524 Journal of Agricultural Research voLxx.no. 7 



dealing with a form on tobacco sufficiently distinct from F. oxysporum 

 to warrant the creation of a new species. These conclusions were upset, 

 at least for the time being, by the appearance of signs of wilt in one 

 plant of the White Burley tobacco, out of six or eight planted, in soil 

 inoculated with MacMillan's F. oxysporum. Several pots of soil were 

 now prepared in December, 191 9, and were again infested with several 

 strains of F. oxysporum in comparison with my own strains secured from 

 Maryland and Ohio, one of them being a 191 6 isolation of the tobacco- 

 wilt Fusarium which had been transferred from an old, dried culture. 

 Good infection (about 80 per cent) was obtained with the tobacco-wilt 

 strains and with MacMillan's strain but not with Bisby's strains (cultures 

 in better growing condition than MacMillan's) nor with Wollenweber's 

 strains (cultures in poorer condition than MacMillan's strain). Mac- 

 Millan's strain did not, however, prove as virulent as the strains from 

 tobacco, and the symptoms were not identical — that is, the leaves did 

 not uniformly lose their color but presented more of a mottled appear- 

 ance in the early stages of the disease, and the vascular system was not 

 so distinctly discolored. On plating out the stem and midrib of the 

 infested plants from MacMillan's strain in comparison with the others, 

 the characteristic "sub-normal" condition of MacMillan's strain re- 

 appeared, showing that the strains producing the disease were the ones 

 inoculated into the soil. A third series of inoculations was made, using 

 all the strains of F. oxysporum at hand. Infection was again obtained 

 with MacMillan's strain and with two of Wollenweber's original strains 

 but not with the others. 



In view of these results it appears that strains of Fusarium oxysporum 

 may vary considerably as regards pathogenicity, but whether this is a 

 true strain difference or merely one resulting from culturing can not be 

 stated. It was evident, however, that the tobacco-wilt Fusarium had 

 not suffered any loss in virulence from four years in culture, existing for 

 a large part of this time under unfavorable cultural conditions. If F. 

 oxysporum is as common in potato fields as a parasite and as common a 

 soil saprophyte as literature would lead us to believe, it is quite surpris- 

 ing to us that wilt of tobacco has not been more generally noted, provided 

 we assume the tobacco-wilt may be caused by F. oxysporum, since 

 tobacco and potatoes are frequently grown in close proximity and are 

 frequently rotated. This would be even more surprising when we add 

 that certain varieties of tobacco are apparently more susceptible to the 

 wilt than is the potato. 



As has been stated, no infection has been secured on potato with the 

 tobacco-wilt Fusarium, although this may sometime be accomplished. 

 In the early work attempts were also made to get infection on tomato, 

 cowpeas, and cabbage, but without results. Excellent infection has, 

 however, been secured upon Nicotiana glauca (California tree-tobacco) 



