June s. 1916 Siher-Scurf of Irish Potato 345 



may shorten and thicken to form a short and many-celled mass of 

 hyphae, from which the conidiophores subsequently arise. In severe 

 cases of infection the cells appear to be disintegrated by the invasion to 

 such an extent that only two or three instead of six or more cork layers 

 remain above the living parenchyma. In experiments with potato roots 

 grown under sterile conditions and inoculated with conidia and mycelium 

 of the fungus, the mycelium grew on the surface, but did not penetrate 

 the parenchyma, which indicates that the roots are less subject to infec- 

 tion than the tubers. 



So far as the author has been able to determine, the fungus hyphse 

 confine their activity to the corky layers. In no case has it been found 

 in the living parenchyma. This superficial infection causes a loosening 

 of the corky and epidermal cell layers, so that these subsequently slough 

 off. In this manner transpiration may proceed with greater facility and 

 thus affect the parenchyma layers. 



That 5. atrovirens prefers this relatively heavy corky layer is further 

 apparent from the fact that it grows very sparingly on the cut surface 

 of the tubers where the loosened surface cells are invaded. Further- 

 more, its very limited presence on roots, stems, and stolons also indicates 

 that it prefers the heavier, corky layers of the potato tuber. 



EFFECTS OF THE FUNGUS ON THE HOST 



The progress of the disease after tuber infection may be divided into 

 two stages, the early and the late. In the former the infected areas are 

 light-brown and have a glazed appearance, the latter characteristic 

 becoming especially pronounced when the infected surface is moistened. 

 Sometimes the margins of these areas are slightly fimbricated. The 

 discoloration, which is found on newly infected tubers at harv^est time, 

 is often so inconspicuous as to pass unnoticed, even on close examina- 

 tion, unless the tubers are washed. When infected tubers are placed in 

 moist chambers, the brownish areas become olive-colored, owing to the 

 formation of conidiophores and conidia. The late stage is characterized 

 by the shrinking and shriveling of the diseased areas and sloughing off of 

 the epidermis and may be subdivided into two stages : The spot or patch 

 infection (PI. XLV, fig. 2) and general infection (PI. XLV, fig. i). In 

 the former slightly sunken isolated areas on the surface show the shrivel- 

 ing, and late in the storage season these areas become shriveled and 

 sunken. 



In the case of general infection the entire surface is covered with 

 infected areas and the epidermal and corky layers may shrink to such 

 an extent that distinct folds or ridges appear. In the red-skinned 

 varieties the color is completely destroyed. This again largely only 

 mars the appearance and not their food value, but still they must be 

 sold at a sacrifice. Potatoes stored under moisture and temperature 



