Ralstonia solanacerum in water

Ralstonia solanacearum is the causative agent of bacterial wilt, which can be destructive in important agricultural crops. It is a quarantine organism in the EPPO region (EPPO A2). Recently a tropical (phylotype I) variant of the pathogen, a strain of R. pseudosolanacearum according to the recent taxonomy, was found in symptomatic rose planting material and cutting roses grown in glasshouses in the Netherlands. In 2016, the survival of Ralstonia in drain water was studied. In 2017, the survival in other substrates including soil, cocopeat and rockwool will be studied.

In 2016, the survival was determined of five Ralstonia solanacearum in drain water at different temperatures up to 112 days, including two Phylotype I strains able to cause wilt in roses. All R. solanacearum strains were able to survive for a long period (at least 112 days) in drain water at 12, 20 and 28 C, but less long at 4 C, as determined by dilution plating on SMSA followed by a colony TaqMan assay or a Bio-TaqMan assay. Populations declined gradually in not-sterile drain water to a low level of approximately 100 cfu/ml after 112 days. In sterile drain water populations hardly declined at 12, 20 and 28 C after 112 days and only a relatively small decline was found at 4 C for some of the strains. Samples that were negative in the dilution plating assay after 112 days, most of them samples that were stored at 4 C, were stem-inoculated in tomato plants to detect cells in a viable but non culturable state (VBNCs). No symptoms developed and no R. solanacearum was detected by dilution plating. Consequently, no indications were obtained that R. solanacearum was able to form VBNCs in drain water.

The  rose strain can survive for at least three months in drainage (Van der Wolf et al., unpublished results). Limited information is available on the survival of R. pseudosolanacearum in other substrates used in the rose production system such as soil, rockwool or cocopeat. In the tropics, R. solanacearum is soil-borne and can survive for many years in soil. In temperate climate regions, R. solanacearum Phylotype II can survive in the field for at least 12 months although populations decline in time to very small densities (van Elsas Jan et al., 2000). Experiments with soil in microscosm held at different temperatures indicated a survival time of 3-6 months (van Elsas Jan et al., 2000). Freezing and thawing as well as drought had a negative influence on survival in soil. For a medium based on cocopeat only a short survival period was found, but the role of cocopeat in the medium was unclear (Singh et al., 2014).