The agar diffusion method was used to test the antibacterial activity of 12 plant species against Agrobacterium tumefaciens, the bacterium that is responsible for crown gall disease. Leaf, root, or flower extracts were prepared, but not all parts were used for each of the 12 plants listed. Plant extracts from leaves exhibited higher antibacterial activity than those from flowers and roots. Furthermore, the type of solvent had a significant influence on both the antibacterial activity and the flavonoid and polyphenol content. Acetone and alcohol extracts contained higher contents of these compounds than water extracts. The strongest bacteriostatic effect was of the leaf extracts of eucalyptus (Eucalyptus nicholii L.) and St. John’s wort (Hypericum perforatum L.). Based on HPTLC analysis, eucalyptus extracts contained, among others, chlorogenic acid, hyperoside, and quercetin, while St. John’s wort extracts contained rutin, hyperoside, and quercetin. The tansy leaf extracts (Tanacetum vulgare L.) were also rich in flavonoids and phenolic acids, such as kaempferol-3-glucoside, luteolin, chlorogenic acid, cynarine, and rutin. However, a moderate inhibitory effect against the tested bacterium was found in tansy extracts, as well as hop (Humulus lupulus L.), wormwood (Artemisia absinthium L.), peppermint (Mentha piperita L.), yarrow (Achillea millefolium L.), and nettle (Urtica dioica L.) extracts. The least effective were the root extracts of dandelion (Taraxacum officinale F.H. Wiggers coll.) and valerian (Valeriana officinalis L.), as well as the flower extracts of chamomile (Matricaria chamomilla L.) and marigold (Calendula officinalis L.). Given the lack of effective chemical products and the unavailability of commercially resistant cultivars, the use of plant-based extracts for protecting against crown gall appears to be of particular interest. The preliminary results are promising and suggest that eucalyptus and St. John’s wort extracts are the most promising for controlling A. tumefaciens.
Jacek et al. (Sat,) studied this question.