Development of a real-time PCR assay for Pseudomonas cichorii, the causal agent of midrib rot in greenhouse-grown lettuce, and its detection in irrigating water

Midrib rot is an emerging disease in greenhouse production of lettuce caused by Pseudomonas cichorii, and probably introduced through contaminated irrigation water. Concentrations of 100 CFU mL(-1) are enough to induce the typical midrib rot symptoms. A sensitive real-time PCR assay was developed, based on a 90-bp amplicon from the pathogenicity gene cluster hrcRST and a Taqman Minor Groove Binding probe. Specificity of the assay was tested with 39 P. cichorii strains, including the type strain, and 89 strains from 83 other Pseudomonas species. The relationship between detection signals and P. cichorii DNA concentrations was linear over 6-logs. Detection threshold with excellent reproducibility was 500 fg of DNA or about 70 genome copies. Sample preparation and DNA isolation were optimized to allow detection in 1 L water samples. The assay was first evaluated with greenhouse irrigation water spiked with serial dilutions of P. cichorii. The calculated cell numbers obtained with real-time PCR were 10-fold lower than plate counts of actual spiked cells. However, the assay consistently detected 100 CFU per reaction, corresponding to the detection of 1 CFU mL(-1) of irrigation water, which is well below the concentration needed for midrib rot infection. Finally, the assay proved to be valuable for detecting infective P. cichorii concentrations in the irrigation water of a commercial lettuce production greenhouse.