THE RHIZOCTONIA DISEASE OF POTATOES. I99 



caused by Alternaria solani. Although the field had been well 

 sprayed with bordeaux mixture it was thought that the early 

 dying of the tops might be the combined results of the factors 

 mentioned. While the possibility of some below ground trouble 

 was suggested Rhizoctonia was not considered. 



When dug it was found that the tubers produced on the field 

 in question averaged small and that many of them showed vary- 

 ing amounts of Rhizoctonia sclerotia upon their surfaces. How- 

 ever, iieither of us had an opportunity to make a detailed ex- 

 amination of them. A barrel of the cleanest tubers which could 

 be storted out were shipped to Orono to be used as seed in the 

 greenhouse for growing plants for various experimental pur- 

 poses during the winter. 



Even with this selected seed it was found that when the 

 tubers were planted in the greenhouse in pots of steam sterilized 

 soil the sprouts frequently failed to reach the surface. Exami- 

 nation showed that such sprouts were usually strong and vigor- 

 ous when they started but later they had either been partially or 

 wholly cut off. Fig. 62 shows a single sprout from an uncut 

 tuber planted in a pot of sterilized greenhouse earth. It will be 

 noted that it is entirely cut off at the base and another, smaller 

 lesion occurs near the apex. These lesions in such cases were 

 brown in color and more or less covered with coarse, brownish 

 fungous threads, apparently Rhizoctonia. Both colorless and 

 brown threads were observed. 



The stems of young potato plants grown in the greenhouse 

 from this seed frequently showed an abundance of the fruiting 

 or Corticium stage of the fungus. This consisted of a whitish 

 to yellowish gray felt, sometimes rather delicate, growing up 

 around the base of the stem. As a rule this growth extended 

 but a short distance above the surface of the soil, but in some 

 instances it ran up 4 or more inches on the stems and leaf 

 petioles and lower sides of the leaf blades. Fig. 64 illustrates 

 an instance of this kind, the lighter color of the under sides of 

 the 3 lower leaves being due to a dense coating of the fungous 

 threads. 



Lesions similar to those described later as occurring below 

 ground on plants in the field were, in some instances, found on 

 plants in the greenhouse. Frequently plants were noted which 

 produced apaprently healthy and normal tops, except there was 



