Cause:
Erysiphe necator (formerly Uncinula necator), a fungal disease common to all areas of the PNW. The disease tends to be more severe on the westside of the Cascades but is a chronic problem in arid districts where over-the-canopy irrigation is used for early-season frost protection or watering. Vitis vinifera (European) cultivars commonly are susceptible to powdery mildew. Other hosts include Boston ivy, Virginia creeper, and Ampelopsis (porcelain berry). The fungus may overwinter as a group of thin threads called hyphae, inside the vine’s dormant buds and/or as small black bodies (chasmothecia) on the exfoliating bark of the vine.
Buds on new shoots can be infected 4 to 6 weeks after shoots start growing but not after bud scales become suberized. These new infected buds remain quiescent until the next growing season. The fungus infects developing buds during the growing season. Shortly after budbreak, the fungus becomes active and covers the emergent shoot with a large white mass of threads or mycelium (“flag shoots”). Flag shoots have rarely been observed in western Oregon or eastern Washington.
Chasmothecia on exfoliating bark release sexual spores during rainy weather above 50oF from budbreak through bloom. This weather also favors infection that results in individual powdery spots, called colonies, on the surface of leaves growing close to the bark.
Many asexual spores (conidia) are produced on the surface of powdery mildew colonies. Under optimal conditions of mild temperatures and high humidity, a single spore can germinate, infect the plant, and produce a new colony and a new crop of spores in 3 days. Temperatures over 85oF inhibit conidia germination. Free water from rain and/or irrigation can wash conidia off of a colony, burst conidia, or result in poor or abnormal germination of the conidia.
Grape berries are highly susceptible from the time calyptras (hoods) fall off to shortly after bloom when berries are about pea size (BBCH 73 to 75). Susceptibility of the fruit drops rapidly after that time. Grapes do not get new infections on fruit after 8% Brix but can still have sporulation up to 15% Brix. Leaves and canes, however, can be infected up to and past harvest.
Symptoms:
Powdery mildew can attack all above-ground plant parts. In early stages, whitish or grayish patches are on leaves and, if severe, ultimately cover both surfaces. Colonies are more easily detected in full sunlight. Later in the season, the mildew darkens and is peppered with minute black dots (chasmothecia). On fruit, the fungus at first may look grayish or whitish but later has a brownish, russeted appearance. Infected fruit cracks and drops from the cluster. Even blossoms sometimes can be infected, causing them to dry up or fail to set fruit. When green shoots and canes are infected, the affected tissues appear dark brown to black in feathery patches. Patches later appear reddish brown on the surface of dormant canes.
Flag shoots are difficult to detect. Some young shoots may be covered with a large white mass of threads or mycelium. Others may have only a hint of thin threads on the shoot. Shoots generally are delayed in bud break and appear stunted and somewhat yellowed compared to healthy shoots.
Early infection of clusters leads to small berries, split berries, berries with gray patches, poor yield and bad tasting wine.
The white cast to the vines on the right are due to powdery mildew. Other vines have been treated with various fungicides.
On the fruit, the fungus at first may appear grayish or whitish, but later is has a brownish russeted appearance. Berries may split and shrivel.
Cleistothecia squashed open to show several asci with ascospores.
The first mildew colonies generally can be found on the underside of leaves as very small diffuse spots.
An early start to the epidemic can destroy clusters before bloom.
Cultural
control:
Prevent excess vigor through proper selection of rootstocks, training systems, and fertility for the vineyard site.
Practice timely sucker control.
Keep canes cut back close to the top wire of vertical trellises.
Removing leaves to control bunch rot helps fungicide cover the clusters, which helps control powdery mildew.
Chemical
control: Begin applications at 6 inches shoot growth and continue at regular intervals through the growing season. Strongest materials and shortest intervals should be used from prebloom (BBCH 57) through bloom and continued for 2 weeks after bloom. A postharvest application may help control late-season infections in some years. Thorough coverage of all actively growing tissue is essential for good control. Do not extend application intervals in the Willamette Valley past 2 weeks.
Research in California has shown that sulfur sprays at budbreak initiate a release of ascospores and have some utility in overall disease management. Although this may occur in the more arid areas of the Pacific Northwest, it may not have the same utility in the wetter areas west of the Cascade Mountains.
