98 MAINE AGRICUIvTUKAlv EXPERIMENT STATION. I914. 



The beginning of these pits is usually indicated by a slight 

 circular browning on the surface of the potato in the region of 

 a lenticel. As the diseased area increases it becomes slightly 

 depressed and brown in color. Usually the margin is somewhat 

 more sunken forming a sharp boundary line between the healthy 

 and diseased tissues. Occasionally diseased areas like this will 

 reach the size of froim one-eighth to one-fourth of an inch 

 without any other superficial changes. More frequently the 

 tissues surrounding the lenticel soon begin to shrink away leav- 

 ing an opening in the center of the diseased area. This may 

 widen out into a broad, shallow pit, or if the disease penetrates 

 any depth into the flesh it may develop into a canal following 

 the center of this area. Figure 52 represents a potato af- 

 fected in this manner. 



Rhizoctonia is very common in New England potato soils an>l 

 the sclerotia of the fungus are frequently seen on the surfaces 

 of potato tubers. These are smalil black, or dark brown, o'ften 

 irregular shaped bodies, comm.only flattened and closely attached 

 to the skin of the tuber. They can usually be removed with 

 more or less difflculty by means of a stiff brush or by pressure 

 wath the thumb nail, leaving the skin of the tuber beneath, 

 entirely sound and uninjured. These sclerotia are in reality 

 ■very compact masses of densely woven threads of the fungas 

 and serve as a resting stage to carry it over winter, but the 

 housewife usually considers them to be closely adhering parti- 

 cles of dark colored soil. If the sclerotia are rounded, fairly 

 regular in shape, of proper size, and appear on potatoes more 

 or less covered with dirt they may be taken for pustules of pow- 

 dery scab. Their true nature is readily shown if the tubers are 

 washed or the dark spots are scraped off with a knife or by 

 means of the thumb nail. Fig. 51 illustrates a potato covered 

 by sclerotia of Rhizoctonia, and photographed after being 

 washed. 



PVart disease. This disease which is caused by Chryso- 

 phlyctis endobiotica Schilb., and is also known as potato canker 

 and potato tumor is not known to occur on the mainland of 

 North America. When both diseases occur, together in England 

 it is said that the warty stage of powdery scab may sometimes 

 be mistaken for the true wart disease, but it is stated that if 



