First report of Acidovorax avenae ssp avenae on rice in Italy

Rice panicle sterility and rice grain discolouration have recently increased their incidence in Italy. Early diagnosis for both diseases is extremely difficult since diseased plants are symptomless. In 2003 incidence of rice panicle sterility in Lombardy (northern Italy) was 1.8%, and the estimated crop loss was 1.5%. The spatial distribution of sterile panicles suggested a seed-borne bacterial etiology. Grain discoloration can have multiple causes; our data disagreed with a suggested fungal etiology, highlighting the possible role of bacteria. In this study, we tested the hypothesis that rice panicle sterility and rice grain discolouration are associated with Acidovorax avenae ssp. avenae (Aaa), a destructive seed-borne bacterium in tropical and subtropical environments. Fifty-three sterile panicle samples and 46 rice samples with more than 4% discoloured grains were analyzed for the presence of Aaa. Light tan coloured colonies typical of Aaa (N >200) were purified from sterile panicle and discoloured rice samples. All strains were tested for poly-b-hydroxybutyrate (PHB) accumulation and identified by PCR. For PCR, DNA of each Italian strain (IAa) and known rice strains CAa4 and CAa6 were tested by classical PCR using Aaspecific primers Oaf1/Oar1 and rice Aaa-strain-specific primers Aaaf3/Aaar2 (Song et al., 2004). All Italian strains were positive for PHB, and were PCR positive. Strains IAa93, 210, 258, and CAa4 were tested for pathogenicity in corn and in rice seedlings and all caused soft rotting. This is the first report of Acidovorax avenae ssp. avenae on rice in Italy.