224 
Journal of Agricultural Research 
Vol XXV, No. 5 
TYPES OF INFECTION 
The value of infection work lies not so much in the fact that one 
acquires the technic of infecting the host, as in the knowledge of the host- 
parasite relationship to be gained by a study of the plant structures 
under most favorable conditions. During April and the first week in 
May, 1922, as an infected plant became noticeable, it was dug up so as to 
include the root from which it had sprung, and also, in many cases, all 
other plants connected with the same runner. Pieces of the canes and 
roots at various points were fixed in Flemming’s Medium Fluid and later 
sectioned and stained. After a careful study of many phases of the 
question, it was seen that infected plants could be arranged in three or 
four groups, based upon the manner of the primary attack by the fungus 
and the reaction of the plant to the attack. 
A. This group includes those plants which were, as shoots, infected in 
or near one or more axial buds below the growing region, so that the shoot 
grew into a normal cane which blossomed the next spring and would have 
borne fruit if allowed to live. Rust appeared on the leaves (of the old 
cane) at a few of the lower nodes as the result of a more or less local 
infection. New shoots, systemically infected, would arise from the base 
of the old cane, the one originally inoculated, provided the hyphae had 
penetrated downward into the underground part of the stem or root. 
There should be, therefore, in this group three subclasses based on the 
extent downward to which the hyphae had penetrated: (1) Those 
infections which were so localized as to be unable to reach the underground 
, parts; the result would be a local infection which would disappear with the 
natural death of the cane at the end of the season. (2) Those cases where 
hyphae penetrated through the cambium or phloem down the stem 
beneath the soil sufficiently far to stimulate the development of new 
shoots into which the mycelium could penetrate and live over another 
year. By merely pulling up such canes so that they break off at the root 
would be all that is necessary to free the plant from the rust. If such a 
plant is allowed to live through the spring, however, the rust would be 
able to establish itself permanently in the crown and roots. (3) The 
third type would include all cases where the root has been reached by the 
fungus the first season. In such types the connecting horizontal root 
must be destroyed. 
B. The small number falling in the second group includes those plants 
in which the fungus soon became established in the growing region of the 
shoot or axial bud, with the result that the whole cane or branch became 
systemically infected. Such types can easily be recognized the following 
spring by the appearance of the new shoot which arises at the end of the 
old cane, or its branch, by proliferation of the terminal bud which has 
remained alive over winter. 
C. The third group includes those types in which the root was directly 
infected so that the fungus entered the phloem. In such cases the root 
lay near the surface of the ground. Such a type of infection results in 
the formation of a witch’s broom at once and the rust becomes fairly 
well established. Since the mycelium in such cases has probably not 
penetrated either way in the root more than a few inches, the parasite 
can be destroyed by pulling out the section of the root which bears the 
infected shoots. Each of these types of infection will now be considered 
in detail. 
