94 MAINE AGRICULTURAL EXPERIMENT STATION. I914. 



disease he can usually recognize it without trouble, even in 

 this stage, on account of the size, grouping, and appearance of 

 the spots. Unless several spots have run together and coalesced 

 they are usually oval to circular in shape, small, seldom move 

 than one-sixteenth to one-eighth inch in diameter and frequent- 

 ly several occur together in clumps. Occasionally they may ex- 

 tend diagonally across the surface of the potato in more or less 

 n-regular, parallel lines. The ragged, turned back margins of 

 the ruptured epidermis already mentioned are usually still m 

 evidence after the top of the pustules have been rubbed off. 



The above description of the appearance of the disease upon 

 the affected tubers applies to all the specimens which the writer 

 has thus far seen which have been collected from the store- 

 houses and cellars in Maine during the present winter. How- 

 ever, this is by no means the only form of alteration which 

 the parasite may produce in the normal appearance of the host. 

 In severe attacks, especially in moist soil, a distinctly warty ap- 

 pearance may develop, quite different from the ordinary type 

 of powdery scab. These warts are several times larger than 

 the scab pustules aleady described and are usually smooth and 

 roundish, although somewhat irregular, varying in color from 

 a light to dark brown in the case of the few specimens which 

 the writer has seen, and more frequently occurring at the ter- 

 minal or "Seed" end of the tuber. Figure 45 represents tubers 

 produced in the plant pathology greenhouse at this Station, 

 under rather moist conditions from a seed potato only 

 Slightly affected by the disease. According to Home* "The 

 warty condition is not so evident after the potatoes have been 

 removed from the soil for some time. The warts become flat- 

 tened and discolored, so that raised, more or less chocolate- 

 colored scars remain in their stead." The same author states 

 that this form of the disease is known in some parts of Great 

 Britain as "Corky-end." Still another stage is recognized' of 

 \vhich the writer has seen only one tuber which latter came 

 from Canada. This is a condition where considerable portions 

 of the surface of the tuber become destroyed and eroded, lea^■- 

 ir.g a distinct, hollowed-out, cankerous area. This is illustrated 

 in Figure 44. It is apparently a very advanced stage of the 



* Home, A. S. On Tumor and Canker in Potato. Jour. Royal Hort. 

 Soc. 37 : 362-389. 191 1. 



