Aug. 4 , 19 23 
Systemic Infections of Rubus with Orange-Rusts 
227 
LOCALIZED SYSTEMIC INFECTION MAY BECOME PERENNIAL 
Slight infections which may become constitutional if the canes are not 
destroyed are represented by No. 334 F (PI. 4, A). The shoot from which 
this plant had developed was originally infected near the soil level after 
inoculation April 23, 1921. A normal cane developed during the summer. 
Two or three new shoots, systemically infected, grew from the base of the 
cane the following spring, and rusted leaves were also noted at the lowest 
nodes of the old cane May 3, 1922. There were no hyphae in the root, 12 
inches nearer the parent plant, which was set out in 1920. This figure 
illustrates a common habit of growth of such horizontal roots which be¬ 
come larger after having given rise to a new cane. Fully two-thirds of 
the successfully inoculated plants of the Iceberg and Mercereau varieties 
had much the same type of infection as the one just described. If one 
merely pulls the cane from its horizontal root the spring following infec¬ 
tion, all of the structures containing the parasite might be destroyed, 
but if in this case the plant had been allowed to live, the parasite would 
have invaded new roots and shoots and become thoroughly established. 
Should the fungus have found conditions more favorable during the sum¬ 
mer for the extension of its mycelium downward, the type of infection 
illustrated by No. 112 C, described below, would have been found. 
Plate 4, B, illustrates a somewhat vigorous infection of the same type. 
The tip end of the shoot originally inoculated soon died, but this did not 
interfere with the activities of the parasite. An axial bud below imme¬ 
diately developed and is now represented by the large cane which is 
about to blossom, only the basal nodes of this old cane being infected. 
Three systemically infected shoots arise from the part beneath the soil. 
If the primarily infected old cane is broken off or cut away during the 
winter or in the spring before the sap begins to flow, a larger number of new 
shoots such as are shown in PI. 5, A, will develop, making a witch's broom 
type of infection. 
PRIMARY INFECTION BECOMES ESTABLISHED IN THE ROOT SYSTEM 
Supposing the fungus succeeds in reaching the root crown in its course 
down the stem, it is known from evidence presented previously that it 
travels along the root and readily makes its way into the buds, which in 
due time grow up as plants infected secondarily. To what extent does 
the fungus work back along the root toward the parent plant ? Examina¬ 
tions of sections of roots from a number of independently infected plants 
show that the fungus does invade the roots in both directions, but that 
the most progress is usually made outwardly. Occasionally, the reverse 
is true; the diagram of plant 112-C in figure 6 represents the conditions 
May 22, 1922, in a plant which was inoculated May 5, 1921, when it was 
a small shoot. The old canes and branches which grew in 1921, are 
indicated by shaded lines; the new shoots are shown in outline. The 
infection has ceased to be local and the fungus has established itself firmly 
in the root as a systemic parasite. All the leaves on the new shoots and 
leaves at several nodes of the old canes show rust. Buds are pushing 
out at the base of the old cane 3 inches below the surface of the soil. 
Six white shoots (C, C') are developing; a witch's broom is being formed 2 
inches farther back along the root. Sections of the root (C, C'), midway 
between the main stem and the group of small shoots, show mycelium in 
the medullary rays in the phloem near the cambium, and some in the 
cortex. Sections of the root 8 inches to the right of C' show no mycelium. 
