Lettuce Chlorosis Virus Disease: A New Threat to Cannabis Production

Challenges to Cannabis sativa Production from Pathogens and Microbes-The Role of Molecular Diagnostics and Bioinformatics

The increased cultivation of Cannabis sativa L. in North America, represented by high Δ9-tetrahydrocannabinol-containing (high-THC) cannabis genotypes and low-THC-containing hemp genotypes, has been impacted by an increasing number of plant pathogens. These include fungi which destroy roots, stems, and leaves, in some cases causing a build-up of populations and mycotoxins in the inflorescences that can negatively impact quality. Viroids and viruses have also increased in prevalence and severity and can reduce plant growth and product quality. Rapid diagnosis of the occurrence and spread of these pathogens is critical. Techniques in the area of molecular diagnostics have been applied to study these pathogens in both cannabis and hemp. These include polymerase chain reaction (PCR)-based technologies, including RT-PCR, multiplex RT-PCR, RT-qPCR, and ddPCR, as well as whole-genome sequencing (NGS) and bioinformatics. In this study, examples of how these technologies have enhanced the rapidity and sensitivity of pathogen diagnosis on cannabis and hemp will be illustrated. These molecular tools have also enabled studies on the diversity and origins of specific pathogens, specifically viruses and viroids, and these will be illustrated. Comparative studies on the genomics and metabolomics of healthy and diseased plants are urgently needed to provide insight into their impact on the quality and composition of cannabis and hemp-derived products. Management of these pathogens will require monitoring of their spread and survival using the appropriate technologies to allow accurate detection, followed by appropriate implementation of disease control measures.

A Guide to Cannabis Virology: From the Virome Investigation to the Development of Viral Biotechnological Tools

Cannabis sativa cultivation is experiencing a period of renewed interest due to the new opportunities for its use in different sectors including food, techno-industrial, construction, pharmaceutical and medical, cosmetics, and textiles. Moreover, its properties as a carbon sequestrator and soil improver make it suitable for sustainable agriculture and climate change mitigation strategies. The increase in cannabis cultivation is generating conditions for the spread of new pathogens. While cannabis fungal and bacterial diseases are better known and characterized, viral infections have historically been less investigated. Many viral infection reports on cannabis have recently been released, highlighting the increasing threat and spread of known and unknown viruses. However, the available information on these pathogens is still incomplete and fragmentary, and it is therefore useful to organize it into a single structured document to provide guidance to growers, breeders, and academic researchers. This review aims to present the historical excursus of cannabis virology, from the pioneering descriptions of virus-like symptoms in the 1940s/50s to the most recent high-throughput sequencing reports. Each of these viruses detected in cannabis will be categorized with an increasing degree of threat according to its potential risk to the crop. Lastly, the development of viral vectors for functional genetics studies will be described, revealing how cannabis virology is evolving not only for the characterization of its virome but also for the development of biotechnological tools for the genetic improvement of this crop.

Translating virome analyses to support biosecurity, on-farm management, and crop breeding

Virome analysis via high-throughput sequencing (HTS) allows rapid and massive virus identification and diagnoses, expanding our focus from individual samples to the ecological distribution of viruses in agroecological landscapes. Decreases in sequencing costs combined with technological advances, such as automation and robotics, allow for efficient processing and analysis of numerous samples in plant disease clinics, tissue culture laboratories, and breeding programs. There are many opportunities for translating virome analysis to support plant health. For example, virome analysis can be employed in the development of biosecurity strategies and policies, including the implementation of virome risk assessments to support regulation and reduce the movement of infected plant material. A challenge is to identify which new viruses discovered through HTS require regulation and which can be allowed to move in germplasm and trade. On-farm management strategies can incorporate information from high-throughput surveillance, monitoring for new and known viruses across scales, to rapidly identify important agricultural viruses and understand their abundance and spread. Virome indexing programs can be used to generate clean germplasm and seed, crucial for the maintenance of seed system production and health, particularly in vegetatively propagated crops such as roots, tubers, and bananas. Virome analysis in breeding programs can provide insight into virus expression levels by generating relative abundance data, aiding in breeding cultivars resistant, or at least tolerant, to viruses. The integration of network analysis and machine learning techniques can facilitate designing and implementing management strategies, using novel forms of information to provide a scalable, replicable, and practical approach to developing management strategies for viromes. In the long run, these management strategies will be designed by generating sequence databases and building on the foundation of pre-existing knowledge about virus taxonomy, distribution, and host range. In conclusion, virome analysis will support the early adoption and implementation of integrated control strategies, impacting global markets, reducing the risk of introducing novel viruses, and limiting virus spread. The effective translation of virome analysis depends on capacity building to make benefits available globally.

