Grape Powdery Mildew (Uncinula necator)
AKA (Erisiphe necator)
If you grow grapes in coastal California, (or almost anywhere in the world for that matter), you are probably going to see a white powdery substance on the leaves. This is grape powdery mildew. While this is common, it is a disease that will damage your grape plants and your crop if left untreated.
What is powdery mildew?
Powdery mildew is a fungal disease that affects a wide range of plants. The specific fungus that causes powdery mildew on grapes is Uncinula necator.
How do you get grape powdery mildew?
- The fungus that causes grape powdery mildew is all around us and it is therefore close to impossible to avoid. It is spread by the wind.
- The fungus is thought to have originated in North America. Therefore, North American grapes that co-evolved with it are more resistant than European grapes.
- A small amount of the fungus stays dormant on grapes in the winter, and when conditions are right it spreads rapidly over the plant.
- Like any fungus, it does best in cool moist environments. This is why the disease is particularly problematic in coastal areas. A wet growing season and/or excessive marine layer will exacerbate the disease.
- Ideal temperatures for the growth of grape powdery mildew is between 70° and 85°F. Welcome to California.
- High temperatures and sunlight are inhibitory to powdery mildew.
Where can you see powdery mildew first?
- Because the disease is a fungus, you will first find the disease on the undersides of leaves. This is because the undersides of leaves are protected from the sun. The fungus is also more common in the deeper parts of the plant where there is poor air circulation.
What will powdery mildew do?
- Grape powdery mildew will suck the life out of your plant, damage leaves and spoil your grapes.
- The disease can and will cover the entire plant if not controlled.
- It can cause leaf curling, withering, red blotchy areas and distortion of leaves.
How do you treat powdery mildew?
- Because the disease lives on the surface of the plant, topical spray treatments are very effective.
- I recommend that you get (and label) a separate garden sprayer for each of your different garden needs. That way you don’t have to worry about spraying residue of something like an herbicide on a plant that you would actually like to keep around. It is also nice to have one with an angled spray nozzle so you can easily get the bottoms of the leaves.
- The first few treatments of the season are the most important, and should be applied at bud break and during early growth.
- There are many different anti-fungal sprays that can be used to fight powdery mildew.
- A few of the organically acceptable methods include sulfur, Serenade, Sonata, and Stylet oil. There are many other good options out there, I happen to use a copper based spray.
- Many professional wine growers rotate through different types of fungal sprays to prevent the fungus from developing a resistance to one specific treatment.
- Most spray treatments for grape powdery mildew are on a near weekly basis depending on your micro-climate. Complete spray coverage is important to get the hard to reach parts of the plant that are also the most susceptible to fungal disease.
- A Powdery Mildew Index (PMI) was created to determine how frequently you need to spray grapes for powdery mildew. The University of California, Davis has the one most widely available models, which requires data entry for leaf wetness and temperature. However, for the home grower, it is a lot easier to just spray on a regular interval as suggested on the fungicide label.
- I found that the most important element for success is to spray the buds in the dormant season with a mix of horticultural oil and anti-fungal spray. This is process is kind of like treating for peach leaf curl. For example, if you wait till the leaves are already out and infected then it is an uphill battle all season long.
- That being said, you can also spray all season long for powdery mildew and it will help while in the case of peach leaf curl, spraying after bud break doesn’t reverse the damage.
- Specifically: I am using a mix of “Bonide All Seasons oil” and Bonide Copper Fungicide. I just use the recommended concentrations of both in the same spray bottle.
More on horticultural oil:
- Overall, horticultural oil is an awesome alternative to systemic pesticides and it kills all kinds of sap sucking pests.
- If you are really interested in learning more about the fine details of oil insecticides, the “Using Oils as Pesticides” article is a rather complete outline from “agrilife extension” of Texas A&M University.
Wine Industry Experience:
- My good friend Stasi Seay has extensive experience in the wine growing business as a viticulturalist. She has recently contributed some farming perspective to the issue of grape powdery mildew. Please read her 6/18/13 insight in the comment section below.
Prevention of powdery mildew?
- While you can’t change the weather, there are some other things that you can do to battle this disease.
- Planting grape rows in a North to South orientation will result in a more even sun exposure on the plant. This will reduce the areas where fungus will thrive.
- Pruning out overlapping and clumping branches will help with aeration and sun exposure.
- Use drip irrigation and keep your plants clear of sprinkler irrigation.
- Plant grapes away from humid micro-climates such as close to a pool or hot tub.