Feasibility of discriminating powdery mildew-affected grape berries at harvest using mid-infrared attenuated total reflection spectroscopy and fatty acid profiling

Background and Aims: Powdery mildew (Erysiphe necator) reduces the quality of winegrapes, and objective methods for assessment are required. Mid-infrared spectroscopy and fatty acid analysis were investigated for rapid diagnosis of affected berries. Methods and Results: Colonisation by E. necator reduced berry diameter and mass and increased the surface area : volume ratio. Mid-infrared spectra (1800-1185 cm-1) contained information on E. necator and compounds related to infection, but spectral similarity of visually healthy and partly infected berries confounded differentiation between these groups. Fatty acids in E. necator and berries were identified and quantified by gas chromatography. Six saturated even-chain fatty acids were prevalent in E. necator, arachidic acid being most abundant. Following stepwise linear discriminant analysis, four saturated fatty acids distinguished 97% of healthy berries and assigned approximately 75% of partly and fully infected berries to their original groups. Arachidic acid concentration, which changed amongst healthy, partly and fully infected berries (P = 0.001), correctly classified 90% of healthy berries. Conclusions: Analysis of fatty acids allowed discrimination of healthy and infected berries whereas mid-infrared spectroscopy proved less informative. Arachidic acid concentration increased with disease severity. Significance of the Study: Fatty acid analysis offers a new approach for objective measurement of powdery mildew. Arachidic acid is proposed as a biomarker for powdery mildew on grapes.