Powdery Mildew in Fruit; Warning Signs + Strategies for Treatment/Processing

9 Feb

Powdery Mildew - Oidium - Uncinula Necator might be an issue that arises during harvest this year. So what are the warning signs of infected fruit and how can it be treated?

What to Expect in Powdery Mildew Infected Fruit:

When processing fruit infected by powdery mildew, must aroma and chemical composition may change, causing the problems below in juice/wine:
Loss of yield and quality of grapes causing downgrading
Increased fixed and volatile acidity
Increased alcohol, phenolics and pH
Reduction in sugar and nitrogen
Increased fermentation duration
Reduced colour causing browning
Increased B-glucans causing filtration and clarity problems
Increased chance of botrytis
Bitter palate as well as other off tastes
Reduction in varietal characters

Strategies for Processing & Treating Infected Fruit:

Use a higher rate of sulphur dioxide (SO2) at harvest (in the range of 60-100 mg/L depending on severity). Contact us for guidance on this
Minimize skin contact and extraction, avoid cold soak
Assess for Bitter palate as well as other off tastes during processing
If moldy characters are observed, fine with bentonite and or carbon.
Cold settle or float to achieve lees separation.
Use additional enzyme to aid settling.
Rack and process separately the lees (higher solids) fraction.
Inoculate yeast – no wild ferments
Check yeast assimilable nitrogen (YAN) levels
Monitor VA, pH, TA, RS and alcohol