Limonene-enhanced botanical fungicides: A sustainable component of integrated tomato powdery mildew management in the tropical region

Tomato powdery mildew, caused by Oidium neolycopersici, significantly reduces yields in humid tropical regions such as Tanzania. Overreliance on synthetic fungicides raises environmental and health concerns, creating a need for safer alternatives. This study evaluated the efficacy of ethanolic extracts from garlic (Allium sativum), custard apple (Annona squamosa), and fish bean (Tephrosia vogelii), applied individually and in combination with the adjuvant limonene, through two consecutive screen house experiments. The first experiment assessed individual extracts at different concentrations, identifying A. sativum at 0.4% as the most effective, reducing disease severity to 39.17%. In the second experiment, this optimal concentration was combined with limonene, which significantly improved efficacy, lowering disease severity to 21.33% – a 45.6% increase in disease suppression. The combined application of all three botanical extracts with limonene provided the highest level of control, reducing disease severity to 14.00%, closely matching the performance of the synthetic fungicide hexaconazole (3.50%). This ternary combination also produced the highest fruit yield among botanical treatments (1579 g per plant), comparable to hexaconazole-treated plants (1694 g per plant). The findings demonstrate that combining botanical extracts with limonene enhances disease control and yield, offering an effective, eco-friendly alternative to synthetic fungicides. Such locally available solutions present a sustainable approach to managing tomato powdery mildew in Tanzania and similar agro-ecological zones.