Comparison of methods for quantification of botrytis bunch rot in white wine grape varieties

Botrytis bunch rot (BBR), caused by the fungus Botrytis cinerea, is one of the most economically damaging diseases in wine grapes in Australia and New Zealand. Although quantification of BBR is important for both grape growers and wine makers, the widely used visual estimation method for assessing BBR is time consuming and can be highly subjective. Various BBR quantification methods were compared with the visual estimation method using naturally infected white wine grape bunches. These methods included: near-infrared spectroscopy (NIR), mid-infrared spectroscopy (MIR), quantitative PCR (qPCR) and image analysis software (RotBot). All quantification methods correlated well with visual estimation and each had advantages and disadvantages. Scanning of samples using NIR and MIR was quick and highly sensitive. Sample preparation for these methods was less labour intensive than the DNA extractions required for qPCR. Running the qPCR reaction also took longer than NIR and MIR scanning. One advantage of qPCR is that it quantifies B. cinerea DNA, which is directly related to fungal mass. It is unclear what NIR or MIR is quantifying and the relationship with BBR is purely empirical. The RotBot method required no sample preparation or expensive equipment and the software was able to rapidly batch process images. However, this method only quantifies visible BBR symptoms. All methods tested were found to be reliable, accurate and objective alternatives to the visual estimation method. The choice of method simply depends on the trade-off between sample preparation time and accuracy.