320 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. 



size they may still make good wine. The markings, however, are very 

 objectionable on table and raisin grapes and decrease their value. 



Tokay and Muscat grapes often show the dark, fern-like markings 

 characteristic of the disease similar to those shown on the canes. ( See 

 fig. 3.) When abundant, these markings, which are accompanied by a 

 thickening of the skin, injure the appearance of the raisins. Where 

 the skin of the Muscat grape has been attacked by Oidium it will re- 

 main smooth after drying and not show the fine wrinkling character- 

 istic of the finest raisins. 



A vine of which the leaves are badly attacked has a peculiar color 

 and wilted appearance. (See figure on cover.) This appearance is 

 only shown when the conditions of temperature and moisture are favor- 

 able to the growth of the fungus on the exterior leaves. This can occur 

 only at the beginning of the season in the warmer and drier parts of 

 the State. When the air is hot and dry the fungus cannot grow on the 

 outer leaves. The proper favorable conditions, however, may still exist 

 in the interior of the vine. For this reason vines may look perfectly 

 healthy from a little distance, while the fruit and the bases of the canes 

 may be covered with Oidium. A vine which is badly attacked has a 

 moldy smell readily perceptible to many people. 



DESCRIPTION OF THE FUNGUS. 



Mode of Attack. The botanical name of the fungus is Uncinula 

 spiralis (Berkeley and Cooke), and it belongs to the family or group 

 of Erysipheae. All the members of this family are parasites which 

 live upon the outer surfaces of leaves and other green organs of plants. 

 The white "mildew" which they form on these surfaces consists of the 

 branching threads or mycelium, which constitute the vegetative body 

 of the fungus. This mycelium grows only on the surface, unlike the 

 mycelia of Peronospora and other parasitic fungi which enter the tis- 

 sues of the host plant. It obtains its nourishment from the epidermal 

 cells of the plant attacked, by means of suckers haustoria which 

 penetrate these outer cells. The outer cells thus attacked become dark 

 colored. The fact that the Oidium does not penetrate the tissues of the 

 vine much facilitates its control and renders its destruction possible at 

 any stage without injury to the host plant from the remedies applied. 



Summer Form of the Fungus. The mycelial threads of the Unci- 

 nula spiralis are extremely fine not over two ten-thousandths of an 

 inch in diameter. They elongate a f nd grow over the surface to which 

 they are attached by their haustoria and at intervals send out upright 

 branches hyphae thirty to forty ten-thousands of an inch long. 



