Grape Powdery Mildew: Symptoms, Causes and Management

By , On , In Best Practices

One of the most common and a widespread disease on grapevines in the interior B.C is grape powdery mildew. Grape powdery mildew is often one of the most common reasons why wine grape growers will apply fungicides to their crops.  Luckily, this disease can be almost entirely controlled through preventative applications in the growing season.

Early powdery mildew is destructive as it can hamper vine growth and fruit quality, reduce berry size and sugar content and in severe cases, the fruit may be unusable. It should be monitored and prevented as early in the season as possible, to avoid worst case scenarios.

Symptoms:

Powdery mildew symptoms are often seen on foliage, fruit, flower parts and canes, but can infect all green plant tissues in the vine. The first symptoms are often evident on the underside of the leaf surface and can be seen as a white or green-white powdery patch but white and grey-white patches of fungi are often evident on the upper surface of the leaf too. These patches will usually enlarge, until an entire coat of white/grey powder can be seen on the upper leaf surface. In severely diseased leaves, mottling or distortion may be caused as well as curling and withering of the leaves. Monitor lateral shoots, as they are often very vulnerable. Infections on young shoots are often more difficult to see, often appearing as dark brown or black patches and remaining this way.

The first three to four weeks after bloom is usually the time in which the berries are the most vulnerable to infection. Shoots, petioles and other cluster parts are usually vulnerable all season, so these should be monitored and checked often. Cluster stem infections often go unnoticed, but can be incredibly damaging to the plant, where they often wither and dry up, resulting in berry drop. Berries that are infected are often misshapen and have either patches of fungal growth or an entire white powdery growth coverage.

Managing the Disease in your Grapes:

Luckily, there are a few steps that can be taken to manage the disease. Some of these will be discussed below:

  • Protect grape foliage from the primary infection period by applying fungicides from early shoot until after bloom
  • Use proper water volume to ensure optimal coverage of the growing vines. You might need to use higher water volumes when applying a contact fungicide such as sulphur or oil
  • Gaining a good control early on in the season is essential, to minimize the effects of the disease and prevent a difficult to control epidemic later in the season
  • Start your fungicide program early: target the first application when new growth is 5-10cm and then continue on a 7-14 day schedule, depending on environmental conditions. End when the berries begin to soften, around the beginning of the veraison stage

Resistance Management:

Recently, growers have had certain struggles with disease control, as their trusted fungicide products that they often use are no longer effective. This happens despite any efforts made to control the disease early on. Resistance to sterol-inhibiting and strobilurin fungicides has been seen in areas of Eastern North America and as a result of this. You should follow proper fungicide resistance management practices to limit the resistance build-up in your vineyard.

Preventative steps to reduce fungicide resistance build-up include:

  • Use the recommended dose rates only and alternate between the different fungicide groups
  • Ensure that the spray that you are using is properly calibrated to ensure thorough coverage. When a product has a high risk of resistance, limit its use to 2 per season, per chemical group
  • If resistance is suspected, discontinue the use of the product and contact Growers Supply Co. for help
  • Use non-chemical control methods, such as increasing air drainage and light penetration, to balance resistance management

Remember to start your program early and apply sprays based on the spray intervals for each product. Do not spray after an event has occurred, rather spray before any rain or high humidity events.  This will reduce the risk of infection by having unprotected plant material. Always remember that Growers Supply Co. is here to help, regarding information and any products you may need.

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