OIDIUM OR POWDERY MILDEW OF THE VINE. 327 



mento and San Joaquin valley will ripen their fruit perfectly if they 

 escape the spring infection. Late ripening varieties, however, such as 

 some of the table grapes, may be attacked during the cooler weather of 

 autumn and in some seasons any variety growing vigorously in moist 

 places may be attacked during summer and autumn. 



CULTUEAL METHODS OF CONTEOL. 



The Oidium can be checked to some slight extent by cultural meth- 

 ods. Anything which permits the sun and air to get to all parts of 

 the vine more completely will lessen the danger from mildew. Rows 

 of trees which shade the vines on the south side can often be removed 

 with advantage. Vines planted wide apart are less subject to the dis- 

 ease, as they dry off more quickly in the morning. For the same reason, 

 the rows of trellised vines should run north and south in order to facil- 

 itate the drying action of the rising sun. Low trellised vines' are often 

 less subject to Oidium than ordinary vines, as the foliage is spread 

 over a larger surface and not massed around the head as in vase prun- 

 ing. On the other hand, high trellised vines are usually more subject, 

 as the wind and sun do not penetrate them so well. 



Drainage of the wet places in a vineyard and methods of pruning 

 and training which spread the vine out so that no part is surrounded 

 by still moist air will tend to diminish the intensity of the disease. 



SULFUE TEEATMENT. 



Mode of Action. No cultural method is capable of completely pro- 

 tecting the vines from Oidium except under the most favorable condi- 

 tions and with the most resistant varieties. Some special treatment, 

 therefore, is nearly always necessary. Of all the methods tried, there 

 is none so effective as the dusting of powdered sulfur over the vines. 



Sulfur acts by means of the fumes it gives off when the tempera- 

 ture is sufficiently high. These fumes destroy the mycelium and 

 summer spores of the fungus, and if present in sufficient abundance 

 will destroy every vestige of Oidium in the vineyard with the exception 

 of the perithecia. 



The exact nature of the fumes given off is uncertain. Some authors 

 believe them to consist of sulfurous acid gas identical with that pro- 

 duced by the burning of sulfur and to be due to slow oxidation. The 

 odor perceptible on a hot day in a sulfured vineyard, however, has not 

 the pungent nature characteristic of the fumes of burning sulfur, but 

 resembles that of hydrogen sulfid, which other authors believe to be 



