Antifungals from forest trees: Usefulness in the control of etiological agents of late season soybean diseases

Abstract

Septoria brown spot and Cercospora leaf blight are late season diseases caused by Septoria glycines and Cercospora kikuchii, respectively. New antifungals are required against these diseases because the chemical controls currently used have detrimental impacts on wildlife and human health. In this work, 48 extracts originated from the leaves, bark, sapwood or heartwood of four forestry species were assayed by the disc diffusion method against S. glycines and C. kikuchii. Although 18 extracts showed antifungal activity, only 5 were active on both fungal species. The leaf methanolic extract of Blepharocalyx salicifolius showed the lowest minimum inhibitory dose (MID) and the highest diameter of growth inhibition (DI) on both fungal species (MID = 200 μg, DI = 14.2 mm, C. kikuchii; MID = 400 μg, DI = 12.2 mm, S. glycines). Pinocembrin was identified as the main antifungal constituent of the methanolic extract. Both the methanolic leaf extract of B. salicifolius and pinocembrin synergized in vitro the effect of the fungicide difenoconazole. Preventive applications of the extract and the mixture extract + difenoconazole (2.4 mg/mL + 0.006 mg/mL) strongly reduced disease severity generated by S. glycines and C. kikuchii 21 days after inoculation of the soybean plants. This effect was significantly stronger than that generated by difenoconazole. Our results suggest that the application of the methanolic extract of B. salicifolius, alone or in mixture with difenoconazole is a promising strategy to be incorporated in the chemical control of S. glycines and C. kikuchii.