Effective canopy management – coupled with an effective spray program – is the best weapon against downy and powdery mildew, delegates at the recent Australian Society of Viticulture and Oenology (ASVO) seminar heard.
‘Effective canopy management has a number of benefits. It allows light penetration, improves air movement to allow leaves to dry, and reduces humidity. Most importantly, it also allows spray to penetrate’, Barbara Hall, a Senior Research Scientist with the South Australian Research and Development Institute (SARDI) told delegates.
Mrs Hall said both downy mildew and powdery mildew could be managed by an effective protectant spray program.
‘This is particularly critical with powdery mildew as there are no effective eradicants to manage the disease if it “takes off".'
She said applications for powdery mildew should start early – 2 weeks after bud burst in warmer areas and when shoots were 10–20cm long in cooler areas.
‘The critical time for protection for both diseases is around flowering.’
Mrs Hall said there were several effective fungicides that could be applied post infection for downy mildew, but it was critical that they were applied before any symptoms appeared. She said even metalaxyl was not completely effective in managing established infections and using any fungicide on established infections would increase the likelihood of resistance developing.
‘Weather is a key driver for downy mildew. It requires quite specific weather conditions for a primary infection. The 10:10:24 guide (10 mm rain at 10oC over 24 hours) still holds for many areas, however a primary infection can occur at lower temperatures especially in cooler regions. Secondary infection needs high humidity/rain overnight with about 13oC. The temperature is a key factor in the time between infection and symptom expression. At 10oC it may take about 3 weeks for symptoms to appear. At 20–24oC it will be 5 days’, Mrs Hall explained .
Mrs Hall said powdery mildew, on the other hand, didn’t require such specific weather conditions.
‘It can grow happily anywhere from 8–32oC, likes humidity over 40 per cent and likes low light. However, like downy mildew, the temperature will affect how quickly the disease cycles. Between 16–30oC it will only take 5–7 days for the fungus to sporulate and cause new infections. The disease starts before or around flowering, earlier in warmer areas, and can rapidly “take-off” if not managed.’
Mrs Hall said the key message was that while weather conditions could contribute to increased disease risk, the main drivers of epidemics were humans – and how they reacted to those changes in conditions.
‘The key for growers is to improve canopy management. Couple this with an effective spray program where applications are timed correctly and you have the best chance of managing risk.’
Mrs Hall has worked in management of powdery mildew for more than 35 years, and currently leads a national collaborative research project investigating fungicide resistance in powdery mildew, downy mildew and botrytis. The project is funded by Wine Australia, in collaboration with SARDI, Curtin University, and The Australian Wine Research Institute.