The Oidium appears sooner on poor soil and on exhausted vines, and vines in which the flow of the sap for some reason or other has been checked are more subject to the mildew than those which are yet in full growing vigor. Elevated vines on trellises which are much exposed, and vines which are so covered up that the air has little access, are the first ones to be attacked, and those which will suffer the most. The powdery mildew affects all varieties of grapes, but some kinds more than others. The Muscats are among those which suffer considerably, and if not sulphured would in severe cases neither set nor bear suitable grapes. The Malaga is less affected, and so is the Sultana. In new districts the Uncinula does not appear until the vines are older. Thus in the Fresno district the earliest vines did not suffer from mildew until they became five years old, but now the mildew would destroy the grapes every year, in case they were not treated with sulphur.
Remedies.
—The most common and perhaps the best remedy is powdered sulphur. The latter is applied either with the dust can or “dredger,” or with bellows. The dust can is used when the vines or vine shoots are yet small, and the bellows when the vines are larger. The first sulphuring should be done when the young shoots are six inches long, immediately before the bloom, and the second time when the berries are well set. Sulphuring as a regular vineyard operation will be more fully discussed further on.
DOWNY MILDEW OR PERONOSPORA.
General Notes.
—The downy mildew is a fungus known botanically as Peronospora viticola. Its native country is the United States, but its greatest damage is done in Europe. It appears as white, downy spots on the underside of the grape leaves, which are gradually destroyed, and later on attacks the berries, which shrivel and spoil. In California the downy mildew occurs frequently on wild native grapevines, but only very rarely on the cultivated Asiatic vines. Dr. H. W. Harkness, the eminent mycologist, found it only once on cultivated vines in the Sacramento river bottom. These vines were growing close to native vines, from which the fungus had spread. There is no fear that this fungus will ever spread and cause damage in our State as long as the vineyards are given plenty of air. In France the Peronospora has caused much damage, but is now being combated with bluestone and lime solutions, according to the following formula: Slake thirty pounds of lime in seven and a half gallons of water, also mix sixteen pounds of bluestone (copper sulphate) in twenty-five gallons of water. Mix the two together, and either sprinkle the foliage with it, or dilute it further with say five hundred gallons of water and spray the vine leaves on both the upper and lower sides. According to Dr. Harkness the efficiency of this spray cannot always be relied on. So far no other fungi have appeared in the raisin districts of this State, nor have we reason to fear that any will attack the vines.
Downy Mildew (Peronospora), Greatly Magnified. a. The Fungus Growing out of a Stoma of the Vine Leaf. b. Transversal Section of Vine Leaf, showing Fungi and their Tuberous Mycelium.