OIDIUM OR POWDERY MILDEW OF THE VINE. 



319 



These marks, at first brown, later black, are usually branching or 

 fern-like in form, showing the points of attack of the fungus. If the 

 canes are attacked when young, they fail to mature properly and often 

 turn black over their whole surface. It is this form of Oidium which 

 has sometimes been mistaken for Anthracnose in California. 



Later, the blossoms and fruit may be attacked. When the blossoms 

 are attacked they fail to set, and if the berries are attacked when very 

 small, they will fail to develop and will drop off. If the fruit reaches 

 nearly full size before being affected, it continues to develop, but irreg- 

 ularly. The parts of the epidermis injured by the fungus become hard- 

 ened and cease to grow. This results in irregularity in the form of the 

 berry and in severe cases in cracking. (See fig. 4.) 



Fig. 4. Berries badly affected and cracked. (After Foex.) 



If this cracking occurs early, the grapes dry up completely before 

 ripening. If later, they may develop sugar and can be used for wine 

 making, but the crop is diminished in volume. In moist seasons the 

 berries which are cracked usually become the prey of blue mould and 

 are completely destroyed. 



After the grapes have lost the green color due to the presence of 

 chlorophyll in the skin and have commenced to ripen they are not at- 

 tacked by Oidium. The markings and blotches which are often seen 

 on ripe grapes are due to injuries .to the skin caused by the presence 

 of the fungus on the grapes before they ripened. If the attack has not 

 been early or serious enough to prevent the grapes reaching normal 



