Oidium or powdery mildew of the vine . Fig. 4. Berries badly affected and cracked. (After Foex.) If this cracking occurs early, the grapes dry up completely beforeripening. If later, they may develop sugar and can be used for winemaking, but the crop is diminished in volume. In moist seasons theberries which are cracked usually become the prey of blue mould andare completely destroyed. After the grapes have lost the green color due to the presence ofchlorophyll in the skin and have commenced to ripen they are not at-tacked by Oidium. The markings and blotches which are often seenon ripe grapes
Oidium or powdery mildew of the vine . Fig. 4. Berries badly affected and cracked. (After Foex.) If this cracking occurs early, the grapes dry up completely beforeripening. If later, they may develop sugar and can be used for winemaking, but the crop is diminished in volume. In moist seasons theberries which are cracked usually become the prey of blue mould andare completely destroyed. After the grapes have lost the green color due to the presence ofchlorophyll in the skin and have commenced to ripen they are not at-tacked by Oidium. The markings and blotches which are often seenon ripe grapes are due to injuries to the skin caused by the presenceof the fungus on the grapes before they ripened. If the attack has notbeen early or serious enough to prevent the grapes reaching normal 320 UNIVERSITY OF CALIFORNIA—EXPERIMENT STATION. size they may still make good wine. The markings, however, are veryobjectionable on table and raisin grapes and decrease their value. Tokay and Muscat grapes often show the dark, fern-like markin
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