Observations and Experiments on Cereal Rusts. 147 
existence at very distant places, makes spring infection of wheat 
inexplicable, and has suggested the possibility of infection of 
wheat due to teleutospores being of greater importance. 
Lastly Eriksson* inclines also to the view that propagation 
to and from the aecidial host is very slight. 
On the other hand one learns from the contributions of 
Plowright}, Stakman and Piemeiselt, Freeman and Johnson §, 
Lind ||, Giissow], Broadbent ** and others that there is a definite 
connection between the aecidium on barberry and the distri- 
bution of black rust on graminaceous hosts. To avoid repetition, 
this factor and the next two will be discussed in detail later in 
the text. 
The present work was taken up chiefly with the object of 
finding out (i) if one could satisfactorily explain the fresh attacks 
of different rusts on wheat in spring, taking the uredospores 
surviving on self-sown plants during winter as the source of 
infection; (ii) whether the amount of such uredospores could, 
with a reasonable amount of certainty, be held responsible for 
the more or less spontaneous outbreak of rusts over large areas 
of winter wheat; (iii) if one could find the cause of the more or 
less fixed sequence in the appearance of the three rusts; and also 
(iv) the relative importance of some of the other factors already 
referred to. 
3. Back Rust (Puccini4 GRAMINIS Pers.). 
(a) Incidence. 
This rust does not appear on graminaceous hosts until late 
in the season. In the year 1921 it was first noticed towards the 
end of June on Couch grass (Agropyron repens). In the vicinity 
of Cambridge it is not common, and it has not been found 
on any other local wild grass. Amongst the cereals it was ob- 
served on wheat alone in the year 1920 and only on barley last 
year (1921). 
(1920-21.) There was plenty of the uredo-stage on wheat 
(self-sown) and Couch grass in October but later on no fresh 
uredo-sori were observed. In January rgzr one could find only 
old and disfigured uredo-sori. No black rust was observed at 
* Eriksson, J., Bot. Gaz. xxv, p. 26 (1898). 
t Plowright, C. B., Gard. Chron., N.S. xvi, p. 234 (1882). 
{ Stakman, E. C. and Piemeisel, F. J., Journ. Agr. Res. x, p. 429 (1917). 
§ Freeman, E. M. and Johnson, E. C., U.S. Dept. Agr. Bur. Plant. Indus. 
Bull, p. 216 (1911). 
|| Lind, J., Danish Fungi as represented in the herbarium of E. Rostrup 
(1913). 
{| Gissow, H. T., Phytopathology, 11, p. 178 (1913). 
** Broadbent, W. H., Journ. Minis. Agric, XXVIII, p. 117 (1921), 
1Io—2 
