First Report of Bacterial Blight of Strelitzia augusta Caused by Pseudomonas syringae pv. lachrymans

Abstract

White bird of paradise tree (Strelitzia augusta Thunb.), originally from South Africa, is a tender perennial cultivated as an ornamental plant and is used in gardens in Italy. During February of 2004, a new blight disease was noticed on potted S. augusta at different ages (6 months to 4 years) in several commercial nurseries of eastern Sicily. Field inspections revealed disease incidences as high as 40%. Initial symptoms were small, water-soaked leaf spots that expanded throughout the veins in dark brown streaks. Stem cross sections revealed browning of the vascular tissues, which might involve the entire stem. In some cases, the necrosis extended to the apical bud, causing death of the plant. Thirty explants from infected tissues were washed in sterile water and plated on plate count agar (PCA) from which two types of bacterial colonies were consistently isolated. Pathogenicity tests were performed on S. augusta plants. Twenty-four plants were inoculated (12 per bacterial isolate) using two different procedures: spray with a bacterial suspension (106 CFU/ml) and wounding with an infected needle on the midribs. The same number of noninoculated plants was used as controls. All plants were maintained at 24 to 26°C with 95 to 100% relative humidity until symptoms occurred 4 days later. Just one of the two tested bacterial types was pathogenic. The symptoms were similar to those previously observed in the field. No symptoms were observed in the plants spray inoculated with the bacterial suspension, proving that the bacteria were unable to infect in the absence of a wound. The controls showed no symptoms. Koch's postulates were fulfilled by the reisolation of the infective strain which was sent to the CBS (Centraalbu-reau voor Schimmelcultures) and identified as Pseudomonas syringae pv. Lachrymans/pisi using the Biolog MicroLog3 4.01C program (Biolog Inc., Hayward, CA). Further pathogenicity tests have been carried out on zucchini and pea pods to characterize the pathovar using 48SR1 of P. syringae pv. syringae and B4 of P. syringae pv. pisi as reference strains. Necrotic, sunken, water-soaked spots surrounded by a chlorotic halo, reported in the literature as typical symptoms of P. syringae pv. lachry-mans (Smith & Bryan) Young, Dye & Wilkie (1), were observed on zucchini when inoculated with our strain. Our P. syringae strain did not cause the typical symptoms of P. syringae pv. pisi on inoculated pea pods. The results of the pathogenicity tests and the inability of the P. syringae strain isolated from S. augusta to utilize homoserine, used to discriminate pv. pisi from other pathovars of P. syringae, allowed us to identify the strain as P. syringae pv. lachrymans. Low temperature damage and late transplant may have promoted the spread of the disease in the nurseries. Under these conditions, the economic importance of this disease for the crop can be considered high. To our knowledge, this is first report of P. syringae pv. lachrymans on S. augusta. Reference: (1) K. Pohronezny et al. Plant Dis. Rep. 62:306, 1978.