Acta Hortic. 1207. ISHS 2018. DOI 10.17660/ActaHortic.2018.1207.25 Proc. V Int. Symp. on Tomato Diseases: Perspectives and Future Directions in Tomato Protection Ed.: E. Moriones, R. Fernández-Muñoz and C.R. Beuzón 193 Aqueous plant extracts as seed treatments on tomato bacterial speck disease F. Karabuyuk 1,2 and Y. Aysan 1,a 1 Plant Protection Department, Faculty of Agriculture, Cukurova University, 01330 Adana, Turkey; 2 Karaisali Vocational School, Cukurova University, 01770 Adana, Turkey. Abstract Bacterial speck disease caused by Pseudomonas syringae pv. tomato (Pst) is one of the most important seed-borne bacterial diseases of tomato. The use of disease- free seeds is a significantly effective method to manage the disease. In this study, the antibacterial activity of aqueous extracts from 25 medical and aromatic plants was investigated by using the paper disc method. In-vitro inhibition of bacterial growth was recorded with extracts from six different plants (eucalyptus, coriander, garlic, ginger, Istanbul thyme and Izmir thyme). Based on their antibacterial activity, these extracts were selected for seed experiments. Disease severity was evaluated in cotyledons 3 weeks after germination, using a 0-3 scale, taking as a reference the necrotic symptoms developed by positive control seedlings. The effect of seed treatment on disease incidence, disease severity and seed germination was recorded. Under these experimental conditions, five of the six aqueous plant extracts assayed reduced disease incidence and severity, by approximately 63-100 and 57-100%, respectively. Additionally, seed treatment did not reduce seed germination. The results of this study indicate that aqueous plant extracts of the Turkish endemic plants Kastamonu garlic (Allium sativum ‘Kastamonu’), Izmir thyme (Origanum onites) and Istanbul thyme (Origanum vulgare subsp. hirtum) and a Eucalyptus sp. reduce seed-borne bacterial speck disease of tomato, and are promising biological seed treatments that could be used in organic farming for disease control. Keywords: Pseudomonas syringae pv. tomato, seed treatment, antibacterial effect, plant extracts, organic farming INTRODUCTION Bacterial diseases are a significant problem in both greenhouse and open-field tomato farming. Bacterial speck disease caused by Pseudomonas syringae pv. tomato (Pst) is one of the most important seed-borne bacterial diseases of tomato (Jones and Miller, 2014). Seed- borne disease can significantly affect crop yield and quality all over the world. Pst can survive on tomato seeds for up to 20 years (Bashan et al., 1982). Several strategies can be adopted to control Pst, such as copper and antibiotic treatments. However, the pathogen has become resistant to copper compounds and antibiotics; there is therefore a need to develop alternative strategies (El Astal, 2004). Seed treatment is the most important strategy to control seed-borne infections of Pst (Mirik and Aysan, 2005). Plant secondary metabolites, such as essential oils, and plant extracts are known to possess insecticidal, antifungal, acaricidal, antibacterial and cytotoxic activities (Rhouma et al., 2009). There have been several studies on the effect of plant extracts and essential oils obtained from medical and aromatic plants, including Aloe vera, anise, caper, cumin, clove, coriander, dill, eucalyptus, garlic, gentian, Istanbul thyme, Izmir thyme, lavender, marguerite, mint, myrtle, mustard, oleander, onion, purple coneflower, radish, rosemary, sage and ginger, for management of soil-borne and foliar plant pathogens (Mangamma and Sreeramulu, 1981; Mirik and Aysan, 2005; Balestra et al., 2009; Kotan et al., 2010; Talibi et al., 2011; Mbega et a E-mail: aysanys@cu.edu.tr