First outbreak and occurrence of citrus blast disease, caused by Pseudomonas syringae pv. syringae, on orange and mandarin trees in Turkey

In the spring of 2004, severe outbreaks of a disease resembling citrus blast (Whiteside et al., 1988), were observed on trees of orange (Citrus cinensis cv. Washington) and mandarin (Citrus rediculate cv. Marisol) in the Turkish Mediterranean regions of Adana and Antalya. Characteristic disease symptoms were first seen on leaves as water-soaked lesions and black areas on the petiole wings. Later lesions extended to the mid vein of leaves (Fig. 1) and to the twigs surrounding the base of the petiole. Finally, the leaves dried and rolled, while still firmly attached (Fig. 2), before eventually dropping without petioles. The necrotic areas on twigs further enlarged and the twigs were eventually killed within 20-30 days (Fig. 3). The damage was serious in a 50 hectare citrus orchard in Antalya, with a disease incidence of nearly 100%.

Twelve isolates of a bacterium, consistently isolated from infected leaves, petioles and twigs, which formed fluorescent colonies on King's medium B, were purified and used for further studies. All isolates were gram, oxidase, pectolytic activity, starch hydrolase, arginine dihydrolase and nitrate reduction negative; and levan, gelatin hydrolase positive. They produced hypersensitive reactions (positive) on tobacco leaves (Nicotiana tabacum cv Samsun N.). All produced acid from glucose, glycerine, arabinose, mannitol, sorbitol, sucrose and xylose but not from lactose and maltose. The test results conformed to the characteristics of P. syringae pv. syringae (Braun-Kiewnick & Sands, 2001) as the causal organism of citrus blast and were similar to those of reference strain NCPPB 2307 of P. syringae pv. syringae used in this study. Fatty acid analysis (Atatürk University, Erzurum, Turkey) confirmed the bacterial strains as P. syringae pv. syringae with similarity indices of 81 to 94%.

Pathogenicity of the strains was tested on one-year-old mandarin. Inoculations were made using a hypodermic needle with a suspension of bacteria in a saline buffer (108cfu per ml), into areas on the wing of the petiole. Saline buffer alone was used for negative controls. Test plants were incubated in clear polyethylene bags for 24 hours in a controlled climate room at 25oC, 70% relative humidity and 16-hour per day light period. Inoculated plants showed water-soaked or black areas, which quickly moved in both directions along the mid vein of the leaves, 7 to 9 days after inoculation. From these P. syringae pv. syringae was readily re-isolated. Bacterial canker on apricot caused by P. syringae pv. syringae has been reported previously in Turkey (Kotan & Sahin, 2002), but this is the first outbreak and occurrence of citrus blast disease in Turkey.