Journal of Agricultural Research 
Vol. XXIV, No. 12 
980 
However, the investigations of Stakman and others {28, 2g, 21, 20, 26) 
have proved that there are a number of biologic forms of this rust. 
A total of 37 different biologic forms of Puccinia graminis tritici have 
been isolated at the Miimesota station by Stakman and Levine (25a). 
No one form has been found to attack all varieties of Triticum species and 
no wheat variety has been found to be resistant to all rust forms, except 
Khapli emmer, a variety of T. dicoccum. The exact distribution of 
each biologic form is not known (19). Perhaps not half of the 37 forms 
are present in any one wheat-growing district. For example, only 14 
forms had been collected up to December, 1921, in western Canada (2j). 
However, as it is possible that some forms may spread into new dis¬ 
tricts, the problem of breeding for resistance means working for a variety 
resistant to all known forms of rust. 
The study of disease resistance in wheat dates back over a century. 
According to Biffen (j), Thomas Elnight, in 1815, suggested that dis¬ 
ease-resistant varieties might be raised, and Farrar, in 1889, stated 
that susceptibility to rust was hereditary in wheat. Biffen (j), in 1905, 
crossed Red King, a variety susceptible to Puccinia glumarum (Schm.) 
Erikss. & Henn., with Burt, a resistant variety. The F^ progeny was 
susceptible and the Fj progeny segregated into susceptible and resistant 
in a 3: I ratio. Later, Biffen (4) stated that the relatively immune 
hybrid forms of wheat bred true and that the immunity was independent 
of any discernible morphological character, that is, the factors for resist¬ 
ance were inherited independently of other factors. More recently, Biffen 
(5) reported the production of a rust-resistant wheat which likewise 
possessed other desirable economic characters. 
Some excellent results in breeding for disease resistance in crop plants 
have been reported by various workers, but the problem is rarely as 
difiScult as in the case of stem rust of wheat. Gaines {12) crossed two 
varieties of wheat highly resistant to bunt and obtained in the Fg some 
plants which were more susceptible than either parent and a large 
percentage which were immune. He concluded that the two varieties 
had different factors for resistance. Garber (ij), in 1921, found that 
resistance to P. graminis avenae ^rikss, & Henn. was a dominant char¬ 
acter, a ratio of 3 resistant to i susceptible being obtained in Fg. 
A definite grouping of the wheat subspecies according to their reaction 
to P. triticina Erikss. was made by Vavilov (51) in 1914. The use of inter¬ 
specific crosses in breeding for rust resistance is liable to some complica¬ 
tion owing to the linkage of certain factors. In 1917, Freeman (10) 
reported c^es of linked quantitative characters in wheat crosses. In 
1919, Hayes, Parker, and Kurtzweil (16) found that in crosses of varieties 
of T. vulgare with varieties of T. dicoccumy resistance was dominant. In 
crosses of T. vulgare with T. durum susceptibility was dominant and in 
Fg there was strong linkage between rust resistance and the durum 
character. The significant fact, however, was that some crossing-over 
occurred. It was apparent, therefore, that there was a possibility of 
transferring the resistance of durums or emmers to common wheats. 
Puttick (54), in 1920-21, studied the reaction of the Fj generation of 
a cross between a common and a durum wheat to two biologic forms of 
P. graminis tritici to which these varieties reacted reciprocally. Fj 
seedlings were inoculated with one rust and later the infected leaves 
were cut off and the plants were inoculated with a second rust form. 
All gradations between complete susceptibility and immunity to both 
