PARASITIC BHIZOCTONIAS IN AMERICA 309 



favorable for damping-off. The fungus could be seen extending in all 

 directions over the surface of the sand. 



The fungus found on the diseased alfalfa seedlings was compared 

 with a fungus obtained from mature alfalfa plants sent from Iowa. 

 Altho the mycelium of the two forms was characteristic of Rhizoctonia, 

 it differed in many respects, particularly in the color of the hyphse. 

 The form on the mature plants was undoubtedly Rhizoctonia Cro- 

 corum, while that on the seedlings was the common Rhizoctonia Solani. 



Rhizoctonia was first reported on the roots of alfalfa from Nebraska 

 in 1890, by Webber, 137 as Rhizoctonia Medicaginis DC. This fungus 

 was next mentioned on alfalfa as Rhizoctonia violacea, by Heald, 57 

 who found it causing a root rot in a single locality in Nebraska in 

 1906. In 1908 it was reported by Freeman, 42 under the name Rhizoc- 

 tonia violacea, as spreading rapidly in the alfalfa fields in Kansas. 

 Freeman described the disease as beginning in different parts of the 

 field where at first a single plant dies. From these centers of infec- 

 tion the fungus grows in all directions thru the soil, killing the plants 

 as it proceeds. Thus circles of steadily increasing radii are formed, at 

 the edges of which plants in all stages of the disease are found. The 

 great majority of the plants within the affected areas die, while those 

 which survive are not vigorous and always lose their main tap roots. 



The first external sign of the disease is a yellowing of the plant, 

 which soon after wilts and dies. The roots of a dead or dying plant 

 are found to be covered with a violet or brownish red mat of mycelial 

 strands, or hyphse. In a few cases the tap root is completely rotted. 

 In less severely affected plants, the cortex of the roots slips off easily 

 when the plants are lifted from the soil, leaving only the central woody 

 cylinder. This condition is due to the fungous threads which grow 

 thru the cortex as far as the cambium layer, which they kill. The 

 fungus forms sclerotia, which may live in the soil for several years. 



Stewart 126 in 1908 mentioned a root rot and damping-off of alfalfa 

 in the field in New York. His description of the disease 1 agrees in 

 some respects with the one given by Freeman. Later he also noticed 

 the damping-off of alfalfa seedlings in the greenhouse. He was not 

 certain that Rhizoctonia Crocorum was present in New York, and was 

 of the opinion that the fungus causing the damping-off of seedlings 

 in the greenhouse was different from the one found in the field. 



Heald, 58 in a later article (1911), described more fully the disease 

 occurring in Nebraska. At that time he regarded the fungus as iden- 

 tical with Rhizoctonia Medicaginis DC. of Europe. 



From the above accounts it is certain that there are two species of 

 Rhizoctonia in this country able to attack alfalfa R. Solani, widely 

 distributed, causing only a damping-off of seedlings, and R. Crocorum, 

 with a limited distribution, attacking as a rule only mature plants 

 in the field. At present this latter species has been reported on alfalfa 

 from Nebraska, Kansas, Iowa, and Virginia. 



