408 THE FUNGI WHICH CAUSE PLANT DISEASE 



discussion is problematic. Much culture and inoculation work is 

 needed. Some of the various hosts upon which a Rhizoctonia 

 apparently closely allied to that of Corticium vagum solani have 

 thus far been found in America are: 



Sugar-beet, bean, carrot, cabbage, cotton, lettuce, potato, 

 radish, sweet potato, pmnpkin, watermelon, garden pea, com, 

 purslane, Solanum verbascifolium, egg plant, pig-weed, spiny 

 pig-weed, Heterotheca subaxillaris, Richardia, Crotalaria, Cy- 

 perus rotundus, Heterotheca lamarckii and Phytolacca decandra, 

 Pinus sps., ^*^' **^ Picea, Pseudotsuga, carnation and alfalfa. 



The sterile myceliiun was noted in Europe on potato many years 

 ago ; its existence in America has been known since 1890 (Duggar ^^) . 

 Its identity with the genus Corticiimai was demonstrated in 1904 

 by Rolfs ^ both by observing the connection between the myce- 

 lium and the basidia on young potato plants and by culture of 

 the typical Rhizoctonia stage from the basidiospores. The .parasi- 

 tism of the organism was proved by inoculations made with pure 

 culture by Rolfs.^ 



The sterile mycelivun (Rhizoctonia) occurs in two forms on 

 the potato, a light-colored actively parasitic form usually some- 

 what deep in the affected tubers and a darker mycelium growing 

 superficially on the host or over the soil. In artificial culture the 

 manner of branching is typical, the young branches running 

 nearly parallel to the main thread and bearing slight constrictions 

 at their bases. 



A key to the species in France is given by Bourdot and Gol- 

 zin.i" 



C. laetum (Karst.) Bres. 



Plant body at first salmon-colored, soon fading to a dirty-white; 

 context, of hyphsB which are nodose, septate, irregular, 4-10 fi, 

 basidia clavate, 35-50 x 7-12 n; spores oblong ovate, subde- 

 pressed on one side, hyaline, 10-14 x 6-8 /»■ 



On fig and apple in Louisiana, and in Europe and in the Northern 

 United States on Alnus, and Corylus. It causes the limb blight 

 of the fig, "• "* gaining entrance through dead twigs. While the 

 fungus is usually a saprophyte, once it gains entrance to the host 

 it follows down the branch, covering it with its bright salmon- 

 colored fructification and causing sudden wilting and dying of the 



