In our last blog we went into the details with the primary infection of Downy mildew and in this one we will focus on secondary infection.

Secondary Infection

involves the leaf to leaf, and leaf to bunch movement of inoculum. This requires oilspots in the foliage and a warm humid night. Sporulation (the production of sporangia on the undersides of oilspots) occurs if relative humidity (RH) is ≥ 98% at temperatures ≥ 13ºc during at least 4 hrs of darkness.

The sporangia will germinate and release zoospores which are spread to other leaves and bunches in wind and rain. If they land in a drop of water, they will cause infection if the leaves remain wet for 45ºc-hrs, as is required at the same stage for primary infection. Many 1000’s of sporangia are produced from each oilspot. As a result, one oilspot can trigger a huge multiplication of disease overnight – an initial 20-50 primary oilspots can produce 100,000 new oilspots after the second incubation period.

downy mildew2Photo (agric.wa.gov.au): Symptoms of Downy Mildew infection 

Spray timing

Because warm wet weather drives downy mildew, for optimum control, apply sprays in relation to primary and secondary infection events. Where possible, access weather forecasts and predicted times of high disease risk and spray before the disease develops. Monitor and interpret the weather that followed to determine if infection occurred and adjust spray type and timing accordingly. If available, seek assistance from regional disease alert services.

You may have access to an automatic weather station (aWS such as our eVineyard hardware for precise and reliable forecasting) that monitors temperature, rainfall, relative humidity, leaf wetness at 15 minute intervals. While eVineyard software provide advice of disease risk for your vineyard based on weather data and your vineyard activities. It also provide e-mail alerts to have helped you minimize the number of sprays needed for effective control of downy mildew, especially in dry seasons.

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Sources:
A bioeconomic model of downy mildew damage on grapevine for evaluation of control strategies; Pascal Leroy, 2012
Evaluation of a Warning System for Controlling Primary Infections of Grapevine Downy Mildew; Rossi, 2010