Jan. 3. 1921 



Fusarium-Wilt of Tobacco 



521 



CAUSAL ORGANISM 



The causal organism can be readily isolated from diseased tissue by- 

 plating out on acid potato agar. The mycelium ordinarily imparts only 

 a dull pinkish tinge to the substratum and seemingly has a more or less 

 characteristically sparse growth and "powdery" surface (PI. 65, A) as 

 compared with the dense cottony development of some forms of Fusarium. 

 The "powdery" appearance is due to microconidia which are formed in 

 abundance, as is characteristic on a number of other media where 

 similar growth is made. "Strains" bearing sporodochia may or may 

 not occur. Where fruiting "strains" have been secured sporodochia 

 have usually been produced in abundance, especially on Melilotus stems, 

 oatmeal agar, and occasionally on potato agar and other media. True 



Fig. 1. — Camera-lucida drawings of spore forms of Fusarium ozysporum var. nicotianae, n. var: A, macro- 

 conidia; B, microconidia; C, chlamydospores; D, conidiophore of the sporodochial stage. 



pionnotes have not been observed in the cultures during a period of 

 four years on various kinds of media. " Pseudo pionnotes " or reduced 

 pionnotes could, however, be made to appear. Blue sclerotia and some- 

 times salmon-colored sclerotia are produced. 



An examination of the conidia from well-developed sporodochia of the 

 Maryland strain ordinarily shows a preponderance of 3-septate conidia, 

 which, together with the shape and size of the spores (fig. 1) and the fact 

 that the fungus produces a wilt disease, placed it readily in the section 

 Elegans, according to Wollenweber's classification (13). A more careful 

 study of the size and shape of the conidia brings out a close resem- 

 blance to Fusarium oxysporum Schlecht., according to recent descrip- 

 tions of this species. Studies were therefore undertaken to establish 

 whether the tobacco-wilt Fusarium is identical with Fusarium oxysporum 



