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.