Managing Brown Rot Disease of Citrus Fruit Using Plant Extracts

ABSTRACT Brown rot, caused by Phytophthora nicotianae and P. citrophthora , is a major disease of citrus fruit, leading to significant economic losses globally. Conventional fungicides are commonly used to manage this disease, but concerns have been raised due to environmental persistence, human toxicity and the emergence of resistant strains. This study evaluated the spore germination and mycelial growth inhibition of both Phytophthora species by extracts of 31 plant species in vitro and in vivo. DCM/MeOH extracts were tested against both pathogens using a microtiter plate assay and an amended medium assay. All bioactive extracts were further fractionated into seven fractions using solid phase extraction (SPE), which were subsequently tested against both pathogens. Phytochemical profiling was determined by ultra‐performance liquid chromatography quadrupole time‐of‐flight mass spectrometry (UPLC–QTOF‐MS) analysis. Extracts of seven plants ( Artemisia afra , Dombeya rotundifolia , Eucomis regia , Olea europaea , Cannabis sativa , Peltophorum africanum and Mentha longifolia ) inhibited both P. nicotianae and P. citrophthora with a MIC < 1 mg/mL and MFC < 2.5 mg/mL as compared to azoxystrobin (6.0 mg/mL). In the lemon fruit in vivo assays, A. afra reduced brown rot severity by > 50% compared to untreated controls, with superior performance to azoxystrobin, the standard fungicide. UPLC–QTOF‐MS profiling of bio‐active fractions identified scopoletin ( A. afra ), oleuropeinic acid ( O. europaea ) and bergenin ( P. africanum ) as putative contributors to the observed activity. These findings highlight A. afra as a promising natural product and suggest that other extracts warrant further evaluation for use as alternative oomyceticides in citrus post‐harvest protection.