Beet curly top virus-beet leafhopper dynamics in hemp in Colorado

Abstract

Hemp (Cannabis sativa L.) production within North America has dramatically increased in recent years following legislative changes in the 2014 and 2018 Farm Bills that allowed legal paths for its production. However, due to previous restrictions on this crop it has been understudied in the U.S. since its production declined, and ultimately was eliminated. Restrictive laws largely prevented any research regarding management of this crop. Among the understudied issues were those associated with disease identification and management. One newly described disease of the crop in Colorado beet curly top virus (BCTV). This viral pathogen is transmitted by an insect vector, the beet leafhopper. To better understand the diversity and prevalence of BCTV strains infecting hemp in Colorado, beet curly top virus (BCTV) was detected at high incidence (81%) in leaf samples from 12 counties in 2019. Two different strains of BCTV, BCTV-Worland and BCTV-Colorado were found present in single or mixed infection in hemp leaf samples. Phylogenetic analysis revealed BCTV sequences from hemp formed a distinct group along with BCTV-Colorado and BCTV-Worland strains. To determine other potential viral and viroid pathogens in hemp, shotgun metagenomic analysis was performed. Virome analysis revealed the presence of seven viruses and one viroid. Of these, cannabis cryptic virus, cannabis sativa mitovirus, citrus yellow vein associated, opuntia-like virus and hop latent viroid sequences that had high sequence similarity with their corresponding sequences in GenBank. In contrast, tobacco streak virus sequence was highly variable compared to sequences in GenBank suggesting a new genotype of this virus. The data presented here has important implications for the epidemiology and management of the various diseases of hemp and will lead to the development of integrated pest management strategies designed to interrupt transmission cycles and facilitate efficient crop production. Beet leafhopper abundance was monitored throughout the hemp season to understand timing of emergence and flight patterns in the north and western regions of Colorado as well as identify timing of population peaks. Virus incidence in hemp and weed species were assessed using PCR analysis. Beet curly top virus was detected earlier in western field sites of Colorado before being detected in northern survey sites. Of the 41 different weed species surveyed, the weeds that most often tested positive for BCTV, contributing to transmission prevalence were Lactuca serriola (prickly lettuce), Taraxacum officinale (dandelion), and Cichorium intybus (chicory). Life history assays were conducted using viruliferous and non-viruliferous beet leafhoppers in both sugar beet and hemp plants to understand if there were any fitness advantages or costs associated with being a carrier of the virus. Viruliferous beet leafhoppers reared on sugar beet produced more offspring than non-viruliferous treatments. There was no difference between viruliferous, and non-viruliferous beet leafhoppers reared on hemp, suggesting that these virus mediated differences in life history are induced in host plant interactions with the vector. However, beet leafhoppers were able to survive 7-day periods and successfully oviposit and develop on hemp. Understanding migration timing and patterns will result in a more thorough understanding of the pest ecology of the beet leafhopper, which will lead to targeted control strategies to incorporate into integrated pest management tactics to interrupt BCTV transmission cycles, in turn improving yield and farming efficiency.