Diversity of the virome associated with alfalfa (Medicago sativa L.) in the U.S. Pacific Northwest

Alfalfa ( Medicago sativa L.) is one of the most extensively cultivated forage legumes in the world. It is currently the third most valuable field crop in the United States with an estimated value of over $9.3 billion. Alfalfa productivity is limited by various infectious diseases that can reduce forage yield and quality and shorten stand life. The crop can frequently be infected with a diverse array of pathogens and other organisms that have distinct life cycles, biology, and mode of action. Among them are many coinfecting viruses, that greatly contribute to the heterogeneity of within-host pathogenic communities, representing a ubiquitous and abundant background for all other host–pathogen interactions. Regrettably, the impact of viral diseases, their role in alfalfa health and involvement in the severity of multi-pathogen infections are often underestimated and not well understood. As high-throughput sequencing approaches have been developed, opportunities to delve into these complex interactions can be realized. In this work, we have characterized a diversity of viral populations in several commercial alfalfa production fields located in the U.S. Pacific Northwest. At least 45 distinct viruses have been identified in all alfalfa samples. Among them some were known to infect the crop prior to this study, and others were designated as emerging, novel and viruses integrated into the alfalfa genome. Known viruses included alfalfa mosaic virus, pea streak virus and bean leafroll virus, while among emerging and novel agents were alfalfa virus S, cherry virus Trakiya, several rhabdoviruses and others. Additional biological and impact studies will be needed to determine if newly identified viruses, especially those that have not been reported from alfalfa before, should be considered pathogens of this crop.