Tomato Rootstock Resistance to Bacterial Wilt (Ralstonia solanacearum): Effects of Genotype and Cold Stress on Epidemic Profile

Tomato (Solanum lycopersium L.) is the second most important vegetable crop in the world. In the U.S., fresh market tomato production farm value was $0.86 billion in 2012, with Florida and North Carolina (NC) producing 38.6% of national production (34.7% and 3.9% respectively). In both states (2007), 99% of tomato growing acres were harvested for fresh market. Bacterial Wilt (BW) caused by Ralstonia solanacearum (Rs) is a devastating disease of tomato in tropical and subtropical environments around the world with losses of 70-100% in the Southeast U.S. in heavily infested fields. Vegetable grafting of commercial scions onto disease resistant rootstocks has been developed as a viable management strategy for BW in tomato, particularly for fresh market. Bacterial wilt resistance in tomato rootstock germplasm, however, is quantitative and partial. A greenhouse resistance study using 10 variably resistant tomato rootstocks lines (not grafted; mixture of open-pollinated and commercial hybrids), Florida 47 susceptible commercial control, and two NC Rs isolates was carried out from October 2013-February 2014. Rootstock lines including ‘HI7997’ and ‘HI7998’ and hybrid lines ‘Cheong Gang’ (Seminis) and RST-04-106-T (DP Seeds) were highly resistant to both NC strains, quantitatively. Other rootstock lines including ‘CRA66’, RST-04-105-T (DP Seeds), and several BHN selections had intermediate resistance. Rootstock hybrid ‘Maxifort’ (DeRuiter) and scion hybrid ‘FL47’ were highly susceptible, as anticipated. Disease scores were complemented stem imprints on semi-selective media and vascular browning scores. Bacterial streaming assays and ELISA assays were used to verify the presence of the pathogen. A severe cold weather event caused plants to be exposed to 9^oC two nights in a row, causing recoverable cold damage after disease had reached a stable plateau. Following cold stress, a second substantial phase of wilting was observed, mostly in resistant rootstocks, which again reached a stable plateau. Analysis of epidemic phases suggests that susceptibility to BW can be early, late, or bi-phase depending on genotype, and possibly Rs isolate. Additionally, it suggests that the late phase epidemic is at least partially induced by environmental stress (cold temperature). This research has implications for BW resistance screening methodology, resistant genotype breeding selection criteria, elucidation of some environmental influences on disease severity and temporal phenomenon, rootstock selection for grafted management of BW, and, potentially, the geographic influence of Rs on tomato genotypes and environments.