191 5 J 



DUGGAR RHIZOCTONIA CROCORUM AND R. SOLANI 417 



and while they are not the only means of penetration they are 

 most important in this connection. 



The hosts upon which the writer has had the opportunity 

 to examine the infection cushions in best condition are alfalfa, 

 carrot, and asparagus. The cushions are distributed over in- 

 fected roots, often 1 mm. apart in alfalfa, .5 mm. in carrot, and 

 3 mm. in asparagus. The external hyphae are for the most part 

 similar to those of the general mycelium, but there occur also 

 branches in which the cells are short and swollen, sometimes 

 resembling a short chain of spores. This form of hypha may 

 have given the suggestion of a conidial stage (see Kuhn 

 ('58), Sorauer ('86), and others. The medullary portion of 

 younger cushions is made up of finer, almost colorless hyphae, 

 and it is this type which enters — strand-like — the cortical 

 tissues of the root, destroying particularly the cambium and 

 younger phloem regions. In the later stages of development 

 it will be found that the cushions seem to extend consider- 

 ably into the cortex, and more of the hyphae are colored. 



In this connection it is well to call attention briefly to some 

 gross changes in the affected roots. By the time the host 

 (alfalfa) reaches the critical stage, the bark slips readily 

 from the root. The disintegration may continue further, 

 however, through the spread of the fungus to the medullary 

 rays and all other parenchyma, so that the root shreds or 

 crumbles when lifted. The late stages of destruction may be 

 assisted by saprophytic organisms. It is difficult to determine 

 if the fungus continues its growth for a short time after the 



death of the root. At any rate, the fungus rapidly disappears 

 with the further decay of the roots. 



In the case of asparagus the cushions are largely super- 

 ficial and the main affected tissues are beneath the shell of 

 thick-walled cells constituting the periphery of the host. In 

 the carrot the invading strands are large, and the host cells 

 in the vicinity rapidly collapse and darken. I have been for- 

 tunate in obtaining affected asparagus roots at intervals after 

 the disease had run its course. In no case could any evidences 

 of spore forms be found which gave promise of genetic con- 

 nection. On the contrary, the fungus gradually disappears, 



