Plant extracts as alternative seed treatments against Fusarium oxysporum associated with Cowpea (Vigna unguiculata (L.) Walp.)

Sammanfattning

Fusarium wilt, caused by the pathogen Fusarium oxysporum, is one of the most destructive biotic stresses affecting cowpea production in sub-Saharan Africa. As synthetic fungicides have multiple limitations, including environmental concerns and inconsistent accessibility, there is a growing need to investigate biological alternatives. Plant-derived extracts from medicinal species represent a promising option, yet their efficacy against F. oxysporum remains insufficiently understood. This study evaluated the antifungal activity of four plant extracts: Ceratonia siliqua, Curtisia dentata, Lantana camara and Schkuhria pinnata. Extracts were first assessed at varying concentration in in vitro experiments. Best performing extracts were subsequently applied as seed treatments to inoculated seeds and evaluated using an ISTA paper-roll germination test. Afterwards, the two most promising treatments were tested in a six-week pot trial. In vitro, all extracts except C. dentata inhibited mycelial growth of F. oxysporum, with L. camara showing the strongest suppression on 7.5 mg mL-1. However, high concentrations of L. camara (10.0 mg mL-1) and S. pinnata (7.5 mg mL-1) caused phytotoxicity and completely inhibited seed germination. Lower concentrations of L. camara (1.25 and 2.5 mg mL-1) enhanced germination but did not prevent infection, which remained high across all treatments. In the pot trial, no significant treatment effects on disease severity or plant growth were observed among the water control, fungicide control (Celest® XL) and the tested L. camara treatments (1.25 and 2.5 mg mL-1). This lack of differences was likely due to wet growing conditions that promoted infection and reduced treatment efficacy. Overall, although L. camara demonstrated clear antifungal activity in vitro, its suitability as a cowpea seed treatment against F. oxysporum was limited by phytotoxicity and environmental growing conditions. Further optimalisation of extract concentration, application techniques and seed inoculation methods is necessary before plant extracts can be developed into practical alternatives for managing Fusarium wilt in cowpea.