THE RHIZOCTONIA DISEASE OF POTATOES. 20/ 



lenticel. ^Microscopic examination of these spots, as small as 

 they could be detected on the surface, invariably showed them 

 to be made up of brown, collapsed cell walls and free starch 

 with Rhizoctonia filaments running through the mass. 



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 

 affected tissue. Occasionally diseased areas Hke this will reach 

 the size of from two to five millimeters in diameter without any 

 other superficial change. More frequently the tissues surround- 

 ing the lenticel begin to shrink away, leaving 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 leading from the center of the 

 aft'ected area on the surface. Such canals are popularly sup- 

 posed by the farmers to be the work of wire worms or some 

 other animal pest, but the canals are quite different from those 

 made by wire worms. 



A distinct wall of rather firm texture exists at the junction of 

 the diseased and healthy tissue. This is firm enough so that 

 when the tubers have been dug a few weeks if the point of a 

 knife is placed under one edge the entire diseased area can be 

 removed occasionally intact, leaving simply a clean pit in the 

 healthy tissues. AA'^hen the tubers are boiled the firmness of 

 this lining of the pits or canals is still more apparent for they 

 readily pull out and remain firmly attached to the skin when 

 the later is removed. 



Rolfs (12) mentions that the Rhizoctonia stage produces two 

 kinds of mycelium, one light and one dark. The latter, he says, 

 develops deeper in the tissues, is more actively parasitic and 

 frequently produces a wet rot of the stem in old seed tubers. 

 We have observed no wet rot or decay of potato plant parts 

 whatever that we could attribute to Rhizoctonia. However, it 

 should be noted that our observations were made under quite 

 different soil conditions. Rolfs was working in the irrigated 

 districts of Colorado, and attributed much of the trouble to 

 over-watering, resulting in the soil becoming wet and soggy. 

 The land on which our most careful field observations were 

 made was a well-drained loam with some mixture of sand. At 

 no time during this season till after the crop was harvested did 



