The aims of the present study were (i) to characterize the dichloromethane extract of the fresh aerial parts of S. somalensis, cultivated in Liguria (Italy), (ii) to quantify carnosic acid production and (iii) to find an eco-friendly alternative approach to control diseases caused by phytopathogenic fungi. The phytochemical investigation yielded several known terpenoids, as well as a diterpene, 4α,9α-epoxy-2H-dibenzo[a,d]cyclohepten-7(5H)-one, not previously described as a plant metabolite before. The results showed a noteworthy quantity of carnosic acid (113.90 µg/mg of dried extract). The potential antifungal activity of the plant surface extract and carnosic acid against five phytopathogenic fungi (Colletotrichum coccodes, Fusarium oxysporum, Sclerotinia sclerotiorum, Botrytis cinerea and Rhizoctonia solani) was considered. A complete inhibition of C. coccodes, S. sclerotiorum and R. solani mycelium growth was observed by carnosic acid at 500 µg/mL. High inhibition values were observed against B. cinerea and F. oxysporum compared to reference active ingredients. Four different B. cinerea strains exhibited a pronounced sensitivity to carnosic acid, as well as those originating from agricultural crop scenarios where a high load of active ingredient for gray mold control was historically adopted. Additionally, the formation and development of the germinative tube in B. cinerea were greatly slowed down.
