In 1999 and 2000, decay of floral buds of Actinidia deliciosa was observed in plantations in the Principality of Asturias, Spain. Bud decay led to a decrease (up to 40%) in the production of kiwifruit. Floral buds with symptoms of browning and necrosis were collected from different areas (Villaviciosa, Grado, and Pravia) and processed for microbiological analysis. A fluorescent bacterium was recovered on King's B medium and identified as Pseudomonas syringae by the LOPAT scheme and Hugh-Leifson reaction (2). Other biochemical features included esculin and gelatin hydrolysis and acid production from mannitol, erythritol, sorbitol, and m-inositol, which are features associated with P. syringae (2). Three isolates from different samples were selected to test pathogenicity using Koch's postulates. Overnight broth cultures of each isolate (109 CFU/ml) were used to infect A. deliciosa in the trials by the following procedures: (i) atomization on branches and buds; (ii) bud injection (1 ml in each bud); and (iii) bud cutting with a scalpel dipped in the suspension. Branches and buds inoculated with sterile water were used as controls. The inoculated parts were enclosed in plastic bags for 48 h. Assays were repeated at least twice. Disease symptoms appeared 2 days later, initially as dark brown spots that developed into an extensive bud rot in all inoculated cases, while no symptoms occurred in controls. P. syringae was successfully recovered from infected samples but not from control samples. The data support the pathogenicity of P. syringae on A. deliciosa. Although P. syringae was previously reported in Italy as the causal agent of disease on floral buds of A. deliciosa (1), to our knowledge, this is the first report of infection of kiwifruit by this pathogen in Spain.
References: (1) G. M. Balestra and L. Varvaro. J. Phytopathol. 145:375, 1997. (2) MAPA, Manual de laboratorio, MAPA, Madrid, Spain, 1991.