First report of knot disease caused by Pseudomonas savastanoi on sweet olive in central Italy.
In April 2012, the presence of hyperplastic outgrowths on trunks, branches, and twigs of sweet olive plants, Osmanthus fragrans, was recorded in two ornamental hedges made up of five and four plants, respectively, in the city centre of Montecatini (Pistoia, Tuscany, Italy). All sweet olive plants were seriously affected by the disease with outgrowths appearing either singly or close together, often forming a single mass that could extend up to 20 cm along the stems, occasionally surrounding the entire circumference. The symptoms observed on Osmanthus fragrans closely resembled those induced by the bacterium Pseudomonas savastanoi on Olea europaea (common olive) and other plant species. Suspecting a bacterial origin of the disorder, young knots were collected from four diseased plants and used for bacterial isolation with standard techniques on nutrient sucrose agar medium. On the basis of biochemical tests, PCR screening, pathogenicity testing, and sequence analyses, the causal agent of knot disease on Osmanthus fragrans was identified as P. savastanoi. This is thought to be the world's first report of O. fragrans as natural host of P. savastanoi, which extends the growing list of cultivated and ornamental plant species affected by this phytopathogenic bacterium.