Attempts at Biological Control of Clavibacter michiganensis subsp. michiganensis On Rockwool-Grown Greenhouse Tomatoes

Two greenhouse experiments were conducted in which tomato plants artificially inoculated with Clavibacter michiganensis subsp. michiganensis (Cmm) were grown in an open rockwool system as spring and autumn crops. Two isolates of the rhizosphere bacteria, Pseudomonas fluorescens strain PSR21, Pseudomonas reactans strain GGS14, a commercial biocontrol agent Aqua Bac Plus (Bacillus spp.) and a proprietary disinfectant containing QAC+Chx, applied at weekly intervals, were evaluated for their efficiency in the suppression of the bacterial canker of tomato. All treatments tested revealed to be ineffective in controlling the disease. The introduction of Cmm bacteria into the fresh rockwool in the first year of its usage resulted in a 100% death of tomato plants, whereas following an artificial inoculation of two- and three-year-old rockwool slabs with Cmm bacteria dead plants amounted to 70 and 58%, respectively. This indicates that in the re-used rockwool a natural microbial suppressiveness to bacterial canker of tomato might be developed in the root zone.