Powdery mildew, also known as oidium, is caused by the fungus Erysiphe necator. This fungus has a narrow host range, attacking only grape plants and a few related species. It is the most common and widespread disease of grapevines in the Okanagan-Similkameen area.
| Susceptible | Intermediate | Least susceptible |
|---|---|---|
| Bacchus Cabernet Franc Cabernet Sauvignon Chancellor Chardonnay Chasselas Gamay Gewurztraminer Grenache Himrod Madeleine Angevine Madeleine Sylvaner Malbec Muller Thurgau Pearl of Csaba Petit Verdot Rkatzeteli Riesling Sauvignon blanc Schonburger Siegerebe Syrah Viognier |
Chelois Chenin Blanc Concord Foch Pinot blanc Malbec Merlot Ortega Pinot Noir Perlett Sheridan Vidal Blanc Weissburgunder |
Auxerrois Malvoisie Melon Pinot Gris Semillon |
Powdery mildew symptoms can be seen on foliage, fruit, flower parts and canes. Mildew usually appears first as whitish or greenish-white powdery patches on the undersides of basal leaves. It may cause mottling or distortion of severely infected leaves as well as leaf curling and withering. Lateral shoots are very susceptible. Infected blossoms may fail to set fruit.
Berries are most susceptible to infection during the first three to four weeks after bloom, but shoots, petioles and other cluster parts are susceptible all season. Infected berries may develop a netlike pattern of russet, and may crack open and dry up or never ripen at all.
Old infections appear as reddish brown areas on dormant canes. Early powdery mildew infections can cause reduced berry size and reduced sugar content. Scarring and cracking of berries may be so severe as to make fruit unsuitable for any purpose.
Be aware that many winemakers have a very low tolerance for powdery mildew on grapes. Research has shown that infection levels as low as 3% can taint the wine and give off-flavours.
The powdery mildew fungus overwinters as cleistothecia (tiny, round, black fruiting bodies) in bark, on canes, leftover fruit and on leaves on the ground. Spores (ascospores) from the overwintering cleistothecia are released in the spring after a rainfall of at least 2.5 mm. For primary infection to occur, the spores require at least 12-15 hours of continuous wetness at 10-15°C to infect developing plant tissue.
Once primary infection has occurred, the disease switches to its secondary phase.
Secondary colonies (white mildew patches) form in 7-10 days, although the disease is not noticeable early in the season. The white patches of powdery mildew produce millions of spores (conidia), which are spread by wind to cause more infections. Free moisture is not needed for secondary infection — temperature is the most important environmental factor.
The disease spreads quickly in early summer when temperatures are moderate. The incubation time (the time between infection and the production of spores) can be as short as 5-6 days under optimal temperatures. Shaded and sheltered locations favour mildew development.
High temperatures and sunlight inhibit powdery mildew. Extended periods of hot weather (> 32°C) will slow the reproductive rate of grape powdery mildew as well as reduce spore germination and infection.
| Temperature of leaf* (°C) | Days for spores to develop, infect vine parts and produce new spores |
|---|---|
| 6 | 32 |
| 9 | 25 |
| 12 | 18 |
| 15 | 11 |
| 17 | 7 |
| 23 | 6 |
| 26 | 5 |
| 30 | 6 |
| 33 (for at least 3 days) | 0 (but 10% can recover in 5 days) |
| 40.5 (for at least 6 hours) | 0 (kills the fungus) |
Start mildew programs before the overwintering fungus can infect new growth. General recommendations are to begin spray coverage between budbreak and early shoot growth. The most important fungicide timings in this area are the blossom spray (generally just before or just after blossom) and the spray immediately following the blossom spray. Protective fungicide treatments prevent infection of grape tissue by fungal spores. Good coverage is important.
The grape powdery mildew pathogen has developed resistance or reduced sensitivity to FRAC Group 3, sterol-inhibiting fungicides (ex. Nova) and to FRAC Group 11, strobilurin fungicides (ex. Flint, Sovran, Pristine) in other areas.
A survey led by Dr. Michelle Moyer at Washington State University and Dr. Walt Mahaffee at the U.S. Department of Agriculture included thousands of samples across Canada and the U.S. In B.C., 419 samples from 2018 and 2019 were analyzed for resistance to group 11 fungicides, and only 18% were sensitive to group 11 fungicides. Learn more about the project on the FRAME Networks website.
To help delay resistance from developing:
A mildew risk model can be used to forecast disease severity of secondary infections. The UC model developed at the University of California, Davis is the one most widely available and is sold with weather instrument software. The UC model requires a data logger for leaf wetness and temperature. Initially, the model predicts primary infection based on hours of leaf wetness and temperature and then switches to the risk phase based only on temperature. The risk indices can be used to help time fungicide applications. When the risk is high, the model recommends that fungicides be applied more often.
In trials in California, the model has reduced the amount of fungicides applied to grapes. In tests in the Okanagan, the primary phase of the model has not been accurate and more research is required to adapt the model to local conditions. However, the risk phase of the model could be valuable for assessing the risk of secondary infection during the growing season.
Daily analysis of the model allows the grower to visualize what the conidial population will be approximately one week later and what the potential disease severity will be two weeks later, allowing them to plan their fungicide program in terms of product and application interval. Several years of data from many different sites around the Okanagan-Similkameen indicate that the risk mode consistently predicts severe powdery mildew and the shortest interval between fungicide sprays in July and August. Typical Okanagan temperatures during the summer months are optimum for powdery mildew. (See the Spread section above.)
For more information on powdery mildew forecasting models, refer to the University of California disease model database. If not using a disease forecasting model, apply powdery mildew control materials as frequently as necessary when severe mildew conditions exist, keeping in mind the leaf temperature and the number of days needed for spores to infect grape parts and produce new spores. Scroll up to see the table displaying effects of temperature on development of powdery mildew.
Protect grape foliage from primary infection by application of fungicides from early shoot growth until after bloom. Good control early in the season to prevent establishment of the disease is the key to preventing a powdery mildew epidemic later in the summer.
Contact your winery at the beginning of the season to determine the acceptable pre-harvest intervals for any pesticides or sulphur products that may be used in the growing season. Some products contribute to the development of off odours and off flavours and may interfere with the fermentation process.
Post-harvest sprays to control powdery mildew are beneficial. Harvest date will determine the need to keep foliage and canes protected. Severe powdery mildew conditions are generally a result of poor control of this organism during the growing season. Additional sprays for powdery mildew under such conditions after harvest will not protect canes.
For more information about chemical control for powdery mildew, see the Pesticides page.
