliability of RT-PCR detection (limited to hop cone tissues). Further analysis of symptomatic plants using next generation sequencing (NGS) analysis revealed the presence of Citrus bark cracking viroid (Pospiviroidae, CBCVd). Until this finding, CBCVd had been described only as a minor pathogen of citrus plants. Research studies confirmed that CBCVd was the causal agent of this new viroid disease of hop, which was called ‘severe hop stunt disease’. Hop is a new and highly susceptible host for CBCVd. Symptoms on hops include plant stunting resulting from a shortening of the internodes of main and lateral branches, leaf yellowing and downward curling, reduced cone production, and dry root rot. The first symptoms appear 4-12 months after infection and plants die within 3-5 years. Since hop is a perennial plant, which requires an extensive and long term support system for cultivation, infections of hop gardens have a high impact on production and cause major economic damage.
New decision on emergency measures to prevent the introduction and spreading of hop stunt diseases was adopted by Slovenian NPPO in 2015. The presence of CBCVd as a causing agent of a severe stunt disease on hops in Slovenia was reported internationally together with emergency measures. CBCVd was added to the EPPO Alert List in 2015 with pictures of symptoms on hops to alert other hop growing countries. In 2017, Citrus bark cracking viroid (Cocadviroid; A2/403) was recommended by the European and Mediterranean Plant Protection Organization for regulation as a quarantine pest.
Beside emergency measures, intensive annual surveys for CBCVd and HSVd have been carried out in Slovenia. In 2015, CBCVd was found on 13 farms (out of 120 in delimited area) infecting 61.5 ha (64.5 ha in 2014). Level of infection in individual field is on average 1.4 % (varies between 0.03 - 21%) with higher rate in hop gardens where the disease had been present already some years. In total, 2629 hop plants were found infected and removed in 2015.
In 2016, CBCVd was confirmed on 17 farms (out of 120 in delimited area) on 92.3 ha. In 2016, the disease with a low level of infection was detected for the first time on 6 farms, where the presence of CBCVd was previously unknown. In 2016, a total of 1856 infected plants (plants showing visual disease symptoms) were identified in the infected hop gardens. Taking into account the average number of plants per hectare (3000), this means a 0.74 % level of infection.
In 2017, CBCVd was confirmed on 101.9 ha. The disease with a low level of infection was detected for the first time on 12 fields, where the presence of CBCVd was previously unknown. On 6 fields the reoccurrence of CBCVd was found after eradication and quarantine period of four growing seasons with non-hosts in rotation. In 2017, an average 0.72 % level of infection was detected, including one field with the level of infection of 20%. Official measures have been implemented on infected farms.
Over long distances, CBCVd in hops can be transmitted by infected planting material or parts of plants. The CBCVd emergence on hops is still unclear, since citrus are not grown commercially in Slovenia. It is assumed that CBCVd transmission to hop occurred from the remains of imported citrus fruits or plants. The initial outbreak took place in a hop garden established on the site of a former waste dump, where such transmission probably happened. Infected citrus fruits and contaminated machinery from areas in which CBCVd occurs are also a pathway of spreading.
In 2018, in spite of implementation of official measures, CBCVd spread to new farms and within already infected farms to new hop fields. Intensive survey programme is still carrying out and more results will be available in October.