Ministry of Agriculture
Powdery Mildew (Uncinula necator)
- Life Cycle
- Disease Management
Powdery mildew, also known as oidium, is caused by the fungus Uncinula 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. Popular wine grape varieties vary in susceptibility to powdery mildew (Table 1).
|'Pinot noir' vines severely affected with powdery mildew. Photo courtesy P. Sholberg, Agriculture & Agri-Food Canada.||Severe powdery mildew on Chancellor grape cluster. Photo courtesy P. Sholberg, Agriculture & Agri-Food Canada.|
|Powdery mildew on Reisling grape. Photo courtesy P. Sholberg, Agriculture & Agri-Food Canada.||Early powdery mildew symptoms on Chancellor grapes. Photo courtesy P. Sholberg, Agriculture & Agri-Food Canada.|
TABLE 1. Level of resistance of grape cultivars to powdery mildew
Pearl of Csaba
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, left-over 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 to 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 (Table 2). 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 to 6 days under optimal temperatures. Shaded and sheltered locations favour mildew development. High temperatures and sunlight are inhibitory to 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.
|Powdery mildew cleistothecia on grape shoot. Photo courtesy P. Sholberg, Agriculture & Agri-Food Canada.||Magnified view of a cleistothecium and ascus containing ascospores. Photo courtesy P. Sholberg, Agriculture & Agri-Food Canada.|
|Severe powdery mildew infection on 'Chancellor' grape leaf, with developing cleistothecia. Photo courtesy P. Sholberg, Agriculture & Agri-Food Canada.||Spore (condia) production of powdery mildew on a grape leaf. Photo courtesy P. Sholberg, Agriculture & Agri-Food Canada.|
|Powdery mildew infection on 'Chancellor' grape foliage. Note whitish mildew and purple discolouration. Photo courtesy P. Sholberg, Agriculture & Agri-Food Canada.||Powdery mildew infection on 'Pinot Noir' grape cane. Photo courtesy P. Sholberg, Agriculture & Agri-Food Canada.|
TABLE 2. Effect of temperature on the development of grape powdery mildew
Temperature of leaf*
|Days for spores to develop and infect vine parts and produce new spores|
|33 (for at least 3 days)||0 (but 10% can recover in 5 days)|
|40.5 (for at least 6 hrs)||0 (kills the fungus)|
- Manage canopies to increase air drainage and light penetration by removing lateral shoots in dense canopies. If necessary remove leaves in the fruiting zone. Dense canopies provide low light intensity, which favours powdery mildew development;
- Use an under-vine irrigation system (drip or micro-jet);
- Manage irrigation carefully. Excessive irrigation leads to excessive vigour and higher disease potential.
- Select varieties that are less susceptible to mildew (Table 1);
Start mildew programs before the overwintering fungus can infect new growth. The first few treatments are the most important, and should be applied starting at budbreak or early shoot growth. Protective fungicide treatments prevent infection of grape tissue by fungal spores. Good coverage is important.
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 and 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.
For more information on powdery mildew forecasting models, refer to the University of California website at: http://www.ipm.ucdavis.edu/DISEASE/DATABASE/grapepowderymildew.html. 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 (Table 2).
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.
Apply fungicides such as Kumulus (sulphur), Nova, Lance, Pristine, Sovran, Flint, Milstop or Serenade at the following growth stages (also see “Fungicide Notes” below for more information).
- When new growth is 5 to 10 cm long
- Just before or immediately after bloom.
- Every 10 to 14 days until grapes begin to soften and red varieties begin development of color and white varieties change from green to white or yellow. If Kumulus (sulphur) is used, shorten the spray interval to 7-10 days.
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.
Dormant spray: Lime Sulphur is effective at suppressing the overwintering population of powdery mildew. It should be applied in early spring before bud break to dormant vines to kill powdery mildew cleistothecia (initial inoculum). Good spray coverage of dormant vines is important.
