First report of Acidovorax avenae subsp. avenae associated with rice panicle sterility in Italy

During the last years sterile panicles have become rather common in rice fields in Italy. Diseased plants are uniformly distributed in the field or are aggregated within distinct areas scattered throughout the field, suggesting a seedborne infection. The objectives of this study were: i) to assess disease incidence and ii) to test the hypothesis that Acidovorax avenae (Aa) was associated with rice panicle sterility. Results of the survey showed that panicle sterility was present in 20 farms on 13 varieties, and the most susceptible were ‘S. Andrea’, ‘Balilla’, ‘Ariete’, and‘Thaibonnet’. Disease incidence ranged from 1 to 52%. To isolate and confirm the identity of the causal organism, spikelets from 53 sterile panicles were surface sterilized, crushed, and soaked in PBS for 4 h at 40°C. Aliquots of 500 μl of the suspension were plated onto sorbitol pyroglutamic acid agar medium, and the plates incubated at 37°C for 3 days. Over 100 light tan coloured colonies typical of Aa were obtained from 19 samples. They were all tested for polyB-hydroxybutyrate (PHB) accumulation, pathogenicity to corn seedlings, and by PCR. DNA of each suspected strain and of four known rice strains of A. avenae subsp. avenae (Aaa) was tested by classical PCR using Aa-specific primers and rice Aaa-strain-specific primers. All suspected and known strains were positive for PHB, caused soft rotting of corn seedling, and yielded the expected amplicons. This is the first record of Aaa from rice in Italy.