342 



FUNGOUS DISEASES OF PLANTS 



appearance which often greatly depreciates its market value. The 

 tissue beneath the spots is dry and brown. 



The fungus. The first studies upon this disease seemed to indi- 

 cate that it was not produced by a fungus, but recent studies .have 

 demonstrated the causal relation of a fungus which seems to be 

 properly a species of Cylindrosporium, as the title suggests. The 

 mycelium is hyaline, septate, and intercellular. Chlamydospores 

 are common in the host tissue. In late stages of the disease a 

 compact stroma develops just beneath the epidermis and finally 



Fig. 165. Cylindrosporium Pom. (Photographs by Charles Brooks) 

 a, spot induced by inoculation of apple ; b, mycelium in agar 



breaks through it to expose spores and sporophores. The spores 

 are hyaline, from one to five celled, and variously curved and con- 

 torted. They are from 2 to 2.5 fi in diameter and from 1 5 to 80 /a long. 

 The chlamydospores and stromata are probably the agencies that 

 carry the fungus over the winter. Under ordinary conditions of 

 preparing separation cultures this fungus does not readily grow, 

 and agar will ordinarily dry out before the fungus becomes notice- 

 able. On this account it has seemed to be a difficult organism to 

 isolate. As a matter of fact, however, under ordinary constantly 

 moist conditions or in liquid media it grows readily. 1 



Infection probably takes place in July or August when the stomata 

 are being torn open and the protecting layers of the lenticels are 

 not yet formed, a season when the metabolism of the apple is 

 extremely great and the transpiration stream necessarily large. 



1 The " Stippen " disease, long known in Europe and now reported from several 

 parts of the United States, is regarded as entirely distinct, and probably not of 

 fungous origin. 



