Potential use of hydroxytyrosol-rich extract from olive mill wastewater as a biological fungicide against <Emphasis Type="Italic">Botrytis cinerea</Emphasis> in tomato

Olive mill wastewater (OMW) is a rich source of antifungal agents and has a merit as alternative to synthetic fungicides. To develop disease control agents against Botrytis cinerea, a hydroxytyrosol-rich (29.27% weight:dry weight) olive mill wastewater (HROMW) and a hydroxytyrosol-rich (52.67% weight:dry weight) extract (HRE) were prepared from fresh OMW using hydrolysis and post-hydrolysis purification processes. The HROMW and HRE showed strong fungicidal activity against B. cinerea with minimal fungicidal concentrations (MFCs) of 14.36–28.72 and 28.72–57.44 mg l⁻¹ (dry weight), respectively. According to EN 1275: 1997 standard method, HROMW and HRE at 1.25% (dry weight:volume) showed powerful disinfectant properties resulting in a reduction in the viability of B. cinerea spores by >4 log units after 30 min of contact time. The HROMW or HRE foliar spray treatments, of the B. cinerea inoculated tomato plants, resulted in disease control percentage of 95 and 91%, respectively, whereas 70% was conferred by benomyl treatment. Although, HRE possessed a similar disease severity reduction than HROMW, it showed great advantage by fortifying plants and by improving their growth and healthy aspect. Indeed, HRE-treated plants showed significantly better: (i) plant height (146.2 and 130.5%), (ii) average stem diameter (135% and 110%), (iii) leaf number (163.6 and 128.5%), and (iv) leaf colour (chlorophyll a content) (144.4 and 103%) than benomyl-treated and non-treated plants, respectively. This study revealed that HRE offers a natural and effective extract to control grey mould.