Detection of latent infection of Erwinia chrysanthemi and Pseudomonas caryophylli in carnation1

Three methods for detecting latent infections of Erwinia chrysanthemi and Pseudomonas caryophylli in carnation stems were studied. The same procedure was used, in all three methods, to prepare the samples and to extract and concentrate bacteria. The samples consisted of 100–500, 1‐cm‐long, pieces of carnation stems. Under the standard procedure, the samples were homogenized in maceration fluid, then centrifuged at low speed to eliminate the larger tissue fragments and centrifuged again at high speed to obtain a final pellet with a maximum concentration of the bacteria to be detected. Immunofluorescence staining (IFAS), direct isolation (DI) and immunoenzymatic test (ELISA) were all applied to the final pellets. IFAS had a sensitivity threshold equal to 8.65 × 104 and 3.33 × 104 bacteria per 100 stem pieces for E. chrysanthemi and P. caryophylli. In simulated latently infected lots IFAS could detect down to 0.2% infected pieces for both pathogens, both at high and at low infection level. Although DI had a similar theoretical sensitivity threshold for both bacteria, it was less successful when applied to simulated latently infected lots. ELISA had the highest theoretical sensitivity threshold for both bacteria and as a consequence could only detect them in simulated latently infected lots with a high infection level. The comparative results are discussed in relation to the applicability of the methods to commercial lots of carnation.