Observations and Experiments on Cereal Rusts. 157 
For the reasons given above the writer has come to the con- 
clusion that the unprecedented late appearance of yellow rust 
in the year 1922 in the locality under report was due to the 
absence of a local source of infection during the autumn and 
winter of 1921-22. Further, that the source of infection for the 
rust which was noticed in June was in all probability wind- 
blown uredospores coming from a place* distinctly cooler or 
such where the uredo-stage (perhaps under protection) had 
escaped injury in the summer of 1921. The fact that weather 
conditions in the summer of 1921 (exceptionally hot as it was) 
were most unfavourable for the infection of aftermath and self- 
sown plants by yellow rust will be discussed later. 
While summing up it may be remarked that negative results 
from the cultures of exceedingly susceptible varieties, even 
when exposed in the open, under conditions of weather un- 
doubtedly favourable for the growth of the fungus go a step 
further to prove that there is no hereditary source of infection. 
(c) Overwintering of uredospores. 
As regards the viability of the uredospores of this rust it may 
be stated that inoculations conducted with material from the 
open several times during the winter and spring of 1920-21 
gave satisfactory results and the spores invariably showed good 
germination. 
6. INFLUENCE OF TEMPERATURE ON THE VIABILITY 
OF UREDOSPORES. 
Marshall Ward; found that the minimum temperature at 
which the uredospores of P. disfpersa on Bromes germinate lies 
between 10-12° C., the optimum being at or near 20° C., and 
the maximum between 26-27:5°C. Johnson has stated that 
the uredospores of P. graminis and P. Rubigovera can germi- 
nate between 2-31°C., the optimum lying between 12 and 
ay C. 
As far as P. graminis is concerned the writer has found that 
it shows better germination at 29-30°C. than at 2-3°C. At 
22-23° C. germination of more than 80% spores of this rust 
has frequently been observed. The uredospores of P. triticina 
on the other hand show better germination at 2-3° C. than at 
29-30°C.; at 22-23°C. it does not show more than 50% 
germination. Lastly P. glumarum shows 20-30 % germination 
* Early in the year 1922 yellow rust of wheat (though rare) was available 
in some parts of England. The writer obtained some material from Harpenden 
in January. 
¢ Ward, H. M., Ann. Bot. xv, p. 560 (1901). 
¢ Johnson, E. C., Phytopathology, 11, Reviews (1912). 
