The Quest to Identify a New Virus Disease of Sunflower from Nebraska

Between 2010 and 2018, sunflower plants exhibiting virus-like symptoms, including stunting, mottling, and chlorotic ringspots on leaves, were observed from commercial fields and research plots from four sites within three distinct counties of western Nebraska (Box Butte, Kimball, and Scotts Bluff). Near identical symptoms from field samples were reproduced on seedlings mechanically in the greenhouse on multiple occasions, confirming the presence of a sap-transmissible virus from each site. Symptomatic greenhouse-inoculated plants from the 2010 and 2011 Box Butte samples tested negative for sunflower mosaic virus (SuMV), sunflower chlorotic mottle virus (SuCMoV), and all potyviruses in general by ELISA and RT-PCR. Similar viral-like symptoms were later observed on plants in a commercial sunflower field in Kimball County in 2014, and again from volunteers in research plots in Scotts Bluff County in 2018. Samples from both of these years were again successfully reproduced on seedlings in the greenhouse as before following mechanical transmissions. Symptom expression for all years began 12 to 14 days after inoculation as mild yellow spots followed by the formation of chlorotic ringspots from the mottled pattern. The culture from 2014 tested negatively for three groups of nepoviruses via RT-PCR, ruling this group out. However, transmission electron microscopy assays of greenhouse-infected plants from both 2014 and 2018 revealed the presence of distinct, polyhedral virus particles. With the use of high throughput sequencing and RT-PCR, it was confirmed that the infections from both years were caused by a new virus in the tombusvirus genus and was proposed to be called Sunflower ring spot mottle virus (SuRSMV). Although the major objective of this project was to identify the causal agent of the disease, it became evident that the diagnostic journey itself, with all the barriers encountered on the 10-year trek, was actually more important and impactful than identification.