Biocontrol of Bacterial Plant Diseases, 1 st Symposium 2005 62 Mitt. Biol. Bundesanst. Land- Forstwirtsch. 408, 2006 Baysal, Ö. 1* , Gölükcü, Ş.B. 1 , Ünlü, A. 1 , Zeller, W. 2 1 West Mediterranean Agricultural Research Institute (BATEM) Plant Pathology, Department P.B. 35, 07100 Antalya, Turkey, * Correspondent author: E-Mail: obaysal@ttnet.net.tr 2 Biologische Bundesanstalt für Land- und Forstwirtschaft, Institut für biologischen Pflanzenschutz, Heinrichstr. 243, 64287 Darmstadt, Germany An early oxidative burst in apple rootstocks treated with DL-β-amino butyric acid (BABA) against fire blight (Erwinia amylovora) Abstract Systemic acquired resistance (SAR) was induced by pre-treatment with the chemical inducer DL-β- amino butyric acid against fire blight disease caused by Erwinia amylovora (Ea 7/74). The plants were inoculated with 10 8 cfu/ml bacterial suspension, and disease development was evaluated up to 14 days post inoculation. Although in vitro growth of bacteria was not affected by DL-β-amino butyric acid treatment, its pre-inoculation application (500 µg/ml) significantly reduced disease severity and bacterial population. DL-β-amino butyric acid treated plants showed significantly higher H 2 O 2 generation, compared to untreated plants. The findings indicate that pre-treatment with the chemical inducer DL-β- amino butyric acid activated H 2 O 2 generation in planta more strongly when the plants were challenged with the pathogen; this may be associated with induction of plant resistance to bacterial pathogens and an effect of BABA in modulation of pathogen defence pathways. Introduction Fire blight is one of the most serious diseases affecting apple and pear trees as well as other Rosaceae plants (van der Zwet and Beer 1992). Fire blight attacks all aboveground organs of the host plants often leading to their death. Chemical control of the disease relies upon the use of antibiotics (such as streptomycin) and copper compounds which prevent bacterial multiplication and further infection. Unfortunately, the antibiotics lead to the selection of resistant bacterial populations and therefore their use is strictly limited or even forbidden in several countries. In addition, none of these chemicals is systemic, and to be effective, they have to be applied on the whole plant surface before the pathogen enters the plant tissues. The induction of plant resistance may be one of the potential methods of reducing the severity of disease caused by the pathogen. Pre-treatment of susceptible plants with avirulent pathogens (biotic inducer) can enhance resistance to subsequent attack not only at the site of treatment, but also in tissues distant from the initial infection sites. Typically, this inducible resistance system known as systemic acquired resistance (SAR) is effective against diverse pathogens including viruses, bacteria and fungi (Ryals et al. 1996). SAR is characterized by a reduction in the number and severity of lesions following challenge inoculation with a normally virulent pathogen. In addition to biotic inducers, certain chemicals with no direct anti-microbial effect can also induce SAR in plants. Natural products such as salicylic acid (SA) and synthetic chemical compounds, such as 2,6-dichloroisonicotinic acid (INA), may serve as good alternatives to classical pesticides depending on their efficacy. Both SA and INA, however, are not tolerated by some plants (Ryals et al. 1999). Potassium salts, acibenzolar-S-methyl, and amino butyric acid (BABA) were reported to induce SAR in plants (Cohen et al. 1999; Narusaka et al. 1999; Oostendorp et al. 2001; Baysal et al. 2003; Baysal and Zeller 2004; Baysal et al. 2005). In our previous study (Baysal et al. 2005) BABA and the effect of its different doses were tested against bacterial canker disease in tomato and the suppressive effect was found to be 52%. The aim of the present study was to test BABA for its ability to induce resistance in apple rootstocks against Ea.