Grape vine powdery mildew: signs and vineyard control
Grape powdery mildew causes pale leaf spots, white powdery growth and web-like scars on berries. The pathogen is Erysiphe necator. Initial pale spots occur on upper leaf surfaces; white webby growth develops underneath. Black or brown web scarring on mature fruit marks former colonies.
Infection and scouting
The fungus overwinters inside dormant buds or in spore-producing bodies called chasmothecia. Reduce early inoculum and subsequent infections to support season-long control.
Examine 10–15 basal leaves from about 20 randomly selected vines, seven to ten days after initial infection. Look underneath for spores. The risk index supplements vineyard inspections.
What the temperature index means
After initial infections caused by ascospores are found, the model's epidemic-start condition is three consecutive days with at least six continuous hours at 70–85°F, measured in the vine canopy. The index starts at zero and gains 20 points on each qualifying day. Before it reaches 60, a day that fails the six-hour condition resets it to zero. At 60, the model moves to its spray-timing phase.
The resulting schedule assumes adequate coverage and calibrated sprayers. Basal leaf removal can improve cluster coverage.
Sulfur and oil timing
For a light summer-oil treatment, ensure no sulfur residue is present. Separate oil and sulfur treatments by at least two weeks and follow the selected product's directions.
For uncertain symptoms, the NPDN state directory locates a receiving diagnostic laboratory.
Treating powdery mildew on grapes: chemical control and spraying grape vines
For chemical control of powdery mildew of grapes, spray grape vines with protectant fungicides promptly and repeat at intervals suited to the fungicide, vine growth and infection risk; oil can also eradicate existing colonies. The section on protectants and resistance below lists how to alternate modes of action.
Winter inoculum and the first spring infection
Chasmothecia are the principal overwintering inoculum in most production areas. Ascospores mature on infected green tissue in late summer and fall. Rain washes the fruiting bodies onto permanent vine parts, including cordons and arms. With abundant moisture on warm winter or spring days, they release ascospores that germinate on leaf undersides.
A separate route begins with mycelium surviving inside dormant buds. Shoots emerging from infected buds can become diseased soon after budbreak. These are flag shoots. Their conidia can spread to adjacent shoots. Distinguishing that source from an infection following a wetting event helps interpret early scouting findings. Conidia continue to form under moderate temperatures; symptoms are delayed at 60–68°F.
Continue the index after its initiation phase
Once the index reaches 60, calculate each day's spray-timing value from the previous day's result. A day with fewer than six continuous hours at 70–85°F subtracts ten points. Six qualifying hours add twenty. Temperatures reaching 95°F for more than fifteen minutes subtract ten. A day combining six qualifying hours with at least fifteen minutes at or above 95°F adds ten overall. The index is bounded between zero and one hundred. These adjustments belong to the established epidemic phase; they are different from the reset-to-zero rule used during initiation.
Keep your spray records in one place. Free to start.
An index of 0–30 indicates low pressure; 40–50 indicates moderate pressure. Select the spray interval for the fungicide group, including biologicals, sulfur or demethylation inhibitors.
Choose protectants and manage resistance
Most powdery-mildew fungicides work best as protectants; oil can also eradicate existing colonies. Begin treatment promptly and repeat at intervals suited to the fungicide, vine growth and infection risk. Under high disease pressure, biologicals and other soft-chemistry materials alone provide inadequate control.
Alternate effective fungicides with different modes of action to manage resistance. Assess coverage on clusters and foliage, including the underside of basal leaves. Combine the disease-pressure record with the calibrated sprayer's deposition observations before adjusting the program.
Match disease-control timing to the harvest system
In California's disease-management program, treatment can end at 12 Brix in wine and traditionally trellised raisin grapes. Continue protecting table grapes until harvest and dried-on-vine raisins until three to four weeks before cane severance.
Find a plant diagnostic lab and prepare a crop sample
Send living symptomatic material and the receiving laboratory's submission form. The NPDN directory provides state laboratory links. The instructions below are those of the University of Minnesota Plant Disease Clinic.
Collection and packaging for Minnesota
Collect symptomatic plants while they are still alive. Include different stages of decline when available. For herbaceous crops, send entire plants where possible, including roots. Dig rather than pull to preserve the root system. Wrap roots in plastic, then wrap the plants in dry newspaper and place them in plastic. A box is preferable to an envelope.
For virus testing at the Minnesota clinic, wrap symptomatic leaves, stems or entire plants in aluminum foil. Samples must not dry out, and extra water should not be added to the bags. Some virus tests are unavailable, so call before sending. Nematode samples also require protection from drying and heat because the nematodes must remain alive.
Mail early in the week to avoid weekend layover. The Minnesota clinic accepts samples from the continental U.S. under USDA APHIS permits. Double-bag and seal samples collected outside Minnesota. The clinic publishes separate mailing and hand-delivery addresses on its submission page.
Complete the current submission form
Use UMN-FORM-012-001, version 1.5. Each contact block includes name, business, address, telephone and email, with email or mail choices for results. The submitter receives the invoice unless another arrangement is made.
Identify the plant or crop, collection date and collection location by county, city or ZIP. Name the test requested if known. Describe symptoms, occurrence pattern, other affected plants, site conditions, pesticide and fertilizer applications, and soil. The reverse side or extra sheets can be used. Leave ID number, received date and notes in the Lab Use Only section for staff.
Next step
Inspect basal leaves and update the vineyard risk index. For uncertain symptoms, find a receiving laboratory through the NPDN directory.