THE RHIZOCTONIA DISEASE OF POTATOES. 211 



Inoculation of tubers after digging. The writers have fre- 

 quently observed dead, spong}^ areas beneath spots of the so- 

 called common potato scab, extending in some distance from the 

 surface. These spots varied from light to dark brown and 

 Rhizoctonia filaments were constantly observed in microscopical 

 mounts of the diseased tissues. Quite recently a large quantity 

 of potato tubers, obtained from many different localities in the 

 State of Maine, have been examined for various tuber diseases. 

 Several cases were observed where a dry, spongy rot had de- 

 veloped over half an inch under the apparently uninjured skin. 

 Many of these tubers were plainly affected with the dry rot 

 caused by Phytophthora. However, when portions of the 

 decayed tissue showing the other type of rot were removed from 

 the interior of these spots, under aseptic conditions, and placed 

 in plates of potato agar pure cultures of Rhizoctonia developed 

 in 4 out of 6 instances. 



In order to test the ability of the fungus to attack potato tubers 

 after they had been removed from the soil, lo clean, sound, 

 healthy tubers about two inches in diameter were thoroughly 

 washed, wet with full strength hydrogen peroxide for about 3 

 minutes, well rinsed in sterilized water and then treated as 

 follows. Slight punctures were made on one side of each of 5 

 tubers with a sterilized needle and small masses of the fungus 

 from pure cultures were laid on the surface over the punctures. 

 Check punctures were made on the opposite side of each tuber 

 to which nothing was applied. Five other tubers were inocu- 

 lated in the same way except that the material was simply laid 

 over a lenticel without puncturing the skin. The potatoes were 

 then placed in a covered jar without providing additional mois- 

 ture. 



In 3 days a distinct, discolored and somewhat sunken area 

 could be seen around a part of the punctures to which the 

 fungus had been applied. At the end of one week one of these 

 discolored areas around a puncture had attained a diameter of 

 6 millimeters. In two weeks this was only slightly larger, two 

 others were somewhat over 3, while a fourth was about two 

 millimeters in diameter. The surface of the diseased area was 

 smooth, somewhat sunken with a slight ridge at the junction 

 with the healthy skin. The latter surrounding the sunken spots 

 was somewhat puckered, due to the shrinking of the tissues 



