Broad-spectrum activity of the volatile phytoanticipin allicin in extracts of garlic (Allium sativum L.) against plant pathogenic bacteria, fungi and Oomycetes

Abstract

The volatile antimicrobial substance allicin is produced in garlic after cellular decompartmentalisation when the tissues are damaged and the substrate alliin mixes with the enzyme alliin-lyase (E.C.4.4.1.4). The effectiveness of garlic extract against a range of plant pathogenic organisms was tested in vitro and in planta in diseased tissues. Allicin in garlic extracts was quantified spectrophotometrically and a rapid bioassay was developed for routine use. The in vitro activity of allicin against a prototrophic E. coli isolate was compared with that of the conventional antibiotics ampicillin and kanamycin.Activity in vitro was shown against the plant pathogenic bacteria Agrobacterium tumefaciens, Erwinia carotovora, Pseudomonas syringae pv. maculicola, P.s. pv. phaseolicola, P.s. pv. tomato, Xanthomonas campestris pv. campestris, the fungi Alternaria brassisicola, Botrytis cinerea, Plectosphaerella cucumerina, Magnaporthe grisea, and the Oomycete Phytophthora infestans. Disease reduction in planta was shown for Magnaporthe grisea-infected rice, Hyaloperonospora parasitica-infected Arabidopsis thaliana and Phytophthora infestans-infected potato tubers. Significantly, the active principle was effective in reducing P. infestans spore germination in vitro and disease in blighted tubers via the vapour phase (fumigation) as well as by direct application at the inoculation site. In Arabidopsis the reduction in disease was apparently due to a direct action against the pathogen since no accumulation of salicylic acid (a marker for systemic acquired resistance, or SAR) was observed after application. The potential for developing preparations of garlic for use as an alternative to synthetic fungicides for organic food production is discussed.