Saline and alkaline soil stress results in enhanced susceptibility to and severity in tomato pith necrosis when inoculated with either Pseudomonas corrugata and/or P. fluorescens

Abstract

Tomato pith necrosis caused by Pseudomonas corrugata and other Pseudomonas species is a common disease occurring worldwide, including Saudi Arabia. We demonstrated how the pith necrosis length induced by P. corrugata strain KSU-RT3 significantly increased (>31%) when it was co-inoculated along with P. fluorescens strain KSU-AT1. The latter exhibited antagonistic activity against P. corrugata in vitro and revealed its ability to induce pith necrosis symptoms on tomato when inoculated individually. The two bacterial strains could grow in the presence of NaCl concentrations of up to 80 mM. The growth of the two bacterial strains was suppressed under acidic conditions (pH 4–5). In vivo, the pith necrosis length was significantly increased by 16.15% under irrigation with saline water. A short exposure of 14 days to high salinity (in the absence of pathogens) was sufficient to render the tomato plants significantly more susceptible to subsequent inoculation with either of the two pathogens or when inoculated in combination. In conclusion, our study showed that acid soil and saline soil watered with fresh water showed decreased infection, only neutral to alkaline soil (up to pH 8) and saline soil watered with saline water showed increased infection. So an important cultural measure could be to keep soil pH reasonably low. More antagonistic P. fluorescens strains should be tested for pathogenicity before large-scale application to avoid the risk of introducing strains, such as our P. fluorescens strain KSU-AT1 that proved to be pathogenic to tomato plants.