Emerging diseases of Cannabis sativa and sustainable management

Cultivation of cannabis plants (Cannabis sativa L., marijuana) has taken place worldwide for centuries. In Canada, legalization of cannabis in October 2018 for the medicinal and recreational markets has spurned interest in large-scale growing. This increased production has seen a rise in the incidence and severity of plant pathogens, causing a range of previously unreported diseases. The objective of this review is to highlight the important diseases currently affecting the cannabis and hemp industries in North America and to discuss various mitigation strategies. Progress in molecular diagnostics for pathogen identification and determining inoculum sources and methods of pathogen spread have provided useful insights. Sustainable disease management approaches include establishing clean planting stock, modifying environmental conditions to reduce pathogen development, implementing sanitation measures, and applying fungal and bacterial biological control agents. Fungicides are not currently registered for use and hence there are no published data on their efficacy. The greatest challenge remains in reducing microbial loads (colony-forming units) on harvested inflorescences (buds). Contaminating microbes may be introduced during the cultivation and postharvest phases, or constitute resident endophytes. Failure to achieve a minimum threshold of microbes deemed to be safe for utilization of cannabis products can arise from conventional and organic cultivation methods, or following applications of beneficial biocontrol agents. The current regulatory process for approval of cannabis products presents a challenge to producers utilizing biological control agents for disease management. © 2021 The Author. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Small hydrophobic viral proteins involved in intercellular movement of diverse plant virus genomes

Most plant viruses code for movement proteins (MPs) targeting plasmodesmata to enable cell-to-cell and systemic spread in infected plants. Small membrane-embedded MPs have been first identified in two viral transport gene modules, triple gene block (TGB) coding for an RNA-binding helicase TGB1 and two small hydrophobic proteins TGB2 and TGB3 and double gene block (DGB) encoding two small polypeptides representing an RNA-binding protein and a membrane protein. These findings indicated that movement gene modules composed of two or more cistrons may encode the nucleic acid-binding protein and at least one membrane-bound movement protein. The same rule was revealed for small DNA-containing plant viruses, namely, viruses belonging to genus Mastrevirus (family Geminiviridae) and the family Nanoviridae. In multi-component transport modules the nucleic acid-binding MP can be viral capsid protein(s), as in RNA-containing viruses of the families Closteroviridae and Potyviridae. However, membrane proteins are always found among MPs of these multicomponent viral transport systems. Moreover, it was found that small membrane MPs encoded by many viruses can be involved in coupling viral replication and cell-to-cell movement. Currently, the studies of evolutionary origin and functioning of small membrane MPs is regarded as an important pre-requisite for understanding of the evolution of the existing plant virus transport systems. This paper represents the first comprehensive review which describes the whole diversity of small membrane MPs and presents the current views on their role in plant virus movement.

Epidemiology and control of emerging criniviruses in bean

During the last two decades, new criniviruses emerged in green bean crops in the south-east of Spain. Bean yellow disorder virus (BnYDV) was first detected in 2003 and caused major economic damage in crops grown in greenhouses. It was characterized as the first crinivirus to infect a member species of the Leguminosae family. Symptoms induced during BnYDV infection include interveinal chlorosis and yellowing on leaves, and reduced fruit yield and quality. Similar symptoms, although more severe, were observed in bean crops in the same region during the fall of 2011. From that moment on, BnYDV was not detected anymore in diseased plants, but instead lettuce chlorosis virus (LCV) was associated with the diseased plants. Previously, LCV was detected only in California, USA, infecting lettuce and sugarbeets. The host range and partial genomic sequences lead to the description of the new strain, LCV-SP. The complete sequence of its genome revealed the virus as a recombinant of BnYDV and LCV, in which the latter had lost two ORFs in the RNA1 of the bipartite genome and had acquired two homologue ORFs from BnYDV. Both viruses are transmitted by the whitefly Bemisia tabaci. When compared with other crinivirus pathosystems, the transmission efficiency of BnYDV to its primary host bean, is among the highest, and its persistence in the vector among the longest, up to 9 days. The host range of BnYDV s restricted to several crop species of the Leguminosae: common bean (Phaseolus vulgaris), pea (Pisum sativum), tirabeque (P. sativum subsp. sativum var. macrocarpon), lentil (Lens culinaris) and faba bean (Vicia faba). LCV-SP is also able to infect green bean plants but not lettuce, its original host, probably following its recombinant nature. Symptoms and epidemiology of the bean criniviruses are compared with similar pathosystems that occur in the same region and that involve cucurbit yellow stunting disorder virus and tomato chlorosis virus, infecting cucurbitaceous and solanaceous crops, respectively. Control of the criniviruses in bean crops will depend on efficient control of the vector. Physical control with greenhouses that prevent viruliferous whiteflies from gaining access to crops reduces BnYDV infection in plants and loss of production. Integrated pest management in beans would be preferred and the use of natural enemies to reduce secondary spread within greenhouses must be investigated.