Post-harvest powdery mildew spray: 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.
|Fungicide||Chemical Group1||Rate/ha||Rate/acre||PHI2 (days)||Notes|
|Vivando (metrafenone)||U8||750 g||300 g||14||Apply at 14-21 day intervals; use shorter interval for high disease pressure or rapid growth phases. Do not apply more than 2 sequential sprays.|
|Quintec(quinoxyfen)||13||300 ml/ha||122 ml/acre||14||Excellent mildew fungicide. Apply on a 14 day interval. Do not exceed 5 applications/ season. Alternate with other fungicides. See Fungicide notes.|
(trifloxystrobin 50% WG)
|11||105-140 g/ha||43-57 g/acre||14||Excellent mildew fungicide. Apply preventively using a 14-21 day interval. Do not use Flint or other group 11 fungicides more than 2 times per season. Alternate with fungicides from other groups. See Fungicide notes.|
(kresoxim-methyl 50% WG)
|11||240-300 g/ha||100-122 g/acre||14||Excellent mildew fungicide. Apply at 14-21-day intervals. Do not use Sovran or other group 11 fungicides more than 2 times per season. Alternate with fungicides from other groups. See Fungicide notes.|
|Pristine (pyraclostrobin + boscalid)||11 + 7||420 – 735 g/ha||170-300 g/acre||14||Excellent mildew fungicide; also provides suppression of botrytis. See label for details on rates and spray intervals. Do not use Pristine or other group 11 fungicides more than 2 times per season.|
|3||200 g/ha||81 g/acre||14||Excellent mildew fungicide. Apply at 21-day intervals. Do not use more than 2 times per season. Alternate with fungicides from other groups.|
|M||4.2 kg/ha||1.7 kg/acre||21/1*||Good mildew fungicide. Apply at 10-day intervals.|
|wettable sulphur (sulphur 92%)||M||2.25 kg/ha pre-bloom
4.5-6.0 kg/ha post-bloom
|910 g/acre pre-bloom
1.8-2.4 kg/acre post-bloom
|21/1*||Use the higher rate when vines are in full leaf. Apply at 10-day intervals. Re-apply after rain.|
|Lance (boscalid 70% WDG)||7||315 g/ha||128 g/acre||14||May also provide some suppression of botrytis bunch rot. Alternate with fungicides from other groups.|
(potassium bicarbonate 85%)
|NC||2.8-5.6 kg/ha||1.1-2.3 kg/acre||0||Apply at 7-14 day intervals.|
|NC||3.0-6.0 kg/ha||1.2-2.4 kg/acre||0||Biofungicide. Disease suppression only. Do not tank mix with other products or fertilizers.|
(calcium polysulphide 22%)
|M||100 L lime sulphur in 1000 L of water.||120 (apply dormant)||Apply 500L of spray mixture per hectare once per season during dormant stage prior to bud swell (early March to early April). Spray to point of runoff, cover completely.|
1Chemical Group: Products with the
same number belong to the same class of compounds.
Alternate products with different chemical groups to
help delay or prevent the development of resistance.
2PHI = Pre-harvest Interval, or minimum number of days between last spray and harvest.
*Sulphur can be used on table grapes up to the day of harvest (1 day PHI), but the pre-harvest interval for wine grapes is 21 days. Excessive amounts of sulphur are detrimental to winery yeasts. It is suggested that the last application to wine grapes be made not later than 30 days before harvest. Check with the winery before application of any fungicides within the last month.
Grape powdery mildew has developed resistance or reduced sensitivity to sterol-inhibiting fungicides (Nova) and to strobilurin fungicides (Flint, Sovran, Pristine) in other areas such as Eastern North America. Avoid over-using these products to prolong effectiveness in this area.
To help prevent resistance from developing:
- Alternate between different fungicide groups. Do not use more than 2 back-to-back sprays of fungicides with the same group number (see table 3 for group numbers).
- Limit the number of sprays of products with a high risk of resistance (Nova, Sovran, Flint, Pristine) to 2 per season per chemical group. Mildew fungicides with a low risk of resistance include Kumulus, sulphur, lime sulphur, Milstop and Serenade. Lance has a moderate risk of resistance.
- Use only recommended dose rates.
- Ensure sprayer is properly calibrated to deliver accurate and thorough coverage.
- Integrate with non-chemical control methods.
- Discontinue use of a product if resistance is suspected and consult your crop advisor.
Sulphur is currently the most common fungicide used for powdery mildew control in the B.C. interior on both conventional and organic vineyards. Kumulus DF is a dry flowable formulation of sulphur. The addition of 730 mL/ha of a wetting agent will help to improve the distribution of wettable sulphur over the plant surfaces. However, the use of these agents may cause more Botrytis infection. Too much wetting agent will cause excessive foaming in the sprayer. Fruit and leaf “burning” may occur if sulphur is applied during slow drying conditions or when temperatures are above 27°C. Do not apply sulphur to Concord, Sheridan or Foch.
Quintec was registered June 2010 for control of powdery mildew in grape. It is a protectant fungicide, best used before visible mildew is present. Use the higher rate during high mildew pressure conditions. Rotate with fungicides from other chemical groups for resistance management. Quintec does not provide control of botrytis bunch rot.
