1915] 



GGAR — RHIZOCTONIA CROCORUM AND R. SOLAN I 433 



examined microscopically, there are no symptomatic differ- 

 ences between the effects of the two organisms. 



Seedlings affected exhibit symptoms somewhat different with 

 age. The youngest seedlings of all delicate plants show what 

 may be called the usual damping off characteristics. Near the 

 base of the stem an hygrophorus or translucent appearance 

 is quickly followed by shrinkage of the tissues and weakness 

 of the stem. The plants topple over, the fungus invades all 

 parts, and spreads rapidly to the neighboring individuals. 

 The cells of the sap-perfusecl tissues are flaccid and injured, 

 some showing this even before the entrance of the hyphae into 

 the cells. Somewhat older plants and the more robust seed- 

 lings of cotton, bean, etc., often exhibit characteristic lesions. 

 Atkinson ('95) gives a description of its effect on cotton 

 seedlings as follows: 



"The trouble is caused by the fungus growing first in the 

 superficial tissues of the stem near the ground and disintegrat- 

 ing them before it passes to the deeper tissues; in other words 

 the fungus never seems to penetrate far in the living tissues, 

 but 'kills as it goes/ and the tissues become brown, depressed, 

 and present the appearance of the plant having a deep and 

 ugly ulcer at the surface of the ground. The fungus does not 

 spread into the tissues either above or below the ulcer to any 

 extent, but literally eats away at that point until it has severed 

 the stem at the affected place or the plant has recovered from 

 its effects." 



DISEASES OF POTATOES 



The potato is the most interesting of the host plants with 

 respect to the parasitism of Rhizoctonia by reason of the 

 many types of disease induced under diverse conditions. The 

 conditions may be in part climatic and, in part perhaps, de- 

 pendent upon the pathogenicity of the particular strain of the 

 fungus or upon the stage and development of the host at the 

 time of infection. It has been noted that when Kiihn first de- 

 scribed the disease of potatoes in Germany he laid emphasis up- 

 on a scab which was often followed or accompanied by decay. 

 This form of the disease was probably less prevalent in the 

 country as a whole at that time, and the more recent accounts 

 indicate that the ' ' black speck scab ' ' or " black speck, ' ' prop- 

 erly the sclerotial stage, is the feature by which the main type 



