Valorization of Quercus suber L. Bark as a Source of Phytochemicals with Antimicrobial Activity against Apple Tree Diseases

Abstract

Cork, an anatomic adaptation of the bark of Quercus suber L. through its suberization process, finds its main application in the production of bottle stoppers. Its processing results in a large waste stream of cork fragments, granulates, and dust, which may be susceptible to valorization. The work presented here explored the use of its extracts to inhibit the growth of phytopathogenic microorganisms associated with apple tree diseases. The in vitro antimicrobial activity of cork aqueous ammonia extract was assayed against four fungi, viz. Monilinia fructigena and M. laxa (brown rot), Neofussicoccum parvum (dieback), and Phytophthora cactorum (collar and root rot), and two bacteria, viz. Erwinia amylovora and Pseudomonas syringae pv. syringae, either alone or in combination with chitosan oligomers (COS). Effective concentration values of EC90 in the 675–3450 μg·mL−1 range, depending on the fungal pathogen, were obtained in growth inhibition tests, which were substantially improved for the conjugate complexes (340–801 μg·mL−1) as a result of strong synergism with COS. Similar enhanced behavior was also observed in antibacterial activity assays, with MIC values of 375 and 750 μg·mL−1 for the conjugate complexes against P. syringae pv. syringae and E. amylovora, respectively. This in vitro inhibitory activity was substantially higher than those exhibited by azoxystrobin and fosetyl-Al, which were tested for comparison purposes, and stood out among those reported for other natural compounds in the literature. The observed antimicrobial activity may be mainly attributed to the presence of glycerin and vanillic acid, identified by gas chromatography–mass spectroscopy. In the first step towards in-field application, the COS–Q. suber bark extract conjugate complex was further tested ex situ against P. cactorum on artificially inoculated excised stems of the ‘Garnem’ almond rootstock, achieving high protection at a dose of 3750 μg·mL−1. These results suggest that cork industrial leftovers may, thus, be a promising source of bioactive compounds for integrated pest management.

Chemical Constituents and Antimicrobial Activity of a Ganoderma lucidum (Curtis.) P. Karst. Aqueous Ammonia Extract

“…Its efficacy was similar to that of non-conjugated Q. ilex aqueous ammonia extract [ 61 ], although it was tested on Prunus amygdalus × P. persica excised stems rather than on Q. ilex ones. The activity of the COS— G. lucidum extract conjugate complex was higher than those of non-conjugated Sambucus nigra L. flower ammonia extract [ 73 ] and the COS– Quercus suber L. aqueous ammonia bark extract conjugate complex [ 74 ], but these were tested against Phytophthora cactorum and Phytophthora megasperma , respectively, so the comparison should be made with caution.…”

“…Given the restrictions that apply to in vivo assays involving P. cinnamomi , the efficacy of the most active treatment in the in vitro tests (i.e., COS− G. lucidum carpophore extract conjugate complex) was investigated by artificial inoculation of excised stems in controlled laboratory conditions. Inoculation was performed according to the procedure proposed by Matheron et al [ 77 ], with modifications as described in [ 61 , 73 , 74 ]. Young stems (1.5 cm diameter) of healthy Q. ilex plants were cut into 10 cm-long sections using a sterilized grafting knife.…”

Antifungal and Antioomycete Activities of a Curcuma longa L. Hydroethanolic Extract Rich in Bisabolene Sesquiterpenoids

“…Regarding the efficacy of CE as a protective treatment against P. cinnamomi, Table 3 presents a comparison with alternative treatments targeting Phytophthora spp. Its effectiveness was found to be comparable to that of a conjugated complex of chitosan oligomers (COSs) with an aqueous ammonia extract of Quercus suber L. bark [53], and superior to that of the aqueous ammonia extract of Sambucus nigra L. flowers [54]. However, it is crucial to note that those treatments were evaluated against Phytophthora cactorum and Phytophthora megasperma, respectively.…”

Phytochemical Screening and Antibacterial Activity of Taxus baccata L. against Pectobacterium spp. and Dickeya chrysanthemi

“…Concerning the choice of the extraction media, the hydroalcoholic medium-a very popular option when characterization is to be conducted by GC−MS [16]-was chosen for the arils. However, it does not allow for the dissolution of polyphenols and other bioactive compounds of interest contained in the bark samples, which can be successfully attained by digestion in an aqueous ammonia solution (widely used in lignocellulosic residue pretreatment [17]), as previously shown in other works involving bark extracts [18][19][20]. Concerning the leaf extraction medium, aqueous ammonia was chosen due to the substantially higher extraction yield in comparison with the hydromethanolic medium (an order of magnitude higher), probably due to the high lignin content (avg.…”