Soil disinfestation with dimethyl disulfide (DMDS) to manage the bacterial wilt of tomato in the tropics

Bacterial wilt, caused by Ralstonia solanacearum, is a major disease that affects tomato crops worldwide. In Martinique, the situation has changed drastically since 1999 with the emergence of a new genotype of R. solanacearum, phylotype IIB/4NPB. This genotype wilts tomato cultivars previously considered as resistant and cannot be controlled by any conventional method. Promising results have been reported based on the use of biofumigants and suppressive crops, among which the aqueous extract of Allium fistulosum, belonging to the Alliaceae family, has been shown to have antibacterial properties against R. solanacearum. This extract controlled the bacterial wilt of tomato when used as a preplant soil treatment. Dimethyl disulfide (DMDS) is one of the main biofumigant molecules of Allium spp. The efficacy of soil fumigation with DMDS in the tomato R. solanacearum pathosystem was investigated in Martinique. First, DMDS was evaluated in vitro inhibition bioassays and then in vivo conditions. The suppressive effect of DMDS on R. solanacearum populations in the soil was studied in a growth chamber and the disease control was evaluated in greenhouse conditions with DMDS. In vitro, DMDS suppressed the growth of R. solanacearum. In the growth chamber, fumigation with DMDS significantly reduced R. solanacearum populations with a clear concentration effect. No pathogen was detected in the soil seven days after treatment with DMDS at 10-2 mol L(-1) whereas DMDS at 10-3 mol L(-1) reduced populations from 5.9.10(6) to 5.5 10(5) cfu g(-1) of dry soil. DMDS fumigation also reduced the incidence of tomato bacterial wilt in the greenhouse. In the untreated control, the disease affected 57% of the tomato plants whereas, with DMDS, only 2% of the plants were affected. These results demonstrate for the first time that DMDS can be used as a soil fumigant in future integrated bacterial wilt-management strategies.