During the winter of 2004, symptoms were observed in commercial cauliflower (Brassica oleracea L. convar. botrytis (L.) Alef. var. italica) fields of “romanesco” type (cv. Navona) in Apulia, southern Italy. These symptoms were noted on inflorescences that were almost ready for harvest, and a bacterial etiology was suspected. In particular, the corymbs showed water-soaked and brown discolored areas which then rotted. The above alterations involved the whole inflorescences, or in some cases, only a few florets. Longitudinal sections of the symptomatic inflorescences or the single floret showed brown discoloration and rotting of the internal tissues. The disease caused severe crop losses (approximately 100% either in the field or after harvest). Bacteria were isolated from water-soaked and soft-rotted cauliflower heads on King's medium B (KB). The strains were purified on nutrient agar and assayed for pathogenicity on subcorymbs from freshly harvested cauliflower heads. Bacterial suspensions containing approximately 108 CFU/ml were then sprayed on the surface of subcorymbs (3 subcorymbs per strain). Furthermore, in other pathogenicity assays, the florets were dipped in 108 CFU/ml bacterial suspensions or small aliquots of inoculum were injected into the peduncle of subcorymbs with a sterile syringe. Cauliflower heads treated with sterile distilled water were used as controls. After inoculation, the subcorymbs were maintained at 25°C and approximately 100% relative humidity for 48 h. All bacterial strains either applied to cauliflower subcorymbs by spray inoculation or dipping reproduced the disease symptoms. Intensity of symptoms varied with the inoculation method. Injection of bacteria caused water soaking and soft rot of cauliflower internal tissues. No symptoms were observed in negative control subcorymbs inoculated with sterile water. All bacterial strains were gram negative and fluorescent on KB. Isolates (17 of 18) showed the LOPAT characters of group Vb (++−+−) fluorescent pseudomonads, and only strain USB1237 showed characters of group IVb (−+++−) (3). The pectolytic activity of the latter strain was confirmed by the pectinase plate assay (4). The identity of representative strains was confirmed by the nutritional profile obtained with the Biolog Identification System (MicroLogTM System Release 4.2; Biolog, Inc., Hayward, CA). Strains USB1224, USB1226, USB1228, USB1231, USB1235, USB1236, USB1238, and USB1239 were identified as Pseudomonas fluorescens with similarity indices of 0.86, 0.52, 0.73, 0.81, 0.73, 0.74, 0.69, and 0.85, respectively. The pectolytic strain USB1237 was identified as a Pseudomonas spp. that is closely related to P. putida (similarity index = 0.45). In conclusion, the above results indicate that P. fluorescens is responsible for head rot of cauliflower. A similar disease has been previously reported on broccoli in different areas (1,2), but to our knowledge, this is the first report of head rot of cauliflower caused by P. fluorescens.
References: (1) C. H. Canaday et al. Phytopathology 77:1712, 1987. (2) P. D. Hildebrand. Can. J. Plant Pathol. 8:350, 1986. (3) R. A. Lelliott and D. E. Stead. Methods for the diagnosis of bacterial diseases of plants. In: Methods in Plant Pathology. Vol. 2, T. F. Preece, ed. Blackwell Scientific Publications, Oxford, UK, 1987. (4) N. W. Schaad et al. Laboratory Guide for Identification of Plant Pathogenic Bacteria. The American Phytopathological Society, St. Paul, MN, 2001.
