206 MAINE AGRICULTURAL EXPERIMENT STATION. I914. 



The second form of injury no doubt is a more severe stage 

 of the first. In this the surface of the potato becomes diseased 

 over large areas and growth on the surface is checked. The 

 tension of the expanding tissues beneath ruptures the surface, 

 eventually forming large cracks. These cracks enlarge as the 

 potato grows and the fungus spreads over the tissues thus ex- 

 posed and doubtless assists in enlarging and deepening them. 

 Finally the potato becomes badly deformed and misshapen. 

 Specimens of this form of the disease which were not especially 

 common are shown in Fig. 71. 



Still another form of the disease starts, apparently, from the 

 lenticels. A critical examination of potatoes from a large 

 number of sources, including those from other states, has con- 

 vinced the writers that it is fairly common. Ordinarily this is 

 an oval pit from 3 to 5 millimeters in diameter as shown in 

 Figs. 72 and 73. It usually contains a mixture of corky remains 

 of dead cell walls, free starch and Rhizoctonia filaments. This 

 pitting was first noted in making a critical examination of a 

 large number of different samples of potatoes received from 

 different parts of the country as illustrating forms of potato 

 scab, particularly abnormal types, in the localities from which 

 they came. 



Potatoes affected in this way and received from Nebraska 

 and Wisconsin were planted in the greenhouse in pots of steri- 

 lized soil and the crop produced later showed different stages 

 of the formation of the pits and the close association of Rhiz- 

 octonia with them. The young tubers showed small holes, 

 apparently formed at the lenticels and largely filled with Rhiz- 

 octonia filaments. The tissues lining the pits were dark brown 

 and water-soaked in appearance. Pure cultures of Rhizoctonia 

 were obtained from the pits in the young tubers produced in 

 the greenhouse. This, however, was previous to undertaking 

 most of the work here recorded, particularly the greenhouse 

 studies described in a later section. 



Potatoes from the field under consideration frequently 

 showed the pitting in various stages of development at harvest 

 time. A study of the development of these pits, as shown by 

 these different stages, showed plainly that they were not of 

 insect origin. The beginning of a pit was indicated in tubers at 

 harvest time by a slight circular browning in the region of the 



