2017 ASHS Annual Conference

Bacterial spot of tomato is a serious disease caused by multiple bacterial species and physiological races within the genus Xanthomonas. The disease affects all-above ground parts including leaves, stems and fruits and has a potential to cause up to 66% yield loss. It is one of the most important diseases of tomato in North Carolina and many other States. Management of this disease is based on integrating multiple tactics including the use of cultural practices and chemicals such as copper, antibiotics (streptomycin) and plant activators. However, the failure of these chemicals and antibiotics to control bacterial spot of tomato has been reported from different parts in North Carolina. Therefore, we designed a systematic sampling protocol and assessed 183 Xanthomonas strains in 2015 and 124 strains in 2016, isolated from different cultivars, grown in different fields and counties of Western NC, for copper and streptomycin sensitivity. Strains able to grow on a specialized bacterial (Sucrose Peptone Agar) media containing 200 ppm of copper sulfate or 100 ppm of streptomycin were considered as copper and streptomycin resistant, respectively. We discovered 94.0% and 96.0% of the Xanthomonas strains were resistant to copper and 44.8% and 24.8% of the strains were resistant to streptomycin in 2015 and 2016, respectively. The high incidence of copper resistance is presumably associated with frequent application of copper in fields. The variation observed in the levels of streptomycin resistance between years might be due to different management practices as well as seed and transplant sources that place differential selection pressure or introduce diverse bacterial populations. All fields sampled harbored bacteria that were copper or streptomycin resistant, which is consistent with growers’ report of control failure. These data indicated widespread existence of copper resistance and a modest level of streptomycin resistance, enabling the outcome to recommend the use of streptomycin in the transplant production phase and discontinue use of copper. These data also document the need to develop more sustainable and alternative strategies to manage the disease. This study will further classify the bacteria for race (strains that differ in ability to parasitize different tomato selections) and genetic diversity. In turn, these data will be helpful for developing effective disease management strategies and to optimize our current breeding program to develop bacterial spot resistance in tomato cultivars.