lOOO 
Journal of Agricultural Research voi. xxiv, No. 12 
Explanation of classes of host reaction and of corresponding types of infection resulting 
from inoculation of seedling wheat plants with spores of Puccinia graminis tritici ® 
Classes of host reaction. 
Immune (immune in the field).^ 
Very resistant (highly resistant 
under field conditions). 
Moderately resistant (highly resist¬ 
ant under field conditions). 
Moderately susceptible (somewhat 
resistant in the field). 
Very susceptible (entirely suscept¬ 
ible under field conditions). 
2. 
3 - 
4. 
Types of infection. 
No uredinia developed but definite hyper¬ 
sensitive areas present. 
Uredinia minute and isolated; surrounded by 
sharply-defined, continuous, hypersensi¬ 
tive areas; hypersensitive areas lacking 
uredinia also may be present. 
Uredinia isolated and small to midsized; 
hypersensitiveness present in the form of 
necrotic areas in circles; uredinia often sur¬ 
rounded by green islands. 
Uredinia midsized; coalescence infrequent; 
development of rust somewhat subnormal; 
true hypersensitiveness absent; chlorotic 
areas, however, may be present. 
Uredinia large or varying from midsized to 
large, numerous and confluent; true hyper¬ 
sensitiveness entirely absent, chlorosis 
seldom present. 
The studies which have been made show that a pure-line wheat 
variety inoculated with a single biologic form frequently shows a varia¬ 
tion in infection from type 3 to type 4 and occasionally from type 2 to 
type 3, due to varying environmental conditions to which the seedlings 
are exposed, even when the conditions of the greenhouse test are kept 
as uniform as possible. These variations apparently are not due to 
changes in the genotype. 
The most striking differences in the infection of Kota and Marquis 
were obtained with Form XIX and Form XXVII and accordingly these 
two forms were chosen for the study. Form XIX was collected in 
Pocatello, Idaho, in November, 1918, and has been cultured through 
approximately 51 urediniospore generations. The Form XIX used to 
inoculate the F4 material was collected in North Dakota in July, 1921, 
and has been cultured through approximately 17 generations. Form 
XXVII was collected in Barges, France, in August, 1919, and has been 
cultured through approximately 45 urediniospore generations. 
Between 15 and 20 seeds of each individual F2 plant were sown in a 
single pot, but, owing to the unfavorable conditions under which the 
F;5 plants were grown, the germination of these seeds was low. No 
pots were used in the study in which the number of Fg plants per pot 
fell below 8. While 8 plants is much too small a number to prove abso¬ 
lutely the genetic nature of the individual Fg parent, it gives a fair 
indication of the genetic class in question. Larger numbers of seeds 
could not have been used without depleting the quantity of seed 
reserved for sowing to obtain Fg families under field conditions. The 
results obtained wiQi the small numbers of seed used are reliable enough, 
however, to furnish a good idea of the approximate number of factors 
involved, and, from ^e plant-breeding standpoint, the information 
obtained is sufficient to answer the main questions for which the 
investigation was undertaken. It is recognized that from the genetic 
standpoint larger numbers of seedlings in each Fg family should be 
inoculated. 
» These descriptions of types of infection in the greenhouse are based on those given by Stakman and 
Levine (d). 
