First report of the melon stem rot disease in protected cultivation caused by Pseudomonas fluorescens

In recent years, melon culture in East China represented by Shanghai district has been severely affected by a stem rot disease. Symptoms were described as the presence of badly water-soaked soft rot associated with spilled brown juice in the later stages. Based on our disease diagnosis study, 30 bacteria were isolated from the stem tissues of affected plants and one bacterial pathogen, named EXXP-01, was highly virulent on melon stem dipped in bacterial suspension. This bacterium has caused significant damage in late autumn melon cultures, and a disease severity analysis presented as decay diameter of leaves and length of the stem rot suggested that the isolated pathogen not only caused extensive damage to the stem of melon, but also posed potential harm to the whole melon plant including seedlings and leaves. The strain EXXP-01 was also found to be most closely related to P. fluorescens based on physiological and morphological characterization, and also the sequencing analysis of 16S rDNA gene and 16S-23S rDNA internal transcribed spacer gene. Moreover, sequence analysis of another housekeeping gene, the RNA polymerase b-subunit gene (rpoB), indicated that P. fluorescens EXXP-01 could be classified as P. fluorescens bv. I. This confirmed that P. fluorescens is the causal agent of melon stem soft rot. Further bactericidal test in vitro suggested that the strain EXXP-01 has strong resistance to various germicides and should be worthy of attention.