Assessing the role of onion thrips (Thrips tabaci) in bacterial leaf blight of onion

Onion thrips (Thrips tabaci) are the major insect pest of onions in North America. Adults and larvae feed on leaves and while they are known vectors of viruses, they have more recently been identified as potential vectors of the onion bacterial leaf blight, caused by Pantoea agglomerans. This is a relatively new pathogen to Michigan’s onion industry and is becoming an important production problem. In 2015, we surveyed onion production fields located in Allegan, Ingham and Newaygo counties in Michigan and documented the incidence and severity of bacterial leaf blight and onion thrips abundance. Our objective was to determine the temporal and spatial relationship between bacterial disease and thrips population dynamics over the course of the 2015 growing season. Five onion plants were collected from 10 plots from two production fields from each of four farms weekly from June through August (400 sampled plants per week). Thrips were collected from each sampled plant and screened for P. agglomerans with Real-Time PCR using species-specific primers to identify P. agglomerans. Onion plant samples were also visually assessed for bacterial leaf blight. A positive correlation between thrips population density and thrips positive for P. agglomerans was established. A second positive correlation between the numbers of thrips positive for P. agglomerans and bacterial leaf blight incidence in the field was established suggesting T. tabaci as a probable vector of bacterial leaf blight. Results of this research demonstrate urgency for further research to study vector biology and dynamics.