Efficacy of Oregano Essential Oil as a Sustainable Biocontrol Agent Against Major Phytopathogens of Tomato and Onion in Karbala, Iraq: From In Vitro to In Vivo Assessment

Abstract

The intensive use of synthetic fungicides in Iraq, particularly in arid agricultural regions such as Karbala Governorate, has raised concerns regarding pathogen resistance and environmental contamination, highlighting the need for sustainable disease management strategies. This study evaluated the antifungal potential of locally sourced Oregano Essential Oil (OEO) against Alternaria porri, the causal agent of onion purple blotch, and Fusarium oxysporum f. sp. lycopersici, responsible for tomato fusarium wilt, in the Al-Rujibah district of Karbala. The chemical composition of OEO was characterized using Gas Chromatography–Mass Spectrometry (GC–MS), while its antifungal efficacy was assessed through in vitro mycelial growth and spore germination assays. Phytotoxicity tests were conducted to determine safe application levels for tomato seeds and onion bulbs, followed by greenhouse experiments to evaluate disease control performance under in vivo conditions. GC–MS analysis revealed that the oil was predominantly composed of carvacrol (57.7%) and thymol (24.5%). The oil exhibited strong antifungal activity, with minimum inhibitory concentrations of 1.0 µL/mL against A. porri and 1.5 µL/mL against F. oxysporum. Concentrations up to 1.0 µL/mL showed no significant phytotoxic effects and were therefore established as the maximum safe threshold for crop application. Greenhouse trials demonstrated that OEO significantly reduced disease incidence and severity, surpassing the performance of carbendazim in tomato wilt management and providing comparable control of onion purple blotch while enhancing plant growth and yield parameters. These findings demonstrate that OEO represents a promising, environmentally friendly biocontrol agent with potential application in sustainable horticultural production systems and integrated disease management programs in Iraq and other arid regions of the Middle East.