Powdery mildew, caused by Erysiphe necator, affects the sensory properties of wines and objective measures are needed for assessing grape quality. The aim of this research was to investigate mid-infrared (MIR) spectroscopy, and fatty acid and powdery mildew-specific quantitative PCR (qPCR) analyses as methods for distinguishing healthy and disease-affected berries.
Intact, fully-developed berries (n=138) were detached from 35 representative Chardonnay bunches collected at harvest maturity in the Waite Campus research vineyard (University of Adelaide). Single berries were assessed using a stereomicroscope, designated visually healthy, partly- or fully-infected, deseeded, homogenised and scanned in the MIR range. A subsample of 61 berries was selected for subsequent fatty acid analysis using gas chromatography and thin layer chromatography, and subsamples of 54 of these berries were subjected to qPCR analysis. Colonisation of berries by E. necator altered their chemical composition, mainly in the spectral region 1800-1185 cm-1. However, analysis of fingerprint-like spectra failed to distinguish partly-infected berries from the two remaining categories. Saturated fatty acids were prevalent in E. necator, arachidic acid (C20:0) being the most abundant. Although total saturated and total unsaturated fatty acids (C14-C24) were similar in healthy and infected berries, the fatty acid profile in individual berries was altered by infection. Arachidic acid content changed in response to powdery mildew severity. A duplex TaqMan qPCR with limit of detection of 3 fg of pure E. necator DNA from conidia, detected 6 fg - 0.1 ng of pure E. necator DNA per 0.5 ng of total DNA in the 53 berries. Increasing E. necator biomass on berries (based on qPCR) was strongly related to C20:0 content (R2=0.76), suggesting that arachidic acid has potential as a biomarker for E. necator and powdery mildew severity.