Dried Residues of Specific Cruciferous Plants Incorporated into Soil can Suppress the Growth of Ralstonia solanacearum, Independently of Glucosinolate Content of the Residues

Abstract

The growth and survival of a rifampicin-resistant strain (22R6) of Ralstonia solanacearum were monitored after incorporation of cruciferous plants into infested soil. Air-dried residues of eighteen cruciferous plants (including two amphidiploid lines) were individually added to soil at a rate of 1.6% (w/w) and the water condition maintained at −10 kPa. Within one month of incubation at 24°C, nine of the 18 residues reduced the population of the strain 22R6; B. rapa sp. rapifera `Ayumi', Brassica oxyrrhina, and an amphidiploid line of Brassicoraphanus, were most inhibitory. In contrast, total soil bacteria increased, regardless of the nature of the residue. After infested soil was incorporated with cruciferous plant residues and pre-incubated for one month in glasshouse, wilt-susceptible tomato were transplanted and monitored for 34-days on residue-treated soil. The pathogen was detected in shoots of only 2-7% of tomato grown respectively in soil with residue of B. rapa sp. rapifera `Ayumi' and Diplotaxis muralis against 47% of tomato in non-residue soil. Data indicated that suppressive effects of the residues might depend on nutritional status and undefined factor in the soil but were not directly dependent on the concentration of glucosinolates (7.2×10^-3-1.1×10¹ μmole g^-1 tissue) in the residues and in vitro tests showed that authentic isothiocyanates were inhibitory to the pathogen only at high concentrations (more than 2 μmole g^-1 soil).