Effect of Acibenzolar-S-methyl and Rahnella aquatilis (Ra39) on Chitinase and β-1, 3-glucanase Activities and Disease Resistance of Apple Plants

Institut fur biologischen Pflanzenschutz, Heinrichstr 243, 64287 Darmstadt, Germany(Received on October 11, 2009; Accepted on February 12, 2010)The effect of Acibenzolar-S-methyl (ASM) and Rahnellaaquatilis Ra39 against apple fire blight disease causedby Erwinia amylovora were tested as a possible alter-native to streptomycin. In vitro studies, no inhibitioneffect against the pathogen was found when ASM wastested. Under greenhouse conditions, application of R.aquatilis Ra39 with the highly susceptible M26 rootstockresulted in a marked disease suppression. Application ofASM and strain Ra39 caused a high decrease of thedisease, 82% and 58% respectively; this was correlatedwith a reduction of the growth of the pathogen withinhost plants up to 64% and 49.5% respectively. Furtherstudies in the field under artificial infection conditionduring full bloom revealed that application of ASM andR. aquatilis Ra39 with Gala variety resulted in a controleffect up to 21 and 29% respectively. In physiologicalstudies, enhanced activities of PR-proteins (chitinaseand β-1, 3-glucanase) were detected, which are wellknown as biochemical markers for systemic acquiredresistance. Application of ASM to apple shoots causedthe highest chitinase activity followed by strain Ra39.The enzyme activity was increased after 2, 4 and 6 daysfrom application. In addition, ASM-treatment causedthe higher β-1, 3-glucanase activity than strain Ra39.Maximum enzyme activity was recorded after 6 daysfrom application and then decreased after 8 and 10 daysfrom application.Keywords :fire blight, acibenzolar-S-methyl, Rahnella aquatilis ,PR-proteins Fire blight has been known as a destructive disease of appleand pear for over 200 years. The disease is caused by thebacterium Erwinia amylovora (Burrill) Winslow et al. It hasbeen reported from 40 countries around the world andcaused serious losses to pome fruits and ornamentals plants,especially Rosaceae family (Vanneste, 2000). In Egypt,during early of 1980, the disease became very severe, andabout 80% of all pear acreage was affected and 50% of alltrees had been eradicated, presumably due to an increase inrain during the blooming period (El-Kazzaz, 1992). At present, the disease is controlled by application ofvarious methods including sanitation, chemical treatment,selection of resistance cultivars and soil management. How-ever, there is no single mean that would totally eradicate thedisease, provide an absolute cure or fully protect host plantsagainst infection. Generally, only copper and antibiotics preparations arerecommended but their use has some limitations. Theantibiotics have lead to the selection of resistance bacterialpopulations. Therefore, their use is strictly limited or evenforbidden in a number of countries. The copper compoundsusually cause phytotoxicity in fruit and therefore, cannot beused for high quality fruits (Paulin, 1996; Sobiczewski et al.,2001). In addition, none of these chemicals is systemic, andto be effective, they have to be applied with through cover-age before the pathogen enters the plant tissues.The newest control method for controlling fire blightdisease is the use of induction of systemic acquiredresistance (SAR). Several biocontrol agents have beenshowed to induce resistance in plants. Biotic and chemicalagents are reported as SAR inducers, such as fungi,bacteria, viruses, metabolites of pathogens, plant extract,etheric oils and synthetic compound (Dietrich et al., 1999). One of synthetic compounds is acibenzolar-S-methyl(ASM, Bion) which activates treated plants to producepathogenesis-related proteins (PR-Proteins) in intercellularspaces. They are able to degrade bacterial cell walls. ASMcause SAR since it mimics the role of salicylic acid(Kessmann et al., 1996; Sobiczewski et al., 2001). It wasfound that the best period of Bion application as fire blightresistance inducer on apples was two days suspectedinfection (Baysal and Zeller, 2004). Several epiphytic bacteria have been also reported toinduce resistance against certain plant diseases (Goodmanet al., 1986). Biocontrol strains, an inducer of resistancewere shown to cause physiological changes in plants. Thus,enhanced level of PR-proteins (chitinase and β-1, 3-glucanase) which can be regarded as markers of inducedresistance were observed. PR proteins have been identified