FUNGI IMPERFECTI 



349 



production in the pycnidium has not been clearly made out, but 

 the spores are unicellular, and, when mature, ooze out from the 

 tip of the flask-shaped body, adhering in masses. They are more or 

 less subglobose or oblong, hyaline, and measure 5-9 /a in length. 

 Upon immersion in water they increase greatly in size and readily 

 germinate. 



When the mycelium has developed to a considerable extent in 

 the root, sclerotia of large .size appear. It is believed that these 

 sclerotia may be properly a phase in the life history of this species, 

 and that they may also be important in the perpetuation or spread 

 of the fungus. This fungus will continue its growth and develop- 

 ment upon the stored roots, and also upon the remnants of the 

 crop left in the field, so that special care must be taken not only 

 with respect to the quality of the roots used at the time of planting, 

 but also to the prevalence of the disease during previous years in 

 the fields where potatoes are to be grown. 



The sunken area and the greenish character of the diseased 

 portion enables one readily to distinguish the effects of this fungus 

 from those produced by the common black mold, Rhizopus nigri- 

 cans Ehr., which is the organism causing the typical soft rot of this 

 crop. The latter disease is not discussed in this work. 



Proceeding from the mycelium within the tissues the Sphaero- 

 nema has been readily cultivated upon various artificial media. In 

 cultures upon sweet potato agar a profuse mycelium is developed. 

 The submerged hyphae are olive brown in color and contain abun- 

 dant oil droplets. All three types of spores are produced, aerial 

 hyphae originating the endogenous spores, and submerged sporo- 

 phores producing the olivaceous conidia. Normal pycnidia develop 

 in culture in a week or more. The disease has been produced in 

 healthy roots by inoculation with the hyaline conidia and with the 

 pycnospores from pure cultures. 



Control. Seed roots for planting purposes should be carefully 

 selected and no slips should be taken from plants in the seed beds 

 showing disease. Rotation of crops is necessary to rid fields of 

 this fungus. Apparently no experiments of interest have been 

 made to determine the possibility of preventing the spread of the 

 fungus in stored roots. Nevertheless, any conditions favoring the 

 accumulation of moisture would be favorable to the organism. 



