Monitoring of Important Virus and Viroid Infections in German Hop (Humulus lupulus L.) Yards

Abstract

Viroids and viruses can cause significant loss of yield and quality in hops. Several viruses are endemic in hop growing regions around the world but are tolerated because of high costs of replanting. A major problem is that viroids and viruses cannot be controlled by chemical plant protection measures. In addition, resistant hop cultivars are not available. But propagation of certified planting material, combined with a systematic replanting and monitoring program can impede the spread of viruses and viroids. Monitoring and close cooperation with plant health services are of utmost importance to detect and eradicate at an early stage first sources of infection with pathogens that have potentially catastrophic impacts on hop production. This is especially true with regard to Hop stunt viroid (HpSVd) and Citrus viroid IV (CVd IV), two dangerous viroids which must not be introduced in Germany?s hop production. Therefore, monitoring was conducted from 2008 to 2013 comprising all German hop growing regions to detect possible primary sources of viroid infections. From 2011 onwards, in this monitoring tests for Hop mosaic virus (HpMV), Apple mosaic virus (ApMV), Arabis mosaic virus (ArMV), Hop latent virus (HpLV) and American hop latent virus (AHpLV) were included to elucidate the virus  situation regarding economically important hop viruses. Hop leaves from breeding yards, field trials and cultivar collections from the Bavarian State Research Center (LfL), from commercial hop gardens and a propagation facility were tested by ELISA and RT-PCR. While viruses are widely distributed, HpSVd has not been detected in Germany’s hop industry. In the present study, nine HpSVd findings from the hop germplasm collection in Huell in 2010 were the only ones in a total of 1,444 samples. This result suggests that eradication measures taken in 2010 were successful. Tests for CVd IV began in 2013 on a small scale and to date this viroid has not been detected  in Germany. Monitoring will be continued in 2014 with the main focus put on HpSVd and CVd IV tests providing knowledge for effective risk management of these dangerous diseases.