194 The Potato 



Rhkoctoniose (Plates X and XII) 



This is a name given to the several symptoms of a 

 disease caused by the fungus Rhizoctonia, which when 

 producing its perfect spores is known as Corticum vagum 

 var. solani. The fungus forms small black irregular 

 masses often the size of a half-pea or smaller on the sur- 

 face of the tuber. These resemble httle chunks of muck, 

 but can be readily detected by washing the tuber, for 

 they do not wash off as soil does although they can be 

 easily scraped off. These bodies, called sclerotia, enable 

 the fungus to pass the winter without injury. In the 

 spring, when planted with the tuber or even when kept 

 in a moist place, a fine mycelium grows out from them. 

 This attacks the sprouts and produces cankers on them, 

 so that the portion above is commonly destroyed. 

 Younger sprouts may then appear from below the can- 

 kered area, but these in turn may be infected, so that the 

 plants are late in making their appearance above the 

 ground or do not appear at all. Affected plants that 

 succeed in appearing may continue to grow and produce 

 well. The fungus, however, commonly attacks and 

 destroys the stolons and in this way reduces the yield. 

 Older plants are sometimes weakened by cankers on the 

 stems, and brown strands of mycelium may be observed 

 over such affected areas. The cankers may be so severe 

 that the vines become yellow in color and the leaves roll 

 upward the same as when the vine is affected with black- 

 leg. Indeed, in such cases the stem is sometimes found 

 to be decayed by a black-rot similar to black-leg. Af- 

 fected vines often have small aerial tubers formed in the 

 axils of the leaves and the stems themselves are thick- 

 ened. Such vines usually have a large number of small 



