574 
Journal of Agricultural Research 
Vol. XI, No. ii 
have a considerable influence on spore size in pure cultures. This 
organism, which is no doubt identical with F. lint, is the only pathogenic 
form of Fusarium isolated from flax during this work. 
strains or flax used 
In undertaking a study of the inheritance of wilt resistance one of the 
most important considerations was to obtain for the crosses strains of 
flax which were highly resistant and strains which were highly suscept¬ 
ible to the disease. A number of both resistant and susceptible strains 
were obtained from Prof. Bolley. Several varieties of common flax 
were also obtained from various places in North Dakota and Minnesota. 
Before the crossing work was begun all of these strains, or varieties, 
were thoroughly tested as to resistant and susceptible qualities on “ flax- 
sick soil” from North Dakota. Plants for these tests, as well as for all 
infection experiments, were grown in the greenhouse. Some of the 
strains proved satisfactory under greenhouse conditions, while others did 
not. All of the resistant strains except North Dakota Resistant 114 were 
discarded, that variety being used exclusively, as it proved to be supe¬ 
rior in resistance to all other strains tested (PI. 44, B, a). This strain is 
designated as No. 4 throughout this work. Prof. Bolley in sending the 
seed of No. 4 says: 
This flax has been growing on the North Dakota State grounds for a number of 
years and ought to *be highly resistant to wilt. 
The most satisfactory strain of susceptible flax used is strain 3 (North 
Dakota Pure-Seed Laboratory No. 14654; PI. 44, B, b ). Bolley states 
that this strain died out completely on his seed plots at the North Dakota 
station and should be well suited for w'ork of this kind. Two other 
strains of common flax have been used to some extent, but have not 
been so satisfactory as No. 3 in giving uniform results. One of these, 
No. 5—a white-flowered variety(Pl. 45, B, b )—was obtained from a 
linseed mill at Red Wing, Minn. This was the only white-flowered variety 
tested, and, although it was found to be entirely susceptible, it does not 
wilt so rapidly and uniformly as No. 3. The other, No. 6 (PI. 45, A, d)> 
is a strain of common flax obtained from Mr. M. S. Kirk, Devils Lake, 
N. Dak. It is similar to No. 3, but is slightly less susceptible. 
definition of The problem 
The first available remedy for controlling fiaxwilt was introduced with 
Bolley’s (4) discovery of the cause of the disease and his selection of 
flax plants for wilt resistance. The variation of such disease resistance 
of individual plants within a variety was the basis for Bolley’s selection. 
This method of selection has likewise been successfully employed in 
obtaining disease-resistant strains of other cultivated plants (15, 18 , 20, 
21). In connection with this improvement by selection, there has 
