EFFECT OF THE ORANGE-RUSTS OF RUBUS ON THE 
DEVELOPMENT AND DISTRIBUTION OF STOMATA 1 
By B. O. Dodge 
Pathologist , Fruit-Disease Investigations , Bureau of Plant Industry, United States 
Department of Agriculture 
In order to obtain a supply of teleutospores with which to carry on 
some experiments on infecting the black raspberry (Rubus occidentalis) with 
the gametophytic stage of Gymnoconia, the writer has frequently sowed 
aecidiospores when the topmost leaves of new canes were still folded, 
while the lower leaves and all those on old canes were fully expanded. 
Although many trials, both in the field and in the greenhouse, have 
resulted in failure to obtain infection, the most abundant production of 
teleutospores followed inoculation of leaves of the latter type. It has 
been shown repeatedly that aecidiospore germ tubes gain entrance into 
a leaf through the stomatal openings. Clinton 2 reports that the stomata 
of leaves of blackberries and raspberries are confined to the lower surface 
except for a few which occur along the margin on the upper side. He 
believes, therefore, that infection by aecidiospores of the orange-rust must 
occur at a time when the side bearing the stomata is exposed to receive 
the spores falling from above, which would be when the leaves are 
beginning to unfold. The ventral surface is then turned outward and 
somewhat upward, and the halves are so folded together as to expose the 
margins perfectly to catch the spores. As the results of our experiments 
did not strongly support the theory that leaves are the most readily 
infected before they unfold completely, a study was made of the develop¬ 
ment and distribution of stomata on various types of leaves from black¬ 
berries and raspberries. The fact that agitation by the wind and the 
work of insects might well carry spores to the underside of fully expanded 
leaves would not minimize the importance of the distribution of stomata 
in facilitating infection. In the course of this work it was found that 
the invasion of leaves by the orange-rust mycelium has a very unusual 
effect on the production of stomata. 
If account is taken of the conditions of the leaves of our blackberry or 
raspberry in nature at the time aecidiospores are being shed, it will be 
seen that several types are exposed to infection. The first leaves to 
unfold are the ones on the normal old canes; the infected old canes are 
also developing their leaves at this time. Practically all leaves on the 
old canes, normal and infected, are of about the same age. New shoots 
from the base of infected and of uninfected plants grow up a little later. 
During the growth period of the new canes there will always be leaves 
just unfolding at the top, and others lower down fully expanded. 
In general, most of the stomata on normal leaves of Rubus are on the 
lower side, as stated by Clinton. A number of species of this genus have 
a few on the upper side at the tips of the serrations. Some blackberry 
leaves examined have a small number irregularly scattered, singly or 
in groups of two or three, on the upper side, especially along the larger 
1 Accepted for publication June 25, 1923. 
* CUNTON, G. P. ORANGE RUST OF RASPBERRY AND BLACKBERRY. Ill. Agr. Exp. Sta. Bui. 29, p. *73- 
300, 4 pi. 1893* Literature, p. 292-295. 
Journal of Agricultural Research. Vol. XXV, No. 12 
Washington. D. C. Sept. 22.1923 
agn. Key No. G-330 
(49s) 
60097—23-3 
