[Cir. 127— n] 



SILVER SCURF, A DISEASE OF THE POTATO.^ 



By I.E. Melhus, Pathologist, Cotton and Truck Disease and Sugar-Plant 



Investigations. 



INTRODUCTION. 



It seems desii'able to call the attention of potato growers to a disease 

 of the potato which, though little known, is becoming widespread in 

 the United States. This is the silver scurf, a tuber trouble caused by 

 a species of SpondylocUidium which attacks and destroys tlie peri- 

 derm, causing discoloration and loss of moisture. According to infor- 

 mation in hand it seems to have secured a foothold throughout the 

 Eastern States without its presence being generally recognized. 

 Introduced in all probability from Europe, it has spread rapidly and 

 will have to be reckoned with as one of the factors in the deterioration 

 of this crop. While the inconspicuous appearance of silver scurf has 

 permitted it to be overlooked, it can readily be recognized after a 

 little experience, and everyone interested should learn to distinguish 

 this disease from the several others which occur on potato tubers. 



APPEARANCE OF DISEASED TUBERS. 



Silver scurf in its earlv stages of infection under moist conditions 

 causes blackish olive patches on the surface of the potato, which may, 

 and often do, cover considerable areas. This more or less superficial 

 growth is the brown-mold stage and consists of the conidioj^hores and 

 conidia of the fungus. This stage is well shown in figure 1. In the 

 later stages of the development of the disease small black sclerotia 

 are formed on the infected areas instead of the superficial growth of 

 conidiophores. The surface of the discolored areas when clean takes 

 on a silvery or glistening appearance, which is due to the raising 

 of the cells of the outer layers by the mycelium of the fungus. This 

 disfiguration of the surface is later followed by depressed patches of 

 greater or less extent. The early stages in the formation of such spots 

 are shouii in figure 2, while the late stages are evident in figure 3. 

 As the disease progi'esses the infected areas increase in diameter and 

 the fungus penetrates deeper into the tuber. As a result it is not 

 uncommon to find the whole surface of a tuber discolored, shrunken, 

 and shriveled like the lower tuber in figure 3, while the outer layers 

 of the periderm are loose and sloughing off. 



'Issued May 17, 1913. 

 [Cir. 11'7] 15 



