Bacterial Rot of Witloof Chicory Caused by Strains of Erwinia and Pseudomonas: Symptoms, Isolation and Characterization

A survey of chicory fields in the open air and on forcing farms revealed the importance and complexity of bacterial rot in witloof chicory crops. In the different witloof production stages, a total number of 294 bacterial cultures was collected, from which 61 were shown to be pathogenic. Inoculation of witloof head leaves in vitro allowed the differentiation between two symptoms of bacterial rot: 30 of our isolates caused a water‐soaked translucent wet rot (slime rot) whereas 31 others caused more localized oval‐shaped, brown to reddish lesions (brown rot). Computer‐assisted numerical analyses, using data‐banks built up in our laboratory from API tests and additional biochemical and phytopathological features, and which include reference erwinias and pseudomonads from chicory and other hosts, allowed the phenotypic characterization of each pathogenic strain isolated from witloof chicory. Taking also into account protein profiles from polyacrylamide SDS gel electrophoresis, we were able to delineate at least seven bacterial groups, responsible for bacterial rot on witloof chicory. Five of them induced brown rots and belonged to the genus Pseudomonas. Two of the Pseudomonas groups were identified as P. marginalis and P. viridiflava. The three other brown‐rot inducing groups formed separate clusters of unnamed fluorescent pseudomonads. More than 50% of isolates which induced brown rots were authentic P. marginalis strains. They occurred in classic type farms using covering soil, as well as in hydroculture farms. The two other bacterial groups induced slime rot and were respectively identified as authentic Erwinia carotovora subsp. carotovora and as a new separate cluster in E. carotovora. About 80% of the isolates inducing slime rots belonged to the new cluster. Their occurrence seemed to be limited to hydroculture farms. Three nonpathogenic erwinias, including two isolated from chicory seeds, were characterized as E. rhapontici. Determinative schemes for the diagnosis of all these groups of pathogenic bacteria occurring on witloof chicory are proposed.