1916} PARASITIC EHIZOCTOXIAS IN AMERICA 377 



5. Studies of the growth of Rliizoctonia Solani Kiihn on media 

 show that the strains are very variable, those from the same host 

 often producing a different growth even on the same media, and that 

 the differences in various cultural characters which are shown by 

 strains from unlike hosts are no greater than the differences which 

 may be manifested by two different strains isolated from the same host 

 oi 1 by the same strain at different ages. 



6. Measurements of mycelial cells of RJiizoctonia Solani Kiihn 

 shoAved such large variations, not only between strains from different 

 hosts but also between different strains from the same host, that no 

 standard could be determined on for distinguishing the different 

 strains. 



7. By means of a local soil survey, it was found that Rliizoctonia 

 Solani Kiihn is abundant in cultivated land, where it may live either 

 011 dead organic matter in the soil or on weeds and other plants. 



8. A certain vigor of mycelium must be attained before RJiizoc- 

 tonia Solani Kiihn is able to attack a plant. A high temperature 

 (88 F.), together with either too little or too much moisture, deter- 

 mines to a large degree the virulence of the strains. It is only under 

 certain conditions that the fungus becomes a dangerous parasite. 



The writer gratefully acknowledges his indebtedness to Dr. F. L. 

 Stevens, Professor of Plant Pathology, and to Dr. J. T. Barrett, for- 

 mer Chief Assistant in Botany, for their kind assistance and encour- 

 agement. He wishes also to thank Professor H. B. Dorner, Assistant 

 Chief in Floriculture ; Mr. C. C. Kces, formerly Assistant in Floricul- 

 tural Pathology; and other members of the Division of Floriculture 

 for assistance rendered during the progress of this work. 



