Enhanced control of bacterial wilt of tomato by DL-3-aminobutyric acid and the fluorescent Pseudomonas isolate CW2

Soil treatments with DL-3-aminobutyric acid (BABA) and the fluorescent Pseudomonas isolate CW2 significantly reduced bacterial wilt on tomato plants caused by Clavibacter michiganensis subsp. michiganensis (Cmm). Applications with 50 ml per pot of BABA (0.5 mg ml(-1)) applied 4 days before inoculation (dbi) or 50 ml per pot of CW2 (2 x 10(8) cfu ml(-1)) applied 2 dbi, reduced leaf wilting index (LWI) by 51.2 and 41.6% and vascular browning index (VBI) by 63.7 and 48.8%, respectively. Combined treatments, BABA applied 4 dbi and CW2 2 dbi (BABA/CW2) or CW2 4 dbi and BABA 2 dbi (CW2/BABA), proved to be more effective in reducing disease symptoms than single treatments: Treatments with BABA/CW2 and CW2/BABA reduced LWI by 86.6 and 70.3% and VBI by 83.6 and 86.7%, respectively. When BABA and CW2 were applied simultaneously 2 dbi disease parameters of bacterial wilt were less effectively reduced compared to combined treatments in sequence (BABA/CW2 or CW2/BABA). CW2 and BABA/CW2 treatments significantly increased fresh and dry weights of roots and shoots of tomato plants compared to the infected control. Roots of tomato plants treated with BABA alone or combined with CW2 (BABA/CW2 and CW2/BABA) revealed the highest total and free salicylic acid (SA) contents. The treatment with BABA and CW2 applied in sequence 4 and 2 dbi, respectively, reduced most effectively bacterial wilt symptoms of tomato, resulted in the highest root and shoot weights and the highest SA contents. The combined application of an inducer of resistance (BABA) with a biological control agent (CW2) might be a promising strategy to improve both the level and reliability of biological control in controlling bacterial wilt in tomato.