Transmission and survival of Ralstonia solanacearum on tobacco machinery.
Bacterial wilt (Ralstonia solanacearum) is an extremely damaging disease of flue-cured tobacco in the southeastern USA. It is generally believed that R. solanacearum infects field-grown tobacco through the roots. Epidemics of bacterial wilt are so common within South Carolina that the organism must be spread in a more rapid and efficient manner than the movement of soil on equipment. Field trials conducted at the Pee Dee Research and Education Center measured the spread of R. solancearum down a row of tobacco from a point inoculation following machine flower removal (topping), leaf harvest or stalk cutting. Machine topping, leaf harvest and stalk cutting increased disease 879%, 1245%, and 800% respectively within a 20 plant row when compared to hand topping, hand harvesting or a non contaminated stalk cutter (cv. K 326, P≤0.001). A survey was conducted to determine if R. solanacearum is a common contaminate on harvesting equipment in South Carolina. Tobacco harvesting equipment was randomly selected within Horry and Marion counties and sampled for R. solanacearum by streaking sterile cotton swabs on the harvester surface, then restreaking the swab onto an enriched Tetrazolium-based selective media (SM3). R. solanacearum-like colonies that developed on SM3 were inoculated onto cv. Rutgers tomato to confirm pathogenicity. Bacteria from diseased plants were reisolated on SM3 and R. solanacearum identity confirmed with an immuno strip test. Pathogenic populations of R. solanacearum were recovered from steel defoliator knives, rubber defoliators and steel guides (51, 50 and 50% of sampled harvesters respectively). The survival of R. solanacearum on mechanical steel topper blades was determined by running the topper through infected tobacco plants and sequentially sampling the steel knives over time for viable populations of R. solanacearum. Sterile cotton swabs were swept across the blade surface, then streaked onto SM3 media. Confirmed pathogenic populations of R. solanacearum could be recovered on SM3 media for up to 6 h after topping R. solanacearum-infected tobacco. The efficiency of machine transmission of R. solanacearum and it's longevity on tobacco machinery may be responsible for the high frequency of bacterial wilt epidemics in flue-cured tobacco occurring in the Southeastern USA and its movement into previously clean fields.