 Tulare County |
Disclaimer: This article is geared towards a Tulare County audience and may not be applicable to other geographical areas.
Reprint freely with credit to: The University of California Cooperative Extension, Tulare County.
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For more information contact: Bill Peacock, Farm Advisor, wlpeacock@ucdavis.edu
| Publ. # IPM1-96 |
Downy Mildew In Tulare County
Downy mildew (Plasmopara viticola) is a major disease in humid grape-growing areas of the world, but it has rarely been seen in California vineyards. In Tulare County, the first documented finding of downy mildew occurred the spring of 1995.
Last year's record breaking wet spring was responsible for the appearance of downy mildew. Hopefully, a return to more typical spring weather will cause its disappearance. Downy mildew probably won't be a problem again for a long time, unless we have another spring like 1995. However, overwintering spores are now plentiful in some Tulare County vineyards, and we're not exactly sure what will happen this spring, especially if significant rainfall occurs after budbreak. Growers should be cautious and closely monitor vineyards from budbreak until berry set for signs of downy mildew. If we have a wet spring, the application of a preventative fungicide (copper, captan, mancozeb) would be advisable for vineyards that had downy mildew last spring, particularly Christmas Rose and Ruby Seedless vineyards. Ridomil/Copper 70W ® provides effective post-infection control if downy mildew infections are found in the vineyard.
Downy mildew makes its first appearance. Last year a grower first observed downy mildew in his Ruby Seedless vineyard near Farmersville on Memorial Day, May 29, 1995. Reports of downy mildew in other vineyards soon followed. It was most commonly found on Christmas Rose and Ruby Seedless, two varieties that are apparently very susceptible to the disease. It was also found on Red Globe, Crimson Seedless, and Fantasy Seedless but not on Thompson Seedless.
In Tulare County, downy mildew was reported in about a 50 square mile area. This area bordered Exeter on the east, Visalia on the west, Tulare-Lindsay highway to the south, and highway 198 to the north. There were unconfirmed reports of downy mildew in vineyards near Orange Cove and Caruthers in Fresno County.
Infections in Tulare County generally were not serious with only an occasional infected flower cluster, shoot tip, tendril, or leaf. However, two Christmas Rose vineyards near Exeter lost the entire crop in a 2 to 3 acre area. This was an alarming demonstration of the destructive potential of this disease.
Growers who discovered downy mildew quickly moved to protect their crop, especially with additional rain forecast. They applied copper (Champ®, Kocide 101®, COCS® dust) which prevented secondary spread of the disease. Ridomil/Copperr was applied to late season varieties (66 days of pre-harvest restriction) and it provided effective post-infection control.
What to Look For. The first obvious symptoms are yellowish-green spots on the upper surfaces of leaves. After warm humid nights, white downy growth appears on the underside of the leaf within the borders of the spots. When young shoots, petioles, tendrils, or cluster stems are infected, they become distorted, thickened or curled. In warm humid weather, they may be covered with white downy growth. Affected tissues soon turn brown and wither from the vine.
Berries are susceptibleto infection until they are pea-size (4 to 5 mm diameter). Mature berries are resistant to infection. Downy mildew is very different from powdery mildew. Downy mildew is a true water mold or algal fungi, classified as a phycomycete, and it must have wet, humid conditions to develop and spread. Powdery mildew is not dependent on rainfall and high humidity to develop. Temperature plays the largest role in the development of powdery mildew, classified as an ascomycete, with optimum temperatures for germination and mycelial growth between 70 °F to 86 °F. Fungicides that effectively control powdery mildew do not control downy mildew, with the exception of copper, see Table.
Control. Primary infections first occur 2 to 3 weeks before bloom, and protection needs to start at this time if rainy, humid conditions occur. Copper fungicides (Champ®, Kocide 101®), mancozeb, and captan fungicides can prevent downy mildew infections when applied before rainfall and periods of high humidity. Mancozeb and captan have been restricted by some raisin processors, wineries, and table grape markets. Ridomil/Copper 70W® can both prevent infections and provide effective post-infection control. Last year Ridomil/Copper 70W®, which is systemic, controlled downy mildew infections within a few days after application. Do not apply Ridomil/Copper 70W® after bloom or within 66 days of harvest.
Disease CycleThe following description of the disease cycle was provided by the New York State Agricultural Experiment Station, Geneva, New York (Disease Identification Sheet No. 5). The fungus overwinters as tiny oospores in leaf debris on the vineyard floor. In the spring, the oospores germinate in water to form sporangia. The sporangia liberate small swimming spores, called zoospores, if standing water is present. The zoospores are disseminated by rain splash to grape tissues where they swim to the vicinity of stomata and encyst. Encysted zoospores infect grape tissues by forming germ tubes that enter stomata and from there invade inner tissues of the plant. At night during periods of high humidity and temperatures above 13o C (55o F), the fungus sporulates by forming sporangia on numerous branched structures, called sporangiophores, that protrude out through stomata. Sporulation only occurs on plant surfaces that contain stomata, such as the undersides of leaves, and it gives the surface of the lesions its white, downy appearance. Sporangia are disseminated by wind or rain splash to other susceptible tissue. There they liberate zoospores into water films formed by rain or dew and these zoospores initiate secondary infections. Infections can occur in as little as 2 hours of wetting at 25oC (77o F) or up to 9 hours at 6oC (43oF). Infections are usually visible as lesions in about 7 to 12 days, depending on temperature and humidity. The number of secondary infection cycles depends on the frequency of suitable wetting periods that occur during the growing season and the presence of susceptible grape tissue (see Figure). |
Effectiveness of Fungicides for Control of Powdery and Downy Mildews1/
| Fungicides | Powdery Mildew | Downy Mildew |
|---|---|---|
| benomyle (Benlate) | 0 | 0 |
| captan | 0 | +++ |
| copper | ++ | +++ |
| Rovral | 0 | 0 |
| mancozeb | 0 | +++ |
| sulfur | +++ | 0 |
| Bayleton, Rubigan, Rally | +++ | 0 |
Key: +++ = excellent; 0 = not effective.
1/Adapted from Cornell Cooperative Extension Publication, "1990 Pest Management Recommendations for Grapes."
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University of California Cooperative Extension - Tulare County
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Revised: April 2, 1998