Facts about powdery mildew
- It is a vineyard fungal disease that can cause crop losses and affect wine quality negatively.
- Powdery mildew affected grapes can contain higher microbial populations.
- Fermentation duration can be increased.
- Wines produced can have earthy, mushroom, cooked tomato and wet fur characteristics.
- Wines can also have undesirable textural changes and increased phenolics and bitterness.
- In addition wines can have increased titratable acidity and protein instability.
Practical advice on how to handle grapes and musts affected by powdery mildew:
- Ideally, affected vineyards should be hand harvested in order to select for healthier bunches.
- In case of mechanical harvesting the worst affected bunches must be removed by hand first before harvesting commences.
- Add a high dosage of SO2 at crushing (60 – 100 mg/ml) depending on how severe the grapes have been affected.
- In the case of white juice, minimise skin contact and extraction by using shorter pressing cycles.
- Assess each press fraction for negative attributes and if present, discard or conduct fining trials (bentonite, casein or PVPP) depending on the severity of these attributes. Suppliers can advise.
- Use additional enzymes for adequate cold settling or flotation.
- Measure the YAN since powdery mildew infection can lower it and adjust if necessary during fermentation.
- Inoculate with a commercial yeast to reduce the risk of undesirable microorganisms.
- In the case of red musts avoid cold soaking and press early.
- Avoid post fermentation maceration and ageing on lees.
- Expect a higher bentonite requirement for white wines. Red wines might require a proteinaceous fining to achieve better balance.