26 ISOLATION AND CULTIVATION OF FUNGI 



made, however, the task of subculturing becomes onerous, a 

 situation which constitutes an argument for the estabhshment and 

 maintenance of "bureaus of culture collections." With certain 

 species it may be found advantageous to incubate them for a few 

 days at a temperature favorable for rapid development and then 

 to place them in storage at 10° C. 



Efforts to retard the drying out of the medium by the use of 

 paraffin on the cotton stoppers or of waxed paper caps or rubber 

 thimbles are quite uniformly unsuccessful because they promote 

 contamination. Spores lodged in or on the cotton stoppers 

 are thereby provided with sufficient moisture to permit them to 

 germinate and the organism to employ the cotton fibers as a 

 nutrient substrate. 



Because of lack of detailed knowledge regarding the nutri- 

 tional requirements of fungi, a choice of culture medium is quite 

 arbitrary. Potato agar, bean agar, corn-meal agar, wort agar, 

 malt agar, and Sabouraud's agar are among those commonly used 

 to maintain cultures. 



Some fungi appear to require comparison with type cultures 

 for certain identification. This situation becomes complex in 

 dealing with an organism which does not remain true to type. 

 In culture, certain fungi gradually vary in mycelial color, texture 

 of the colony, and in sporulation or other characters. Moreover, 

 others gradually lose their vigor, dwindling until they become so 

 depauperate and so different from their appearance when first 

 isolated as to be unrecognizable as the same species. Some mu- 

 tate without known reason and become clearly distinct in ap- 

 pearance from normal colonies. Finally, some pathogenic fungi 

 lose virulence in culture, and no basis has been established for re- 

 storing their aggressiveness. On the other hand, others, such as 

 Taphrwa dejormans and Ustilago zeae, have been maintained in 

 culture for years without apparent loss of virulence. Until more 

 basic knowledge can be gained of the influence of nutritional and 

 environmental factors each mycologist who maintains fungi in 

 culture must be guided largely by his o^vn experiences in culti- 

 vating them. 



Another serious difficulty that will be encountered sooner or 

 later by anyone who maintains cultures of fungi is infestation by 

 mites. These pests devour the fungi and, in crawling from one 



