CAN BIOLOGIC FORMS OF STEMRUST ON WHEAT 
CHANGE RAPIDLY ENOUGH TO INTERFERE WITH 
BREEDING FOR RUST RESISTANCE ? 1 
By E. C. Stakman , 2 Head of the Section of Plant Pathology, Division of Plant Pathology 
and Botany, Department of Agriculture, University of Minnesota, and John H. 
Parker and F. J. PiEMEisEL, formerly Scientific Assistants, Office of Cereal Investi¬ 
gations, Bureau of Plant Industry, United Stales Department of Agriculture 3 
COOPERATIVE INVESTIGATIONS BETWEEN THE AGRICUETURAE EXPERIMENT 
STATION OE THE UNIVERSITY OE MINNESOTA AND THE BUREAU OF PEANT 
INDUSTRY OE THE UNITED STATES DEPARTMENT OE AGRICUETURE 
INTRODUCTION 
Doubt has often been expressed whether it is possible to breed cereals 
permanently resistant to rusts. The opinion is commonly held that 
either the newly developed resistant variety loses its resistant quality or 
that the particular rust in question adapts itself to the new variety. The 
fundamental facts ought to be known if breeding is to be successful. 
There are two main possibilities, besides mutation, as far as the rust is 
concerned. It is possible to assume that a highly resistant variety may 
occasionally be weakly infected by the rust and that, as a result of its 
sojourn on the host, the rust acquires additional virulence, thus enabling 
it to infect the variety with progressively greater ease. If this assumption 
be true, the degree of virulence of a rust on a particular cereal variety 
should be directly proportional to the length of its association with that 
variety, or a physiologically similar one. The second possibility is that 
the rust may be changed by a closely related host variety, species, or 
hybrid. That is, assume the presence in a breeding plot, or in a wider 
area, of varieties completely susceptible (S), moderately susceptible 
(S—), moderately resistant (R—), and highly resistant (R) to a given 
rust. The rust from S might not be able to pass directly to R, but 
might be able to pass to S—, thence to R—, and thence to R. S— and 
R— would therefore act as intermediaries or bridges between S and R. 
The rust having once grown on R could then continue to infect R. 
If such hypothetical cases as cited above actually occur in nature, the 
value of breeding for rust resistance would be problematical. It was 
for this reason that the work reported in this paper was undertaken. 
1 Published, with the approval of the Director, as Paper 120 of the Journal Series of the Minnesota Agri¬ 
cultural Experiment Station. 
8 On leave. 
3 The writers wish to acknowledge valuable suggestions by Prof. H. K. Hayes, Head of the Section of 
Plant Breeding, Division of Agronomy and Farm Management, Department of Agriculture, University of 
Minnesota; efficient assistance by Mr. M. N. Eevine, Field Assistant, Office of Cereal Investigations, Bureau 
of Plant Industry, United States Department of Agriculture, in the work with Puccinia grammis tritici- 
compacti, and by Mr. Olaf S. Aamodt, formerly Scientific Assistant, Office of Cereal Investigations, United 
States Department of Agriculture, for painstaking work with both strains of rust used. 
Journal of Agricultural Research, 
Washington, D. C. 
oi 
(in) 
Vol. XIV. No. 3 
July 8,1918 
Key No. Minn. 31 
