422 F. M. Rolfs 



after the second week, and all the new leaves that developed in the green- 

 house remained free from infection, while the leaves of the four trees in 

 the open continued to develop many new spots. 



The relation of moisture to the spread of the disease is also brought out 

 to a marked extent in the field when a favorable wet period is followed by 

 a dry period. All the leaves formed in wet weather contain many brown 

 spots and usually develop the shot-hole condition more or less, while those 

 formed in a dry period remain free from spots, the contrasts in the foliage 

 of the two periods being sometimes very striking. 



Heavy dews, if the temperature does not fall too low, also furnish favor- 

 able conditions for inoculations, and observations indicate that they often 

 play quite as important a part in the progress of the disease as does rain. 



Shaded situations are favorable for the growth of the organism. Direct 

 sunlight soon kills it. However, since the stomata of the leaves occur 

 only on the lower surface, and the twigs and the fruit are usually fairly 

 well shaded by the leaves, only comparatively few of the organisms ever 

 become exposed to the direct rays of the sun. 



In dry seasons injury from this organism is very slight, especially if the 

 spring has been cold. Infections in such seasons are found only in the 

 center of the tree, where the foliage remains moist from the dew until 

 late in the day. 



While it is true that temperature and moisture are the factors that gov- 

 ern the distribution of the organism, yet soil conditions, fertilizers, and 

 cultivation have a marked influence on the extent of injury done. The 

 soil in which the trees are growing has a considerable influence on the 

 development of the disease. When a general outbreak occurs in an orchard, 

 the trees on the higher, thin ridges invariably suffer more than those on 

 the lower, more fertile land. Trees that have borne the heaviest crops in 

 previous years invariably suffer more than do those that have yielded 

 little or no fruit in previous seasons. Young trees planted in new land 

 properly broken up before planting are more nearly free from the disease 

 than trees planted on land poorly prepared. Trees properly pruned and 

 well cultivated show less disease than do those that are not pruned nor 

 cultivated. 



VARIETAL SUSCEPTIBILITY 



Observations show that the different varieties of the various hosts 

 exhibit considerable variation in their susceptibility to the disease. The 