Vivando was registered in September 2010 for control of powdery mildew in grape. Rotate with fungicides from other chemical groups for resistance management, and do not use more than 2 times sequentially. Not effective on botrytis bunch rot.
Sovran and Flint are strobilurin fungicides which provide good control of powdery mildews. These products are in the same class of compounds (group 11) and are at high risk of resistance. Use the higher rate under heavy disease pressure. Use 14-day intervals under moderate to heavy pressure, and 21-day intervals under low disease pressure. To help prevent the development of resistance, limit the use of group 11 fungicides to a maximum of 2 times per season, and never make more than 2 applications in a row. Alternate with at least 2 applications of fungicides in different chemical groups (see table 3). Caution: Sovran drift may cause severe injury to certain cherry and Asian pear varieties.
Pristine contains 2 fungicides; one is in the same class as Flint and Sovran and at high risk of resistance. Begin applications prior to the onset of disease. Use a 10-14 day interval if using the low rate, or a 21 day interval if using the high rate. To help prevent the development of resistance, limit the use of Pristine (and other group 11 fungicides including Sovran and Flint) to a maximum of 2 times per season. Alternate with fungicides from different groups (see table 3). Pristine also suppresses botrytis bunch rot.
Nova is a strong fungicide for control of powdery mildew, however powdery mildew has developed a reduced sensitivity to Nova in some areas of the world. To prevent the development of resistance, alternate Nova with other fungicides and do not use more than 2 times per season. Nova is locally systemic. Once absorbed by the plant, it can not be washed off by rain. Do not apply Nova with copper fungicides such as fixed copper, copper oxychloride, copper sulphate or other copper containing products because this combination reduces the effectiveness of Nova.
Lance may provide some suppression of bunch rot, in addition to control of powdery mildew. Apply on a 10-14 day schedule. Use the shorter interval when disease pressure is high. Rotate with other fungicides for resistance management.
Milstop is a contact fungicide that controls powdery mildew. It does not provide control of botrytis. It is considered a weaker product than sulphur, but may provide a good alternative for use on sulphur-sensitive cultivars. Apply using a sufficient volume of water to insure complete coverage of all stems and foliage. Use the high rate and short application interval (7 days) when conditions favour the development of powdery mildew. OMRI approved for organic production.
Serenade Max is a biofungicide derived from the beneficial bacterium Bacillus subtilis. It is registered for "suppression" of powdery mildew, and will also provide some suppression of bunch rot and sour rot. OMRI approved for organic production.
Bordeaux Mixture is a broad-spectrum fungicide and bactericide that is not easily washed off by rain. Experience in Ontario indicates post harvest Bordeaux sprays using copper sulphate as a copper source to control late powdery mildew infections also appear to be an aid to increased bud hardiness and improved wood maturity in older vines. Sprays of fixed copper to young vines one month before the first fall frost appear to help “induce” some level of bud hardiness and wood maturity.
Copper sulphate (Bordeaux) is registered for several grape diseases including downy mildew, black rot and dead arm. It is not a strong powdery mildew fungicide, but may be useful for post-harvest control of light mildew problems, or as part of a mildew program on sulphur-sensitive varieties. Do not mix copper with other fungicides or insecticides. Copper applied under cool, slow drying conditions may cause injury, even to so called tolerant varieties. Copper oxychloride is less injurious to plant tissue than copper sulphate. Addition of the lime helps to minimize, but may not prevent injury.
The label rate for copper sulphate on grape is 3 kg copper sulphate + 6 kg hydrated lime in 1,000 L water/ha. Also used is copper oxychloride (50% copper) at 3 kg + 6 kg hydrated lime in 1,000 litres of water.
Making Tank-Mix Bordeaux
- Start water flowing into spray tank.
- When tank is about one-third full and the mechanical agitator is in operation, start washing the powdered copper sulphate into the tank through a screen with water from the supply hose. A wooden spoon is often helpful in working powdered copper sulphate through the screen. Pre-soaking the copper sulphate in a plastic bucket of hot water will speed up the process.
- By the time the tank is two-thirds full, all of the powdered copper sulphate should be in the tank. Then wash the lime (hydrated or builders’) through the screen, using the water supply hose, into the copper sulphate tank. The lime should be as dilute as possible before it meets the copper sulphate solution in the tank, so use lots of water to wash lime through the screen. Pre-soaking the lime before adding to the tank may be preferred to washing powdered lime directly through the screen into the tank.
- Keep the agitator running continuously and apply the Bordeaux Mixture immediately.
Updated October 2010