1915] 



DUGGAR RHIZOCTONIA CROCORUM AND R. SOLANI 



439 



reported by Duggar and Stewart ('01) is not due to typical 



R. Solani 



MYCELIUM AND SCLEROTIA 



The morphological characteristics of the hyphae and 

 sclerotia have been adequately described by several writers, 

 but it may be well to summarize some of the more important 

 features. Upon such hosts as the 

 potato, sugar beet, carnation, and 

 others there is more or less de- 

 velopment of an external web, 

 but never over the general root 

 system such a complete invest- 

 ment of roots by a mantle of 

 hyphae as characterizes the violet 

 fungus. The external hyphae are 



somewhat colored, usually yellow- 

 ish brown, and they are generally 



of two types. One type may be 

 designated as purely vegetative 

 and another as 



external tufts or masses when 

 these occur. All hyphae are prac- 

 tically colorless when young, vac- 

 uolate, more or less irregular, 

 septate with the septa at intervals 

 of 100-200 1*. The diameter of 



constituting the 



8-12 



I* 



Fig. 5. Jthizoctonia Solani (Cor- 

 ticium vagum) : A vegetative 

 hypha and a small strand from 

 artificial culture on potato. 



vegetative hyphae is 

 Branches arise, and when young 

 these are inclined in the direction 

 of growth and are invariably somewhat constricted at the 

 point of union with the main hyphae (fig. 5). As the hyphae 

 mature and become more deeply colored they are more uni- 

 form and rigid, the distances between cross walls are greater, 

 the constrictions where branches arise less marked, and the 

 branches are approximately at right angles to the main hypha. 

 On certain affected plants a short tufted or mealy growth 

 occurs and this is made up of hyphae of very different char- 

 acteristics. In the young condition threads are profusely 



