Degradation of hop latent viroid during anaerobic digestion of infected hop harvest residues

Michael Helmut Hagemann,Elke Sprich,Erich Lehmair,Hans Oechsner,Patrick Winterhagen,Jörg Steinbrenner,Benedikt Hülsemann,Luitgardis Seigner,Ute Born

doi:10.1007/s10658-021-02344-2

Abstract

The citrus bark cracking viroid (CBCVd) was identified as causal agent for a severe stunting disease in hops. Viroids are highly stable parasitic RNAs, which can be easily transmitted by agricultural practices. Since CBCVd has recently been detected in two European countries a growing concern is that this pathogen will further spread and thereby threaten the European hop production. Biogas fermentation is used to sanitize hop harvest residues infected with pathogenic fungi. Consequently, the aim of this study was to test if biogas fermentation can contribute to viroid degradation at mesophilic (40 °C) and thermophilic (50 °C) conditions. Therefore, a duplex reverse transcription real-time PCR analysis was developed for CBCVd and HLVd detection in biogas fermentation residues. The non-pathogenic hop latent viroid (HLVd) was used as viroid model for the pathogenic CBCVd. The fermentation trials showed that HLVd was significantly degraded after 30 days at mesophilic or after 5 days at thermophilic conditions, respectively. However, sequencing revealed that HLVd was not fully degraded even after 90 days. The incubation of hop harvest residues at different temperatures between 20 and 70 °C showed that 70 °C led to a significant HLVd degradation after 1 day. In conclusion, we suggest combining 70 °C pretreatment and thermophilic fermentation for efficient viroid decontamination.

Highlights

Viroids are small parasitic RNAs that replicate in their host plants by interacting with the host transcriptional machinery (Škorić, 2017)
In the first experiment aiming at studying the effect of slurry RNA extract on the qPCR part of the Real-Time quantitative PCR (RTqPCR), citrus bark cracking viroid (CBCVd) or hop latent viroid (HLVd) containing plasmids (DNA) have been used as template
Based on the presented results it can be concluded that the HLVd and presumably other pospiviroids are difficult to eliminate even with the biogas fermentation process

Summary

Introduction

Viroids are small parasitic RNAs that replicate in their host plants by interacting with the host transcriptional machinery (Škorić, 2017). The CBCVd is the causal agent for a severe hop stunting disease, which was first discovered in Slovenia in 2007 (Jakse et al, 2015) Until today this severe hop stunting disease has caused high economic losses and is still spreading (Radisek, 2017). Hop growers have the concern that hop harvest residues used as fertilizers might still contain infectious viroid particles despite ensiling and fermentation, may cause infections. The primary infection of hop plants in Slovenia is thought to be the result of establishing a hop garden on an illegal refuse dump containing citrus plant residues (Radisek, 2017). A current study on the fate of the HLVd during ensiling could not resolve those concerns, because ensiling did not reduce HLVd quantities even after a period of 4 month (Hagemann et al, 2021)

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