Pseudomonas Blight Caused by Pseudomonas syringae on Raspberry in California.

In February 2013 in coastal California (Santa Cruz County), plantings of red raspberry (Rubus idaeus var. strigosus) exhibited symptoms of a previously undocumented disease. Initial symptoms were small (less than 5 mm wide), angular, water-soaked lesions on leaf and petiole tissues of recently emerged foliage. Lesions were observable from both adaxial and abaxial leaf surfaces. As disease progressed, lesions enlarged and coalesced, resulting in significant dark brown to black blighting of the foliage. The foliage of severely affected plants was stunted and wilted. The disease affected 5 ha and incidence was approximately 30%. Cream-colored bacterial colonies were isolated from surface disinfested symptomatic tissue that was macerated and streaked onto King's medium B (KMB) and sucrose peptone agar (SPA). Fungi were not recovered from any tissue that was surface disinfested and placed into acidified potato dextrose agar. Four representative strains were fluorescent on KMB and gram-negative based on lysis by KOH. Strains were positive for levan formation, negative for oxidase and arginine dihydrolase, and did not cause soft rot on potato slices but induced a hypersensitive response in tobacco (Nicotiana tabacum L. cv. Samsun); strains thus belonged to Lelliot's LOPAT group 1, P. syringae (3). All four strains had identical DNA fragment-banding patterns generated by repetitive extragenic palindromic sequence (rep)-PCR using the BOXA1R primer (4). The pattern generated was different than all P. syringae pathovars in genomospecies 1 including P. syringae pv. syringae. According to multilocus sequence analysis conducted by previously described methods, the strains are most closely related to P. syringae pv. aceris and P. syringae pv. solidagae in genomospecies 1 (1). Potted raspberry plants were used to test four strains for pathogenicity. Inoculum was prepared by growing the bacteria on SPA for 48 h and suspending the bacteria in sterile distilled water (SDW) for a final concentration of approximately 10⁷ CFU/ml. Suspensions were sprayed until runoff onto three replicate plants per strain. Control plants were sprayed with SDW until runoff. Plants were enclosed in plastic bags for 24 h and then maintained in a greenhouse (23 to 25°C). After 7 to 8 days, water soaked lesions developed on all inoculated plants; lesions later turned dark brown and appeared similar to symptoms observed in the field. Plants treated with water developed no symptoms. Bacteria re-isolated onto KMB from symptomatic tissues were fluorescent and appeared identical to the bacteria used to inoculate the plants; two selected re-isolated strains were identical to the original strains according to rep-PCR, fluorescence, and LOPAT reactions. The experiment was repeated and disease development and recovery of fluorescent strains on KMB was identical to the first experiment. To our knowledge, this is the first report of Pseudomonas blight of raspberry, caused by P. syringae, in California. Affected plants initially were stunted in growth but later in the summer exhibited no lasting effects from the disease. Pseudomonas blight has been reported in the Pacific Northwest region of the United States, the British Columbia region of Canada, and Serbia (2). References: (1) C. T. Bull et al. Phytopathology 101:847, 2011. (2) Z. Ivanovic et al. Eur. J. Plant Pathol. 134:191, 2012. (3) R. A. Lelliott. J. Appl. Bacteriol. 29:470, 1966. (4) A. S. A. Marques, et al. Genet. Mol. Biol. 31:106. 2008.