Bacterial rot of lettuce caused by Pseudomonas cichorii in Turkey

In the winter of 2002, a severe disease indicating bacterial rot was observed on leaves of lettuce (Lactuca sativa cv. Tasna) grown in four commercial fields in the province of Mersin (Tarsus) located in the eastern Mediterranean region of Turkey. Disease incidence was estimated to approach 25-30%. Initial symptoms were small (1-2 mm in diameter), shiny, dark brown spots scattered on the blades, petioles and particularly along the veins of leaves underneath the second or third head leaves. Numerous lesions enlarged and coalesced, causing moist, greenish or yellowish to black areas, but no soft rot was observed on the infected plants in advanced stages of the disease. These symptoms were consistent with those of bacterial rot described by Hikichi et al. (1996). A fluorescent, gram-negative bacterium was consistently isolated from diseased tissues onto King's B medium (King et al., 1954). Eight representative strains isolated were oxidase, catalase, levan and pectolytic positive, and arginine dihydrolase negative. They all produced postive hypersensitive reactions on tobacco (Nicotiana tabacum cv. Samsun). Fatty acid methyl ester (FAME) analysis identified the strains as Pseudomonas cichorii (Swingle) Stapp with similarity indices ranging from 86 to 98% (Janse et al., 1992). Pathogenicity of the strains was confirmed on five week-old lettuce (cv. Tasna) and tomato (cv. H-2274) plants sprayed with bacterial suspensions containing 10⁸ CFU/ml of sterile water. Inoculated and sterile-water-sprayed control plants were maintained in the growth chamber for 8 days at 25° C and 70% RH. Lesions similar to those observed in the field developed on the inoculated plants within 5-7 days. No symptoms developed on control plants. The bacterium was reisolated from inoculated plants and identified as P. cichorii. This is the first report of the occurrence and outbreak of a bacterial rot disease caused by this bacterium on lettuce grown in Turkey. It is not proven, but is speculated that contaminated seeds and/or transplants may have been the source of introduction of the pathogen to this region.