The reason wine grape growers place rose bushes at the end of their vineyard rows:
a) To alert them to a powdery mildew outbreak;
b) To alert them to an insect infestation;
c) Red roses mark the rows of red wine grapes; white, the white wine grape varieties.
d) They're pretty.
d) is correct. Different strains of powdery mildew attack roses. Insects that bother both would attack the grapes first. And if the color of the rose indicated the varietal, then an apricot colored rose would mean that they are growing grapes for Boone's Farm Strawberry Hill wine.
Sacramento County Farm Advisor and Viticulture specialist Chuck Ingels says the practice may have been tried in Europe a long time ago to detect powdery mildew early, but it doesn't work. The mildew species that attacks grapes is a completely different species from that of the rose powdery mildew; and, they have different temperature requirements. Also, roses are more prone to get aphids; grapes, not so much.
For those at home keeping score:
Powdery mildew species on grapes: Erisiphe necator.
Powdery mildew species on roses: Sphaerotheca pannosa
Viticulture instructor Andy Walker at UC Davis says that roses are planted strictly for aesthetics.
This practice probably started in the early 20th century and continues to today; the myths and the stories about it came along the way.
One blogger took a trip to the vineyards of Italy recently where the winemaker discussed the issue:
"Singore Razzi explained how they grow the grapes for their wine. We wondered why there were rose bushes at the end of each row of grapes and found out that very sophisticated tests were done by scientists on the soil and after those tests, the rose bushes were planted to tell the wine master how the soil is doing. If the roses stay fresh and perfect they know the grapes are doing just as well...when a bush is 'sick' they know those grapes growing in that row are 'sick' also.”
No winemaker is going to rely on roses to tell them about the quality of the wine. But it certainly impresses visitors; and, they probably bought more wine because of this sophisticated-sounding yarn.
So, how do wine grape growers control powdery mildew? With a rather large arsenal of chemical weapons. And for a good reason. Powdery mildew can develop a resistance if the same product is used over and over. According to the University of California Integrated Pest Management Program:
"Alternating fungicides with different modes of action is essential to prevent pathogen populations from developing resistance to classes of fungicides. This resistance management strategy should not include alternating or tank mixing with products to which resistance has already developed. Rotate with fungicides that have a different mode of action. Research has shown that sequential sprays of products with the same mode of action can lead to the development of reduced sensitivity to the active ingredient(s). Some fungicides have two active ingredients and thus two modes of action. When using such materials, do not alternate with other fungicides that contain one of the same modes of action (i.e. they represent the same fungicide class)."
Home gardeners and organic growers have a more limited selection to control powdery mildew.
According to UC IPM:
"Powdery mildew is a perennial problem in grapevines. Sulfur, horticultural oils, neem oil, jojoba oil, and Serenade are registered for controlling powdery mildew in home vineyards.
Begin applying treatments when all buds have pushed. Thereafter, repeat at 10-day intervals if disease pressure is high; otherwise, extend intervals when temperatures are above 90°F until the sugar content of the grapes is 12 to 15%, which is when they begin to soften and approach ripeness and are no longer susceptible to infection.
You can measure the sugar content with a refractometer, if you have access to one, or you can see if sample berries sink in a 15% sucrose solution. (Prepare the sucrose solution by dissolving 8-1/2 teaspoons of table sugar in a half cup of warm water, then mixing in enough cold water to make the total volume 1 cup.)"
More information about these products:
Fungicides. Several less-toxic fungicides are available for backyard trees and vines, including horticultural oils, neem oil, jojoba oil, sulfur, and the biological fungicide Serenade. With the exception of the oils, these materials are primarily preventive. Oils work best as eradicants but also work as good protectants. The fungicides listed here are registered for home use.
Oils. To eradicate powdery mildew infections, use a horticultural oil (such as Saf-T-Side Spray Oil, Sunspray Ultra-Fine Spray Oil) or one of the plant-based oils such as neem oil (such as Green Light Neem Concentrate) or jojoba oil (such as E-rase). Be careful, however, never to apply an oil spray within 2 weeks of a sulfur spray or plants may be injured. Some plants may be more sensitive than others, however, and the interval required between sulfur and oil sprays may be even longer; always consult the fungicide label for any special precautions. Also, oils should never be applied when temperatures are above 90°F or to drought-stressed plants. Horticultural oils as well as neem and jojoba oils are registered on a wide variety of crops.
Sulfur. Sulfur products have been used to manage powdery mildew for centuries but are only effective when applied before disease symptoms appear. The best sulfur products to use for powdery mildew control in gardens are wettable sulfurs that are specially formulated with surfactants similar to those in dishwashing detergent (such as Safer Garden Fungicide). To avoid injury to the plant or tree, sulfurs should not be applied within 2 weeks of an oil spray, used on any plant when the temperature is near or over 90°F (80°F for caneberries and strawberry), and never applied at any temperature to apricot trees.
Biological Fungicides. Biological fungicides (such as Serenade) are commercially available beneficial microorganisms formulated into a product that, when sprayed on the plant, inhibit or destroy fungal pathogens. The active ingredient in Serenade is a bacterium, Bacillus subtilis, that helps prevent the powdery mildew from infecting the plant. While this product functions to kill the powdery mildew organism and is nontoxic to people, pets, and beneficial insects, it has not proven to be as effective as the oils or sulfur in controlling this disease.
, from the University of California Division of Agriculture and Natural Resources.