Induction of resistance to fire blight in apple by acibenzolar-S-methyl and DL-3-aminobutyric acid

Fire blight disease caused by Erwinia amylovora is one of the most important bacterial diseases of apple and pear worldwide. DL-3-aminobutyric acid (BABA; 1.0 mg ml(-1)) and acibenzolar-S-methyl (ASM; 0.1 mg ml(-1)) were applied as foliar treatments on apple seedlings at various days before inoculation (dbi) either alone or in combination at different time sequences. ASM was slightly more effective than BABA in controlling fire blight disease expressed as browning discolouration index (BDI) and stem bending index (SBI). Combined treatments in sequence with BABA/ASM drastically reduced disease expression (BDI 98.1%, SBI 97.8%) for a long period and proved to be more effective than the other treatments. The highest degrees of induced resistance were obtained when BABA and ASM were applied in sequence 4 and 2 dbi or 6 and 4 dbi, respectively. The resistance inducing effects gradually decreased when the period of time between treatment and inoculation increased. Peroxidase (PO) activities increased more markedly in inoculated leaf tissues treated with ASM and BABA/ASM than in BABA-treated plants and in inoculated control tissues. Inoculated or not inoculated apple seedlings treated with BABA alone or combined with ASM (BABA/ASM) revealed the highest levels of both total and free salicylic acid (SA) in leaf tissues. Moreover, in leaf tissue treated with the combination BABA/ASM the lowest number of bacterial cells was determined.