Detection, Survival, and Source of Inoculum of Pseudomonas syringae pv. syringae from Weeds and Plant Debris in Relation to Epidemiology of Bacterial Citrus Blast and Black Pit in Tunisia

Background: Citrus blast and black pit caused by Pseudomonas syringae pv. syringae occur in several citrus growing regions around the world. However, the sources of inoculums of Pseudomonas syringe responsible for this disease are not well defined.

Aims: To determine the survival of P. syringae pv. syringae in weeds and plant debris, evaluate the pathogenicity of Pseudomonas syringae on citrus and to assess the potential for plant debris andweeds serving as inoculum sources for this pathogen.

Settings and Design: This study was carried out in the Department of Biological Sciences and Plant Protection at the Institute of Science, Agriculture, Tunisia. For a period of six months.

Methodology: Strains of Pseudomonas syringae pv. syringae was recovered from plant debris and symptomless weed species growing in citrus orchards. A total of 24 samples of weeds and four samples of plant debris were included in this study. The fluorescent strains cultivated on King’s medium were identified by LOPAT (Levan production, Oxidase, Pectolytic activity on potato slices, Arginine dehydrolase, HR on tobacco leaves) and GATTa tests (Gelatine hydrolysis, Aesculine hydrolysis, Tyrosine activity, Tarataric Acid usage). Pathogenicity test was carried out on orange (cv.Navel) fruits. PCR amplification for the detection of syrB gene was performed with the syrB primers.

Results: Forty six strains isolated from weeds and plants debris were gram negative by KOH test, levan-positive, oxidase-negative, pectolytic activity-negative, arginine dihydrolase-negative and tobacco hypersensitivity-positive. They showed the LOPAT characters of group Ia (+- - -+). Among all, the 46 strains were positive for gelatin liquefaction and aesculin hydrolysis but negative for tyrosinase activity and tartrate utilization (G+A+T-Ta-). Twenty eight of 43 (65.11%) strains were isolated from weeds, 18 of 22 (81.81%) strains isolated from plant debris were, pathogenic on mature orange (cv. Navel) fruits. The PCR amplification with the syrB primers yielded 752-bp fragments confirmed that the syrBgene was present in the 46 isolates. According to biochemical tests (LOPAT and GATTa), pathogenicity assay and PCR amplification with the syrB primers, the forty six isolated bacterial strains were identified as Pseudomonas syringae pv. syringae.