114 MANUAL OF FRUIT DISEASES 



elsewhere in California also suffer extensively from the effects 

 of this disease. 



The nature of the injury induced by powdery-mildew makes 

 an estimate of the financial loss difficult. The leaves and twigs 

 are directly affected, while the fruit escapes almost entirely. 

 Damage may result to the affected host in one or more of the 

 following ways : (1) the leaves are so affected that it is diffi- 

 cult to find normal foliage in humid regions ; this is especially 

 true of susceptible varieties ; (2) affected shoots are stunted ; 

 the terminals as well as the leaves die ; (3) the floral organs are 

 reduced in size ; (4) affected trees may fail to form blossom- 

 buds; (5) diseased trees obviously cannot produce well; 

 (6) trees which are affected year after year decline in normal 

 appearance, owing to the cumulative effect of the disease. 



Symptoms. 



Early in the summer the foliage and young twigs may show 

 signs of powdery-mildew. Leaves are most commonly affected 

 on the lower surface, where a whitish or grayish powder develops. 

 These powdery spots may reach nearly an inch in diameter; 

 several may occur on a leaf, and as a result of their coalescence 

 the lesions involve considerable area. Affected leaves crinkle, 

 become stunted, and are particularly more narrow than normal 

 leaves. Later the mildew may be seen on the upper surface 

 of the foliage. Diseased twigs may be practically covered 

 with mildew. Like the leaves they are stunted, the internodes 

 of a single year's growth sometimes reaching not more than 

 one or two in length. In the winter the terminal shoot may die 

 back; the following spring a new shoot arises from a lateral 

 bud. This phenomenon may be repeated year after year. In 

 the latter part of the summer black fruiting bodies appear as 

 specks scattered through the powdery surface growth. No 

 direct injury is done to the fruit and infections thereon are ex- 

 tremely rare. When the floral parts are affected, they are 

 dwarfed and deformed. 



