12 BULLETIN 1417, U. S. DEPARTMENT OF AGRICULTURE 



were severely wilted, except occasionally when a young lesion was 

 found farther down on the taproots. 



The most successful method used in isolating Ozonium was as 

 follows: Plants just showing the first signs of wilting or healthy 

 appearing plants next to these in a row were dug up and the loose 

 dirt washed off. Bits of tissue with the young lesions present were 

 cut out and dipped in mercuric chloride. 1 to 1,000. for 30 to 60 

 seconds. Instead of washing off the mercuric chloride as is the 

 custom, the excess of mercuric-chloride solution was taken up with 

 filter paper and the pieces dropped on cotton or alfalfa root plugs 

 in test tubes. As soon as the mycelium grew from the bits of 

 tissue on to the plugs, the pieces were removed, to prevent con- 

 tamination with slower-growing organisms. By the use of this 

 method about 75 per cent of the cultures attempted yielded 

 Ozonium. 



Attempts were made to culture the mycelium making up the co- 

 nidial mats, but in no instance was a pure culture obtained. Simi- 

 larly, no pure cultures were obtained from the Ozonium strands 

 when taken from diseased roots. 



THE FUNGUS 



Xo detailed account of the various stages of the fungus will be 

 presented, as in the main the observations made by the writers 

 are in accord with those of Shear (6). Duggar (7). Taubenhaus 

 and Killough (£), and others. 



STERILE MYCELIAL STAGE 



The type of sterile mycelium of the fungus Ozonium omnivorum 

 universally present on the healthy roots of the plants in advance 

 of the wilting zone is the strand hyphae. composed of several or 

 many bands of hyphae, the cells of which vary in shape and size. 

 These strand hyphae vary in color with age. but are usually buff or 

 brownish. They branch off in a more or less regular fashion, but 

 when spreading out over filter paper from an infected cotton root 

 thev may branch to produce a fan-shaped arrangement of strands. 



(Pi. 5. %. i.) 



These strands may be compared to some extent to the rhizomorphs 

 of some of the Basidiomycetes. As has been pointed out before, the 

 function of these strands appears to be in the nature of advance 

 mycelia, which spread through the soil and are responsible for the 

 direct penetration and infection of the roots. 



The acicular type of hyphae is generally associated with the strands 

 found on the roots. In culture this type has been found on cotton 

 and alfalfa root plugs and sterilized soil, but not on any media 

 containing agar. 



The mycelial weft so prominent on the roots of diseased plants 

 consists principally of la^ge Rhizoctonialike hyphae, together with 

 some acicular hyphae and a few strands. The large-celled hyphae 

 also form the major portion of the pseudosclerotia. although the 

 outer surface may be covered by strands with acicular hyphae. 



The large-celled hyphae break up and collapse very readily. Only 

 the young growing tips of these hyphae appear normal. The 



