Evaluation of chemical and biological control methods for their potential to reduce bacterial canker of tomato in a greenhouse stonewool cultivation system

In a three-year experiment three commercial disinfectants based upon: peracetic acid + hydrogen peroxide, alkyldimethylbenzylammonium chloride + a biguanidyne derivate, alkyldimethylbenzylammonium chloride + glutaraldehyde and a natural product (a proprietary grapefruit extract) were evaluated for their effectiveness in reducing the incidence and severity of Clavibacter michiganensis subsp. michiganensis (Cmm) on tomato plants planted to the stonewool slabs with high disease incidence in the previous crop. Each year, at the beginning of the spring growing cycle, the pathogen was introduced into the stonewool production system by injecting 5 ml of Cmm inoculum (2 x 10(8) cfu/ml) at a point just under the drip irrigation emitters. Chemical disinfestation of stonewool slabs prior to autumn crops tended to reduce the severity of internal disease symptoms (xylem and pith discoloration) of apparently healthy plants, decreased wilt rating scores, and increased marketable yields by 2-15% over untreated inoculated control. The grapefruit extract in most cases provided better disease suppression than disinfectants, but its impact on fruit yield was inconsistent in consecutive years. Four isolates of bacteria from the rhizosphere of tomato (GGS14, GS13, GK239, RZ156) and one isolate from the oilseed rape roots (PSR21), introduced in one trial to the stonewool slabs 14 times at weekly intervals, failed to reduce the disease incidence and severity in the spring cultivation. All rhizobacterial strains tested in the presence of Cmm decreased the marketable yield by 7-17% compared with the untreated, Cmm inoculated control.