ORIGINAL RESEARCH Antimicrobial activity of Melia azedarach fruit extracts for control of bacteria in inoculated in-vitro shoots of ‘MRS 2/5’ plum hybrid and calla lily and extract influence on the shoot cultures By Grazia Marino & Francesca Gaggìa & Loredana Baffoni & Chiara Toniolo & Marcello Nicoletti Accepted: 29 October 2014 # Koninklijke Nederlandse Planteziektenkundige Vereniging 2014 Abstract The study aimed to investigate the antibacte- rial activity of Melia azedarach fruit extracts in plant shoot culture. Unripe fruits were collected in different periods and the extracts were obtained through homog- enization in an ethanol (EtOH) solution (40 g fresh weight in 100 ml 50 % EtOH) followed by EtOH evaporation. Their activity was tested against isolates of several bacterial species (Bacillus spp., Pseudomonas spp.) of agronomic interest, and on shoots of ‘MRS 2/5’ (Prunus cerasifera x Prunus spinosa) and calla lily (Zantedeschia aethiopica) inoculated (bacterized) with Microbacterium oleivorans (Mo), Bacillus subtilis (A17), Sphingomonas meloni (Sm) and Bacillus nealsonii (Bn). Shoot number and fresh weight of ex- plants of ‘MRS 2/5’ on the proliferation medium and shoot length on the rooting medium were reduced in cultures derived from shoots bacterized with Mo and A17. Moreover, rooting frequency and root number decreased in A17-bacterized shoots. In calla lily, the number of rooted shoots was strongly reduced also by Sm. The extracts had a bactericidal activity against Mo, A17, Bn and most other Bacillus spp. isolates within 2- year trials. This activity remained unchanged against most of the tested bacterial strains after autoclave- sterilization (20 min at 120 °C). The filter-sterilized (0.22-μm Millipore membrane) extract generally re- duced shoot elongation, and caused the death of some ‘MRS 2/5’ shoots at a 20 % level in the proliferation medium, while all calla lily shoots survived the treat- ment. It strongly reduced Mo and Bn spread in both plant genotypes, and was even bactericidal against Mo in’MRS 2/5’, without showing any negative effect on growth of survived shoots already at the second subcul- ture on standard medium. The HPTLC analysis revealed the prevalence of polyphenols in the extracts from fruits collected in two successive periods; this was consistent with their antimicrobial activity. Keywords Bacteria inhibition . Micropropagation . Persian lilac . Prunus cerasifera x Prunus spinosa . Zantedeschia aethiopica Introduction Bacterial contamination can be a great problem for in vitro plant tissue cultures. Bacterial contaminants may derive from initial explants, after deficient sur- face sterilization, from latent bacteria inside the tis- sue (Kulkarni et al., 2007), or from the laboratory environment, including strains of human origin (Leifert et al., 1994). Non-phytopathogenic bacterial strains may become harmful to plants under in vitro Eur J Plant Pathol DOI 10.1007/s10658-014-0559-6 Electronic supplementary material The online version of this article (doi:10.1007/s10658-014-0559-6) contains supplementary material, which is available to authorized users. B. G. Marino (*) : F. Gaggìa : L. Baffoni Department of Agricultural Sciences, Bologna University, viale Fanin, 44-40127 Bologna, Italy e-mail: grazia.marino@unibo.it C. Toniolo : M. Nicoletti Department of Environmental Biology, Sapienza University of Rome, Piazzale A. Moro, 5-00185 Rome, Italy