Diseases of the Potato 195 



tubers clustered close to the base. The fungus produces 

 its spores during moist weather on a thin, white, frost-like 

 growth ejctending over the base of the stalks. 



This fungus is able to attack many kinds of plants and 

 to live as a saprophyte in the soil from year to year. For 

 this reason it is diflficult, if not impossible, to eradicate it. 



The sclerotia on the tubers can be killed by soaking 

 the tubers in corrosive sublimate solution, and if this 

 treatment is coupled with a judicious rotation of crops 

 on the land, the injury from the disease will be greatly 

 reduced. 



Fusarium wilt 



This disease is caused by the fungus Fusarium oxy- 

 sporum, which gains entrance through the roots from in- 

 fected tubers or from the soil and works upward in sap 

 vessels, interfering in that way with the normal circulation 

 of the sap. The fungus mycelium destroys the roots and, 

 later, fruits on them, producing abundant spores. The 

 diseased condition of its roots and of the sap vessel causes 

 a yellowing, dwarfing and finally a wilting and dying of 

 the vines. The stem pulls up easily and its tissues beneath 

 the outer layer have a light brownish color. This color 

 in the vascular tissues extends to the roots, stolon, and 

 tubers. A section across an affected tuber near the stem 

 end shows a brown ring of sap vessels. This browning of 

 the vascular ring is associated with some other diseases 

 and is always found in tubers in which this Fusarium has 

 developed to any extent, although in the earlier stages of 

 infection no discoloration may be noticed. 



Such diseased tubers should . never be used for seed 

 purposes, as vines coming from them nearly always be- 

 come infected. The only successful way to control the 



