June 23,1923 
1015 
Bioloqic Forms of Puccinia graminis 
Tabl^ I .—Results of inoculating three differential varieties of Avena spp.y with four 
biologic forms of Puccinia graminis avenae 
Summary of inoculation results. 
Bio¬ 
logic 
Form 
No. 
Place of 
collection. 
Victory (C. I. No. 
1145)- 
White Tartar (Minn. 339). 
Monarch Selection (of 
Etheridge). 
m- 
ber of 
trials^ 
Character of 
infection. 
Num¬ 
ber of 
trials. 
Character of infection. 
Num¬ 
ber of 
trials. 
Character of 
infection. 
I 
St. Paul,Minn. 
19 
Normal,heavy 
8 
Infection light. Ure- 
6 
Infection exceedingly 
infection. 
dinia usually small 
light. Uredinia, 
U redinia 
and scattered. Hy- 
when present, mi¬ 
large, nu¬ 
persensitiveness 
nute, scattered and 
merous and 
(sharp chlorosis to 
surrounded by very 
coalescing. 
definite necrosis) or¬ 
sharply defined ne¬ 
dinarily present. 
crotic areas. Dis¬ 
tinct hypersensi¬ 
tive flecl^ also oc¬ 
cur quite frequent¬ 
ly. 
II 
San Marcos, 
16 
.do. 
8 
.do. 
6 
Normal, heavy in¬ 
fection. Uredinia 
Tex. 
large, coalescent 
and numerous. 
III 
Potchefstroom, 
7 
.do. 
5 
Infection moderate. 
3 
Do. 
South Africa. 
Upsala, Swe¬ 
den. 
Uredinia medium 
in size and only 
slightly confluent. 
True hypersentitive- 
ness absent, but 
light chlorotic areas 
usually present. 
IV 
2 
.do. 
2 
Normal, heavy infec¬ 
tion. Uredinia large. 
2 
Do. 
many, and conflu¬ 
ent. 
a From 10 to 15 plants were inoculated in each trial. 
These newly discovered biologic forms are quite as distinct as are 
those of P. graminis on wheat varieties {12). The inoculations were 
made on seedlings in the greenhouse, but, as a result of previous work, it 
is safe to conclude that the reaction of seedlings to rust forms is a fairly 
accurate index of the reaction of older plants. It usually is more difficult 
to obtain infection on older plants than it is on seedlings, but this appar¬ 
ently is due very largely to the fact that the film of water necessary for 
spore germination and entrance of germ tubes through the stomata, is not 
easily maintained on old plants. However, when this difficulty is over¬ 
come, by atomizing the plants several times a day, infection occurs 
normally. 
The new forms apparently are constant. Repeated inoculations and 
cross inoculations have been made with most of them and the results have 
been consistent. There are variations, of course, in the intensity of rust 
development when the environmental conditions are unfavorable for the 
development of host or parasite or both. When light intensity is low, and 
when the temperature is either too high or too low, the rust develops sub- 
normally. If the host plants are weakened by poor growing conditions, 
by mildew, root rots, or other factors, the rust usually does not develop 
well. This variation, however, does not indicate any change in the 
genotypic constitution of the rust forms, but is only the temporary result 
of environment. In order to draw accurate conclusions regarding the 
parasitic behavior of biologic forms it is necessary to grow the host plants 
and the rust fungus within the range of environmental conditions in 
