Observations and Experiments on Cereal Rusts. 143 
PART I. 
INCIDENCE AND MEANS OF OVERWINTERING. 
I. INTRODUCTION. 
The annual origin of rusts on cereals has been the subject 
of numerous investigations. In recent years several valuable 
contributions have been made on different aspects of the 
problem, attaching special importance to one factor or the 
other as the cause of infection. 
The chief sources of ‘‘ primary” infection, as has been pointed 
out, might be (i) uredospores that may survive on the cereals 
through the critical periods, (ii) an inherited germ of disease 
within the seed grain, as suggested by Eriksson*, (iii) sori within 
the seed grain, (iv) aecidiospores on some alternate host, 
(v) sporidia arising from teleutospores, (vi) uredospores living 
during the critical period on other grasses. 
As far as England is concerned, one finds that no special 
study of the problem has been made. In literature one comes 
across only a few general statements based mostly on observa- 
tion of the fact that the uredo-stage of some rusts can be found 
every now and then on cereals or wild grasses even during winter 
and the earlier part of spring—Plowright+, Ward? and Grove§. 
It was desirable therefore that a detailed investigation of the 
problem should be undertaken at an important wheat growing 
area in this country. 
The work was commenced at the suggestion of Mr F. T. 
Brooks in October 1920 and carried out for nearly two years 
at the Cambridge Botany School. 
In this part of the paper I propose to give a brief account of 
the incidence of rusts on wheat in the vicinity of Cambridge 
and also to discuss the relative importance of the various factors 
which go to explain the origin of rust outbreaks year after year. 
Throughout the period under report, field observations were 
made as far as possible once a week at the Cambridge University 
farm and occasionally also at other farms in the vicinity. In 
addition to the rusts of wheat, observations were regularly made 
on the black rust of Couch grass (Agropyron repens). During 
the latter half of the period casual observations were also made 
on the rusts of barley and rye. 
Side by side with field work, extensive culture work was 
* Carleton, M. A., U.S. Dept. Agric. Div. Veg. Phys. and Path. Bull. No. 16 
(1899); Eriksson, J., Compt. Rend. cxxiv, p. 475 (1897). 
{+ Plowright, C. B., Gard. Chron., N.S. xviii, p. 234 (1882). 
t Ward, H. M., Ann. Bot., x1x, p. I (1905). 
§ Grove, W. B., British Rust Fungi (1913). 
