July is, 1914 
Influence of Host on Puccinia 
3 i 5 
Table) V .—Incubation period of each rust 
Incubation of aecia. 
Incubation of uredinia. 
Species of rust. 
Host inocu- 
Period. 
Host inocu- 
Period. 
lated. 
Year. 
Range. 
Aver¬ 
age. 
lated. 
Year. 
Range. 
Aver¬ 
age. 
fViola spp. 
Days. 
Days. 
Days. 
Days. 
Puccinia ellisiana. 
1912 
15 to 24 
20 
Andropogon.. 
1912 
12 
12 
.do. 
1913 
13 to 25 
18 
.do. 
1913 
1914 
17 
13 to 18 
17 
1914 
15 to 25 
.do. 
16 
Do. 
[Pentste mon 
{ sp. 
1.do. 
1913 
15 to 18 
17 
.do. 
1913 
17 
17 
1914 
18 to 31 
23 
.do. 
1914 
9 to II 
10 
Puccinia ellisiana 
[Viola spp. 
1914 
18 to 29 
*3 
.do. 
1914 
IO to 16 
13 
from Pentste- 
■[Pentstemon 
1914 
14 to 21 
IS 
.do. 
1914 
10 
IO 
mon. 
l sp. 
[Pentstemon 
1913 
16 to 17 
17 
12 to 13 
Puccinia afadro- 
S P-. 
\ .do. 
.do. 
pogonis. 
1914 
17 to 21 
19 
8 to 14 
II 
Viola spp. 
1913 
1914 
1913 
1914 
[ .do. 
19 to 22 
aI 
Andropogon.. j 
10 to 14 
12 
When an attempt is made to carry the rust on Viola spp. (Puccinia 
ellisiana) back from Pentstemon to Viola, the incubation period is materi¬ 
ally lengthened. This is shown in Table V under “ Puccinia ellisiana from 
Pentstemon, Viola spp., 1914,” where the incubation stage ranges from 
18 to 29 days, with an average of 23 days, while the same rust on Pent¬ 
stemon ranges from 14 to 21, with an average of 18 days. The same 
lengthening of the incubation period also occurs when Puccinia andropo- 
gonis is carried over to species of Viola. In this case the range is 19 to 
22, and the average is 21 days. In general, the incubation period in the 
change from Viola to Pentstemon is more uniform than in the change from 
Pentstemon to Viola. 
The short incubation period shown in Table V for the urediniospores 
for 1914 is due to the fact that the blades of Andropogon were very young 
and tender when inoculated. This condition of the grass was obtained 
by cutting all the first leaves off and forcing a new lot to develop in the 
greenhouse. On leaves which had developed in cold frames and then 
inoculated the incubation period was perceptibly longer. 
A comparison of the averages of the incubation periods given in Table 
V for all the rusts on violets with those on Pentstemon shows that on the 
violet the general average is 21 days, while on Pentstemon it is 19 days. 
After making due allowance for the other factors known to influence the 
incubation period of rusts, this indicates that the aerial host and not the 
species of rust is the main factor in determining the variability and 
average length of the incubation period. 
