Observations and Experiments on Cereal Rusts. 161 
He also states as the chief argument in favour of his hypo- 
thesis, that normally the fresh outbreak of rust takes four to 
six weeks after sowing, and denies, on the strength of this ob- 
servation, that the source of infection could ever be the uredo- 
spores present at the time of sprouting. 
This interpretation of Eriksson is based on the epinion that 
the rust does not take more than eight to ten days from the 
time of infection to produce pustules. 
All the above phenomena, curious as they seem, are easily 
explicable in the light of the fact that the incubation period is 
not only not fixed at eight to ten days, but is most variable, and 
that temperature more than anything else is the regulating factor. 
While dealing with incidence it was stated that in October 
and a part of November 1920 there were a large number of 
self-sown plants of wheat practically covered with yellow rust 
growing just outside the cages, where winter wheat was sown 
in the first week of October. In the beginning of January 1921 
the rust was exceedingly rare and was found only as old pustules 
on a few withered leaves. 
On the 20th of January 1921 (about fourteen weeks after 
sowing) yellow rust was observed on the wheat crop inside the 
cages. On account of the exceedingly short distance (less than 
ten yards from some beds) between the rusted plants and the 
new crop, there could be very little doubt about the source of 
infection for the young seedlings. 
On the 3rd of February last year, some inoculations were 
tried on wheat seedlings, with material from the rusted plants 
in the cages, primarily to see if infection does take place in the 
open during the cold weather and if so how long the pustules 
take to appear. As a control inoculations were tried in the 
laboratory and the pot removed to a greenhouse at 60—70° F. 
The inoculated spots were clearly marked. 
1. Pot No. r. Seedlings inoculated in the open and kept in 
the open throughout. Pustules on the 36th day. 
2. Pot No. 2. Control. Pustules on the 13th day. 
Minimum temp. on the 3rd and 4th 23-25° F.) : 
Maximum >: = rs 43°8° F.J a he aT 
Unfortunately the germination tests were not very reliable 
as most of the spores sank to the bottom of the watch glasses. 
The experiment was therefore repeated a week later and the 
results were as follows: 
Pot No. r. Ina heated greenhouse throughout. 
Pustules on the 13th day. 
Pot No. 2. In a cool greenhouse throughout (45-60° F.). 
Pustules on the 28th day. 
M.S. II 
