Technological and Educational Institution of Larissa, Department of Plant Production, 411 10 Larissa, Greece

Background and objectives
The antibacterial and antifungal properties of garlic, mainly against human pathogens, are well known [1, 2]. The objective of the present study is the investigation of the possible role of garlic as a potential biocontrol agent against microorganisms of agricultural importance.

Materials and methods
The effect was tested of aqueous garlic extract prepared from cooking garlic powder over five tomato plant pathogens in vitro. Verticillium dahliae (P1),Fusarium oxysporum f. sp. lycopersici (P2l), Phytophthora parasitica (P3), Pseudomonas syringae pv. tomato (P4), and Clavibacter michiganensis subsp. michiganensis (P5), all isolated from diseased tomato plants, were used. Non-sterile aqueous garlic extract was prepared by cooking garlic powder. Sterilized and non-sterilized garlic extracts, respectively, were incorporated into autoclaved potato dextrose agar in petri dishes at concentrations of 25, 12.5, 6.25, 3.125 and 0.0 mg/ml. Another series of PDA dishes, with garlic extract at concentrations of 50, 45, 40, 35, and 30 mg/ml, was also prepared. The plates were inoculated with 3-mm mycelium plugs (fungi) or by streaking (bacteria). The plates were incubated in darkness at 23C. The radial colony growths were measured every 24 h for 6 days and the minimum inhibitory concentration (MIC) was assessed after 4 days' incubation. Counts of spores produced by Fol and Vd were made after 6 days' incubation in darkness at 23C, whereas chlamydospores were counted after an extra period of incubation for 4 months at 4C. Subsequently, garlic extract and fluotriafol (Impact) were tested, alone and in combination, for their efficacy to control disease caused on tomato plants in pots by the same five plant pathogens. An experiment was designed to investigate the effect of four treatments (garlic extract, flutriafol - Impact 12.5 SC , garlic extract+flutriafol, and sterile water, sw) and the five pathogens. A randomized block design was used, with 10 blocks and 20 (4x5) treatment combinations. 25 ml of each treatment was added per plant. Garlic extract and flutriafol were used at a concentration of 30 mg/ml and 2 g/ml, respectively. Prior to this experiment, another experiment was conducted to investigate the effect of the organisms on the plants relative to control. A randomized block design was used, with ten blocks and six treatments (P1-P5; P6=control). The plants were kept for 3 weeks in a greenhouse and then plant growth was assessed

Results and conclusions
Garlic extract at a concentration of 25 mg/ml decreased the radial colony growth of P1, P2 and P3 by up to 42, 48 and 100%, respectively. The MIC for P1 was 30 mg/ml after 24, 48 and 72 h incubation. The MIC for P3 was 12.5, 12.5 and 25 mg/ml, respectively, after the same incubation time. For P2, no one MIC was recorded at all concentrations tested. A decrease in the number of spores produced by P1 and P2 was also observed. The bacterial species P3 and P4 showed an MIC of 30 mg/ml after 72 h incubation. Sterilized aqueous garlic extract lost any fungistatic and bacteriostatic effect. The ANOVA of the in vivo test showed that all organisms tested were pathogenic. There were also significant differences and a significant interaction between pathogens and treatments. Garlic extract, flutriafol and their combination exhibited protection of tomato plants infected with the five pathogens tested.

1. Hughes BG, Lawson LD, 1991. Phytotherapy Research 5, 154-158.
2. Rees LP, Minney SF, Plummer NT et al., 1993. World Journal of Microbiology and Biotechnology 9, 303-307.