RESEARCH ARTICLE Antifungals from forest trees: Usefulness in the control of etiological agents of late season soybean diseases Christian J. Sequín 1 | Diego A. Sampietro 2 | Analía A. Gomez 2 | César A.N. Catalán 2 | Pablo G. Aceñolaza 1 1 Facultad de Agronomía, Universidad Nacional de Entre Ríos, Entre Ríos, Argentina 2 LABIFITO, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucumán, San Miguel de Tucumán, Argentina Correspondence Diego A. Sampietro, LABIFITO, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucumán, Ayacucho 471 (4000), San Miguel de Tucumán, Argentina. Email: dasampietro2006@yahoo.com.ar Funding information Universidad Nacional de Entre Rios, Grant/ Award Number: PID Novel-UNER 2175; Universidad Nacional de Tucumán, Grant/ Award Number: PIUNT D532 Septoria brown spot and Cercospora leaf blight are late season diseases caused by Septoria gly- cines 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 gener- ated 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. KEYWORDS Blepharocalyx salicifolius, Cercospora kikuchii, pinocembrin, Septoria glycines 1 | INTRODUCTION Septoria brown spot and Cercospora leaf blight are soybean diseases caused by Septoria glycines Hemmi and Cercospora kikuchii T. Matsumoto & Tomoyasu, respectively (Carmona, Moschini, Caze- nave, & Sautua, 2010). They cause severe defoliation in the late sea- son of soybean growth (Almeida, Hau, Amorim, Bergamin Filho, & Mariano, 2015) reducing the grain yields and quality. In Argentina, soybean monoculture and the large-scale adoption of no-till seeding have increased the incidence and severity of both diseases (Hartman et al., 2015; Wrather et al., 2010). The control of both S. glycines and C. kikuchii heavily depend on the use of fungicides such as difenoco- nazole, ciproconazole, propiconazole and tebuconazole applied from the beginning of pod development until seed filling either alone or in mixtures with strobilurins or benzimidazoles. The intensive repeated application of these fungicides resulted in the appearance of resis- tance and the application of higher doses for fungal control. This situ- ation increases the cost of soybean production and generates serious concerns regarding the impact of fungicides on human health and the environment (Damalas & Eleftherohorinos, 2011). Fungicides cur- rently in use, particularly strobilurins, also increase the percentage of plants with green stems when the pods are mature making harvest operations difficult (Hill, Bowen, & Hartman, 2013). Antifungal agents with new mechanisms of action are needed in order to overcome these problems. In this context, several reports indicate that forest biomass is a source of high value metabolites including antifungals useful for the control of agricultural diseases (Luís et al., 2016). For example, dichloromethane extracts of Larix species almost completely Received: 27 June 2018 Revised: 12 December 2018 Accepted: 6 January 2019 DOI: 10.1111/aab.12492 Ann Appl Biol. 2019;1–8. wileyonlinelibrary.com/journal/aab © 2019 Association of Applied Biologists 1