Wild Wild West: Emerging Viruses and Viroids of Hemp

Abstract

Hemp (Cannabis sativa) or industrial hemp is a rapidly growing industry in the United States. However, the crop has been poorly studied in the US since its production declined in the late 1950s, and information on the biological and agricultural factors affecting hemp is limited. With the passage of the 2014 Farm Bill, hemp is no longer considered an illegal plant and is now a legal agricultural crop in the US. Within the bill, Section 7606 (Legitimacy of Industrial Hemp Research) provided a formal definition of the crop as "the plant Cannabis sativa L. and any part of such plant, whether growing or not, with a delta-9-tetrahydrocannabinol (THC) concentration of not more than 0.3 percent on a dry weight basis". In addition, the 2018 Farm Bill addressed many issues that had previously impeded development of the crop. Currently, 46 US states have enacted laws removing barriers to its production. In 2019, hemp production was 206, 965 hectares with 16,877 grower licenses across 34 states, according to "US Hemp Report" (www.votehemp.com). This is a greater than 455% increase over 2018 licensed acreage. Colorado, the leading state in hemp production increased production from 4,873 licensed hectares (346 growers) in 2017 to 12,525 licensed hectares (835 growers) in 2018. The US retail sales of hemp products was reported at $1.2 billion in 2018. The most profitable market for North American hemp is oilseed production and cannabidiol (CBD), a nonintoxicant cannabinoid with promising therapeutic use as a pharmaceutical product. Current CBD sales are estimated at $190 million and are projected to reach $2.5 billion by 2022. As we enter this new era of hemp production, plant diseases associated with the crop in the US and associated disease management needs are essentially undescribed. The emergence of phytopathogens and their spread is a rising concern as hemp production increases. Therefore, there is an urgent need to identify emerging and established hemp viruses and viroids, elucidate their diversity and distribution, develop early pathogen detection tools, and provide hemp growers, industry and other stakeholders information and resources to make timely management decision and minimize crop loss.

Emerging diseases of Cannabis sativa and sustainable management

“…Viral and viroid pathogens of cannabis and hemp have been identified by molecular analysis that involves RT‐PCR of RNA templates 22,23 . In addition, next‐generation sequencing (NGS) approaches are revealing the presence of previously unreported viruses in hemp 27 . The number of virus diseases occurring on hemp is steadily increasing as more research identifies new symptomology and spread of these pathogens from other host species 27 .…”

“…Currently, there are no certified disease‐free plants that have been derived through tissue culture. The importance of virus‐free planting materials and early detection for management of viral and viroid pathogens has been emphasized 27 …”

“…Studies conducted on other crop species have identified the mechanisms of resistance to Fusarium and powdery mildew 93–98 and they should be valuable in directing further research into the underlying disease resistance responses in cannabis plants. The search for resistance to viral pathogens is in its infancy 27 . Differences in susceptibility to Botrytis infection have also been observed that may be related to canopy architecture and inflorescence size and composition as it affects microclimate and relative humidity around the infection sites.…”

“…The foliar‐infecting pathogens of cannabis and hemp include powdery mildew (Fig. 3(G)–(I) ), while hemp cultivated outdoors can be severely affected by several leaf‐spotting fungi, as well as a number of recently identified viruses 27 , 28 that have not yet been diagnosed on indoor cultivated cannabis plants (Table S1 ). The stem‐infecting pathogens are varied and diverse in the symptoms they produce and occur both indoors and outdoors (Table S1 ), although outdoor grown plants have a higher prevalence of these diseases as they are difficult to manage.…”

“… 22 , 23 In addition, next‐generation sequencing (NGS) approaches are revealing the presence of previously unreported viruses in hemp. 27 The number of virus diseases occurring on hemp is steadily increasing as more research identifies new symptomology and spread of these pathogens from other host species. 27 There are few viruses currently reported on cannabis but this is likely due to a lack of research efforts.…”