To prevent developing resistant fungi, alternate materials with different modes of action. Use group 3 fungicides at the highest rates and a maximum of four (4) times during the growing season. New York recommends to tank mix group 3 or group 11 fungicides with sulfur or other products with a different mode of action. Several forecasting programs are available to help time applications (see below).
Abound at 10 to 15.5 fl oz/A. Alternate with other fungicides. Do not apply within 14 days of harvest. Sprayers used to apply Abound should not be used on apples. A group 11 fungicide. 4-hr reentry.
Adament 50 WG at 3 to 4 oz/A. Only use on a 14 day schedule. Do not use more than 48 oz/A/season or within 14 days of harvest. Contains a group 3 and 11 fungicide. Do not use on Concord grapes. 12-hour reentry.
AuxiGro WP at 2 to 4 oz/A plus a silicone-based surfactant. Use only in the early season when disease pressure is low and thorough coverage is easier to obtain. Do not use more than 24 oz/A/season. Was not effective in one trial in western Oregon. 4-hr reentry.
Bicarbonate-based products. Might supplement a normal program when powdery mildew is first observed. Do not mix with acidifying agents. Only use early season since thorough coverage is essential and timed when disease pressure is low.
Armicarb "O" (85% potassium bicarbonate) at 2.5 to 5 lb/100 gal water. 4-hr reentry.
Kaligreen (82% potassium bicarbonate) at 2.5 to 5 lb/A. 4-hr rentry.
MilStop (85% potassium bicarbonate) at 2 to 3 lb/A. Oregon and Washington Only. 1-hr reentry.
Monterey Bi-Carb is registered for home use in all states.
Copper formulations are registered but provide only moderate control alone. They are not generally recommended.
Endura at 4.5 oz/A. Do not use more than five (5) times per year or within 14 days of harvest. A group 7 fungicide. 12-hr reentry.
Flint 50 WG at 1.5 to 2 oz/A. Do not use on ‘Concord’ grapes. Rotate with other fungicides that have different modes of action. Do not apply more than twice consecutively, or more than six (6) applications total per season, or use more than 24 oz/A/season or use within 14 days of harvest. A group 11 fungicide. 12-hr reentry.
JMS Stylet Oil at 1 to 2 gal/100 gal water. Necrotic foliage may result if applied within 10 days of any sulfur application. Do not tank mix with copper based products when fruit is present. Do not use during freezing temperatures, above 90°F, or when plants are under heat or moisture stress. Do not use when foliage is wet as good coverage is essential. Brix reductions have been observed in several locations when product is used all season. 4-hr reentry.
Mettle 125 ME at 3 to 5 fl oz/A. Do not apply more than 10 oz/A/season or within 14 days of harvest. Group 3 fungicide. 12-hr reentry.
M-Pede at 1 to 2 gal/100 gal water. Good coverage is essential. Do not use within 3 days of applying sulfur. Do not mix with hard water. 12-hr reentry.
Ph-D WDG at 6.2 oz/A. Do not make more than 3 applications/A/season. May be applied on the day of harvest. Group 19 fungicide. 4-hr reentry.
Pristine at 8 to 12.5 oz/A. Do not use more than two (2) consecutive applications, within 14 days of harvest, or more than 5 times/year. Do not use on labrusca type grapes such as Concord and Niagra due to possible foliar injury. A mix of group 7 and 11 fungicides. 12-hr or 5-day reentry based on activity.
Procure 480 SC at 4 to 8 fl oz/A. Do not use within 7 days of harvest or more than 32 oz/A per season. A group 3 fungicide. 12-hr reentry.
Quintec at 3 to 6.6 fl oz/A. Do not apply more than five (5) times/year, more than 33 fl oz/A/season, or within 14 days of harvest. A group 13 fungicide. 12-hr reentry.
Rally 40 WSP at 3 to 5 oz/A (or Eagle 20 EW at 6 to 10 fl oz/A for landscape use). Do not apply more than 1.5 lb/A/season or within 14 days of harvest.
A group 3 fungicide. 24-hr reentry.
Rubigan EC at 2 to 6 oz/A plus a surfactant (or use Vintage SC at 3 to 6 fl oz/A). Use 2 oz prebloom, 4 oz at full bloom, and 6 oz in postbloom sprays. Do not apply more than 19 oz/A per season or within 21 days of harvest. Do not use with copper-based products. Higher rates early in the season may cause leaf cupping and/or marginal leaf burn. A group 3 fungicide. 12-hr reentry.
