Licorice leaf extract as plant protection agent against tomato late blight – on the way from lab to field

Abstract: Licorice (Glycyrrhiza glabra) leaf extract was tested in the laboratory, on potted plants and under semi-commercial greenhouse and field conditions for control of late blight of tomato (Phytophthora infestans). The leaf extract at 2.5% (w/v) was reducing tomato late blight by 100% in detached-leaf assays. The leaf extract had no curative effect. Protective treatments of potted tomato plants with 1% (w/v) extract concentration reduced late blight to the same degree as the fungicide Fosethyl-Al. Under semi-commercial greenhouse conditions, licorice leaf extract (5% w/v) strongly reduced the disease severity of late blight compared to the control. In a small-scale field trial, licorice leaf extract (5% (w/v)) reduced disease severity of late blight on the last evaluation date by ca. 64%. In a large-scale field trial, the extract (5% (w/v)) reduced diseases severity and disease incidence per plant by 76% and 57%, compared to the control, respectively. Approx. 50% protection was still achieved in treated plots, at the time when control plots were completely destroyed by late blight.