238 DISEASES OF CULTIVATED PLANTS 



sclerotia are less frequent, and may attain to the size of a 

 marble, blackish-purple in colour, and more or less velvety at 

 the surface. These sclerotia give off long strands of dark- 

 coloured mycelium which spread into the soil. 



An ascigerous form of fruit met with on the dead roots of 

 lucerne killed by the Rhizoctonia, and called Leptosphaeria 

 circinans (Sacc.), is considered by some as representing the 

 fruiting condition of Rhizoctonia, but this has not been 

 definitely proved. Hendersonia medicaginis, Sacc. (Bys- 

 sothecium circinans^ Fuckel), is also considered to be a 

 pycnidial form, and Lanosa nivalis (Fr.), a conidial form, 

 but there is as yet no certainty about any one of these. 



The structure and colour of the mycelium and the general 

 habit of Rhizoctonia so closely resembles that of Rosellinia 

 quercina, R. aquila^ R. necatrix, and other destructive para- 

 sites belonging to that genus, that I consider the Rhizoctonia 

 as representing the vegetative condition of a Rosellinia. 



Rolfs, on the other hand, considers that Rhizoctonia is a 

 basidioycete, Cortidum vagum (Berk, and Curt.), var. solani 

 (Burt). Now C. vagum is a saprophyte on the dead bark of 

 various conifers, hence it requires a vivid imagination to 

 assume that a variety of this species has become a rampant 

 parasite on the roots of a large number of different kinds of 

 plants growing in countries where C. vagum is unknown. 

 The C. vagum, var. solani, is described under Hypochnus 

 solani in this book. 



Leptosphaeria circinans. Perithecia erumpent, seated on 

 reddish-violet mycelium, black; asci cylindric-clavate, 8- 

 spored; spores elliptical, 3-septate, 2 interior cells violet- 

 brown, terminal cells small, hyaline, 32-35X10-12 /x. Para- 

 physes slender, hyaline. 



One of the most difficult of parasitic fungi to eradicate, on 

 account of its widespread diffusion in the soil, where, even in 

 the absence of those cultivated crops it especially prefers, it 

 is enabled to continue its existence by attacking weeds of the 

 most varied kind. It has been proved to maintain itself for 

 at least twenty years in land that has produced a crop of 

 diseased lucerne, although during this period no cultivated 

 crop that it was known to attack had been grown. When 

 land is infected cereals appear to be the only thing immune 

 against its attacks. In addition, lime should be used, and 

 the ground kept as free as possible from weeds. 



An important discovery has recently been announced by 



