MORPHOLOGICAL AND PHYSIOLOGICAL STUDIES ON 
THE RESISTANCE OF WHEAT TO PUCCINIA GRAM- 
INIS TRITICI ERIKSS. AND HENN. 1 
By C. R. Hursh 2 
Agent, Office of Cereal Investigations, Bureau of Plant Industry , United StatesDepartment 
of Agriculture, and Assistant Plant Pathologist, Department of Agriculture, University 
of Minnesota 
INTRODUCTION 
Many conflicting statements have been made regarding the nature of 
resistance of wheat varieties to Puccinia gramims tritici Erikss. and 
Henn. It is clear, also, that there are many different opinions re¬ 
garding the degree to which this resistance can be changed. For this 
reason it seemed desirable to investigate the factors which affect the 
virulence of rust attacks. 
There are two conceivable reasons for the resistance of plants to fungous 
attack—morphological peculiarities and physiological resistance. 
There may be mechanical obstacles to the entrance of the germ tubes 
of the rust fungus into the tissues of the host. While the host cells may 
be quite susceptible after the germ tubes once have entered, it is quite 
conceivable that so few may enter that the plant is protected against 
heavy attack. The number of hairs on the surface of the plant, and the 
number, size, disposition, and peculiarities of movement of the stomata, 
conceivably could exert a considerable effect upon the entrance of the 
germ tubes. 
If the pathogene is unable to obtain proper food materials from a 
certain plant, even after it has entered, the possibility of its parasitizing 
that plant is precluded, of course. Furthermore, if toxic substances 
occur in the tissues of the plant, the pathogene will be weakened or killed. 
It is possible, however, that the cells of certain tissues may be quite sus¬ 
ceptible to the attacks of a pathogene but that the number of these cells 
may be so limited as to make it impossible for the organism to develop 
extensively. A plant therefore may be physiologically susceptible and 
morphologically resistant. There appeared to be good evidence from 
field observations that certain varieties of wheat apparently were re¬ 
sistant to certain biologic forms of P. graminis merely because there 
were not sufficient tissues in the host plant in which the rust parasite 
could develop. If the plant develops a large amount of woody tissues, 
it is clear that the area in which rust hyphae can grow is rather limited. 
It is well known that the morphology of wheat plants may be affected 
by temperature, light, moisture, and soil nutrients. Just to what extent 
environmental factors could affect the development of rust indirectly by 
affecting the structure of the plant has not been shown. It frequently 
1 Accepted for publication Nov. 19, 1923. Cooperative investigations between the Office of Cereal 
Investigations. Bureau of Plant Industry. United States Department of Agriculture, and the Department 
of Agriculture of the University of Minnesota. Published with the approval of the Director as Paper No. 
401 of the Journal Series of the Minnesota Agricultural Experiment Station. 
* The writer takes pleasure in acknowledging his indebtedness to Dr. E. C. Stakman, Professor of Plant 
Pathology, University of Minnesota, and Pathologist, Office of Cereal Investigations, Bureau of Plant 
Industry, United States Department of Agriculture, for suggesting the problem and for much helpful 
criticism during its prosecution. 
Vol. XXVII, No. 6 
Feb. 9, 1924 
Key No. G-358 
Journal of Agricultural Research, 
Washington, D. C. 
(38*) 
