Question for James.
How do you assess whether disease pressure is high or low? Why do they call powdery mildew the creeping disease?
How do we assess disease pressure?
History – Powdery survives from year to year in your vineyard. Good control last season helps reduce risk for this season coming.
Susceptibility - Verdelho and Chardonnay are the most susceptible varieties to Powdery – while Shiraz is only moderately susceptible. The variety Merlot seems to have low natural susceptibility.
Vigour - High vigour blocks are more at risk than low vigour vineyards. In high vigour vineyards it is difficult to get good protective spray coverage and tend to have dense canopies. When the canopy is extremely dense protection is difficult. Dense canopies cause shade which favours Powdery.
During overcast weather conditions, with high humidity (both favouring Powdery Mildews lifecycle) we say pressure is high.
The best time to control this disease is in the early season by limit the spread of any Powdery mildew flagshoots and stopping any bunch infection during flowering. That is why we call powdery mildew the creeping disease because it creeps around as the canopy closes over.
A rough rule of thumb is start control when shoots are 20- 30cm long, or 2-3 weeks past budburst.
McLaren Vale growers have been having good success by treating each vineyard patch based on its disease risk. They have been using high pressure programmes for blocks with a history of Powdery and low pressure programmes on block where control has been good.