Effects of different soil amendments on bacterial wilt caused by Ralstonia solanacearum (Smith) Yabuuchi and on the yield of tomato (Lycopersicon esculentum Mill)

Ralstonia solanacearum race 3 biovar 2 (phylotype II) is the causal agent of bacterial wilt of tomato, the most destructive bacterial disease of this crop in Ethiopia for which no effective control measures are available. In this study, the effects of amending topsoil with three different levels (1, 5 and 10%) of cocopeat, farmyard manure (FYM) compost and green compost, and two levels of bacterial inoculations were tested on infection of tomato by R. solanacearum compared to non-inoculated treatments. Non-amended topsoil with and without R. solanacearum were included as control treatments. Survival of and infection by R. solanacearum and yield-associated agronomic responses were used for evaluation along with physico-chemical and biological characteristics of amended soils. Amendments resulted in changes in physico-chemical properties (such as electrical conductivity, organic matter content, total carbon, dissolved organic carbon, NH(4)(+), NO(3)(-)) and microbial activity of the amended soil and the effects were found to be higher at the higher rate of application. Effects on disease suppression and survival of the pathogen in the soil differed depending on amendment type and application rate. Higher disease severity was recorded in soil amended with 10% green compost compared to the control treatment. Complete suppression of R. solanacearum was observed in pots amended with 5 and 10% farm yard manure (FYM), 1% green compost and 10% cocopeat. Absence of disease at the highest rate of FYM was supported by a lower number of culturable R. solanacearum bacteria recovered from rhizosphere soil two months post-inoculation in soil amended with 10% FYM. Soil amended with 10% FYM gave higher root and above-ground dry weight. Moreover, FYM added to topsoil at 5 and 10% gave significantly higher above-ground fresh weight. This study indicated that amending topsoil with different types and rates of amendment can suppress bacterial wilt severity and pathogen survival in the soil. Amendments also enhanced tomato yield, the higher rates of amendments being the most effective except green compost at 10% which gave a 27% lower yield compared with the higher rate of FYM. Among the amendments tested, FYM at 5 or 10% would be an interesting option to manage R. solanacearum in the major tomato-growing regions of Ethiopia. However, the mechanisms of disease suppression at higher rates of FYM need to be investigated