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Scientific Memoirs by
which may constantly be found on the leaves of shrubs subject to drip from trees
whose leaves are also affected by the alga.
In dealing with a blight of this kind, the great thing must necessarily be to
prevent it establishing itself in sites presenting specially favourable conditions
for the development of the alga. Cephaleuros occurs abundantly on most
diverse hosts in tropical regions, but, as a rule, confines itself almost solely to
leaves and produces mere localised mischief in these. The affection of the
leaves is, generally speaking, of no practical importance, but it may serve as a
source of serious mischief where local conditions are such as to favour the
excessive growth and multiplication of the algal elements and their invasion of
axial structures in which their presence may give rise to interference with the
general water-supply of the host-tissues. The presence of the alga in and on
leaves is readily recognisable from the conspicuous orange, lichenoid patches
which the superficial discs and tufts of fructifying filaments form on the foliar
surfaces, and the diffusion of the disease is a gradual process, and one which
can never occur over wide areas with the rapidity with which that of many
blights caused by the presence of parasitic fungi is effected. There can, there
-
fore, be little difficulty in successfully contending with it at the outset, and by
the careful removal and destruction of all conspicuously affected leaves when
it first makes its appearance, and by avoiding planting in sites exposed to the drip
from affected trees, any excessive extension of it, even within otherwise specially
favourable localities, might be effectually prevented. Where this has been
neglected, however, and it has been thus allowed to establish itself thoroughly and
to invade large areas of bark, more drastic measures will be necessary in the
form of free pruning or even total removal of entire shrubs and careful destruc
-
tion of all the diseased tissues. The relatively slow growth and the limited
means of diffusion of the alga are such that it can only, be as the result of the
neglect of such simple preventive measures that the disease can ever become of
any serious importance in any locality, but, given the neglect, it may be the
cause of very considerable mischief.
b. " Thread-blight."—Specimens of this disease have been sent to me by
Dr. King, who obtained them from the Darjeeling district, and by Dr. Prain, who
found it occurring as a cause of considerable injury to the cultivation of tea in
the Andaman Islands. The peculiarity to which it owes its name consists in the
presence of conspicuous white cord-like strands of mycelium which run along
the surface of the shoots from one leaf to another (Plate III, Fig. 5). They
consist of rhizomorphic aggregates of filaments which do not appear to be con
-
cerned in the direct acquisition of nutritive material, but merely to serve as means
of travel from one nutritive site to another. They are purely superficial, and may
readily be rubbed off, leaving the surface of the bark beneath them seemingly
quite intact. This, however, is not the case when the mycelium reaches the foliar