Sovran at 3.2 to 4.8 oz/A. Rotate with other fungicides that have different modes of action. Do not make more than 2 consecutive applications, more than 4 applications/ season or within 14 days of harvest. Do not use organosillicate surfactants. Injury may occur to some sweet cherries, such as Van, if accidentally sprayed. A group 11 fungicide. 12-hr reentry.
Spectracide Immunox at 2 fl oz/gal water. Do not use within 2 weeks of harvest or more than 6 times per season.
Sporan EC (essential spice oils) at 1.5 to 3 pt/A. Zero hr reentry.
SuffOil-X at 1 to 2 gal/100 gal water. Do not use in conjunction with sulfur or Captan. Do not use when plants are under heat or moisture stress. Do not use when foliage is wet as good coverage is essential. 4-hr reentry.
Sulfur products. Do not extend intervals beyond 14 days. Sulfur’s activity is effective from 57 to 830F. It also can burn foliage when applied above 85°F. The temperature relationship is correlated with increases in the daily maximum, within a few days after application. Grapes in California can withstand sulfur applications (at lower rates) above 85°F if there is no major short-term changes in the daily maximum. May injure labrusca types like ‘Concord’. Do not use within two weeks of an oil spray.
Kumulus DF (80% sulfur) at 2 to 5 lb/A. 24-hr reentry.
Microthiol Disperss (80% sulfur) at 3 to 10 lb/A. Use at 7- to 14-day intervals. Do not use a spreader sticker. 24-hr reentry.
Thiolux Jet (80% sulfur) at 6 lb/A. 24-hr reentry.
Tebuconazole-based fungicides are registered. For single active ingredient formulations, do not apply more than 2 lb/A/crop season or within 14 days of harvest. A group 3 fungicide. 12-hr reentry.
Elite 45 DF at 4 oz/A.
Orius 45 DF at 4 oz/A.
Tebuzol 45 DF at 4 oz/A.
Unicorn at 1.75 to 2.5 lb/A. Includes sulfur in the formulation. Do not use more than 20 lb/A/season. 24-hr reentry.
Topsin M WSB at 1 to 1.5 lb/A plus another fungicide.. Do not use within 14 days of harvest or more than 4 lb/A/season. Resistant fungi make this product ineffective and may be prevalent in many vineyards. Always tank-mix with another fungicide. A group 1 fungicide. 2-day reentry.
Trilogy at 1% of spray volume. Do not use after bunch closure, near sulfur applications, below 45°F, above 90°F, or when plants are under heat or moisture stress. Do not use when foliage is wet as good coverage is essential. Poor to fair control as a stand alone product. 4-hr reentry.
Notes:
If you are trying to bring an abandoned vineyard back into production, spraying lime sulfur (48-hr reentry) during the dormant season or micronized sulfur at 100% budbreak may help bring powdery mildew under control with a normal, season-long spray program.
Although OxiDate is registered it will not control this disease due to its short residual.
Forecasting:
There are several forecasting programs for scheduling fungicide applications. The standard Oregon phenology-based program begins applications at 6 to 8 inches shoot growth and continues at regular intervals based on grapevine development. The Gubler-Thomas, UC-Davis program uses leaf wetness and temperature early in the year to predict ascospore infection periods and only temperature during the summer to predict conidial infection periods. The New York (Gadoury) program is based on rainfall and temperature. The Kast (Oi Diag) program incorporates relative humidity along with temperature and rainfall. All programs have been effective at timing fungicides and controlling powdery mildew in western Oregon.
Biological
control:
Serenade MAX (Bacillus subtilis strain QST 713) at 1 to 3 lb/A. Active ingredient is a protein. Ineffective as a standalone treatment based on tests in western Oregon.
4-hr reentry.
Sonata (Bacillus pumilis strain QST 2808) at 2 to 4 quarts/A plus a spreader sticker. May be applied up to and including the day of harvest. 4-hr reentry.
References: Gadoury, D.M., Seem, R.C., Ficke, A. and Wilcox, W.F. 2003. Ontogenic resistance to powdery mildew in grape berries. Phytopathology 93:547-555.
