238 
Journal of Agricultural Research 
Voi. XXV, No. s 
other 48 trials. The number infected in each case is as follows: Cran¬ 
dall, 8; Iceberg, 23; Crystal White, 26; Kittatinny, 30; and Mercereau, 
38. The variety which was shipped to the writer as the Mercereau 
appears to be the most susceptible. Many infections of this variety 
were obtained merely by spraying shoots or root sprouts with sporidia 
without using an artificial humidifier. The results obtained in later 
experiments, 1922-23, but not included in this summary and not given 
in the table, furnish further evidence that our commercial varieties of 
blackberry vary exceedingly in the degree to which they are susceptible to 
orange-rusts. 
The Oregon Evergreen (Black Diamond) blackberry is said to be very 
resistant to orange-rust, yet the writer found that it could be easily 
infected by sowing aecidiospores from wild blackberry on its fully 
expanded leaves, teleutospores developing in about six weeks. This 
variety, however, may be at the same time very resistant to the gameto- 
phytic stage of the long-cycled rust, and to the short-cycled form as well. 
The southern dewberry, Rubus enslenii, which is subject to attack by 
the short-cycled orange-rust, was readily infected by sowing aecidio¬ 
spores of the long-cycled Gymnoconia from black raspberry on the leaves. 
So far as the writer knows, the aecidial stage of this form has not been 
found on Rubus enslenii. 
orange-rust aecidia on canes and FLOWERS 
If the mycelium, lodged in the perennial underground parts of a black¬ 
berry, penetrates a shoot bud and grows up with the cane, this cane, or 
any part of it bearing hyphae, does not blossom. The localized primary 
infections described previously are not included in this category, because 
the mycelium would be unable to reach the growing point and thus grow 
up with the cane. What at first appeared to be an exception to this 
rule was noted at Salem, N. C., where several plants of the wild dewberry, 
Rubus enslenii , infected with the short-cycled orange-rust, bore aecidia 
on the fruit branches, leaf stalks, and even on the calyx of flowers which 
were being formed in the normal fashion. These were cases of primary 
infection by sporidia; this was proved by a study of the distribution of 
the mycelium. 
witches’ brooms on old canes 
The ultimate effect of the orange-rust on most plants if not disturbed 
by pruning is to cause the canes to become dwarfed or spindling, and to 
grow out in large numbers from the infected crown. Such brooms should 
be distinguished from the small ones found at the nodes on certain old 
canes. It has been thought by some that these distortions are also 
caused by the rust which is so often found on the leaves at these points. 
The “double blossom” fungus appears to be able to attack the axial buds 
of canes infected with the orange-rust so that in the spring both parasites 
are found together, but it is the “double blossom” fungus that causes 
the formation of the brooms. These malformations are most common 
in North Carolina and Virginia, and do not often occur in northern 
regions on plants infected with the orange-rust, merely because of the 
absence of the “double blossom” fungus. It is the parasite that lives 
for the most part on the surface of the organs rather than the one found 
within the tissues, that stimulates excessive development of buds in old 
canes. 
