Panicle sterility and grain discolouration : new and emerging bacterial diseases of rice in Italy

In Italy, panicle sterility and grain discolouration have emerged as serious problems throughout the rice growing areas. Surveys showed that the incidence of panicle sterility was highly variable, varying from low, barely observable in some fields, to more than 50% in other ones. In the Lombardy region the estimated average crop loss due to sterility was 1.1% and 0.1% in 2003 and 2004, respectively. Grain discolouration reduces rice quality. At harvest, susceptible cultivars can have more than 30% discoloured grains. The etiology for both diseases is uncertain and early diagnosis difficult because diseased plants are symptomless. We have demonstrated for the first time that both diseases have a bacterial etiology. Bacterial colonies typical of Acidovorax avenae subsp. avenae were purified from sterile panicles and discoloured rice samples. All recovered strains were confirmed as Aaa by classical PCR using Aaa-specific primers. Isolation of Aaa on a semi-selective agar medium did not always work well. However, an improved BIO-PCR assay was used to test 113 sterile panicle samples and 74% were positive for the presence of Aaa, but negative for Burkholderia glumae (Bg). The bacteria associated with discoloured grains were mostly composed of Pseudomonas and Pantoea species. Based on the 16S rDNA sequences, 85% of the pseudomonads population was classified as P. straminea, P. fulva, P. putida, P. psychrotolerans, P. stutzeri and P. fluorescens. Aaa and Pseudomonas strains were tested for pathogenicity on seedlings in the greenhouse and on plants at the booting stage in the field. Strains of Aaa caused soft rotting of seedlings, panicle sterility and grain discolouration and they showed significant intraspecific variation for virulence. Although most Pseudomonas spp. were not pathogenic, strains of P. stutzeri and strains presumptively identified as P. fulva induced grain discolouration incidence significantly higher than the control. In addition, we have found, for the first time, P. ananatis on rice in Italy.