422 MANUAL OF FRUIT DISEASES 



land are more severely diseased than those on light, well-drained 

 soil. Plants exposed to the sun are said to be more affected 

 than those which are growing under shade; it appears that 

 shade has a preventive effect, a phenomenon not easily ex- 

 plained. Moisture alone increases the amount of leaf-spot, 

 yet during an excessively rainy season plants remain quite 

 free from the disease. This freedom from * leaf-spot during 

 a very wet season has been explained on the grounds that 

 cloudy weather offers much the same conditions as those of 

 shade. In a year of continued drought the disease is likewise 

 less common. In general, conditions most favorable to the 

 disease are hot, bright days with occasional showers or heavy 

 dews. 



Symptoms. 



The disease makes its appearance on the leaves (Fig. 123), 

 calyx and fruit-pedicels. In the beginning, the spots are very 

 small, deep purple or red, and are usually first evident on the 

 upper surface of the leaves. These areas rapidly increase in 

 size, and at the same time the color of the central portion 

 changes from a purple to a reddish brown; eventually the 

 lesions become grayish or whitish in the center (Fig. 123). 

 The border remains purplish, shading off to a reddish brown 

 coloration towards the healthy. The single lesions vary from 

 i to T of an inch in diameter, but several spots may coalesce 

 to form a large irregular blotch. Severely affected leaves 

 turn brown, this discoloration beginning at their tips; finally 

 such leaves shrivel and die. The lesions on the other suscep- 

 tible organs are similar to those on the leaves. 



Cause. 



The leaf-spot disease of strawberries is produced by the 

 fungus Mycosphcerella Fragarice. The mycelium of the fungus 

 grows in and between the cells, and through the action of these 

 threads upon the host-cell contents there is a response expressed 

 in the spots as described. After a short time the hyphse 



