Medical Officers of the Army of India.
105
blight is liable to recur. The importance of the production of sclerotia lies in the
fact that it is this which provides for the persistence of the blight from year to year,
in spite of prolonged periods during which external conditions are such as to
preclude its activity. So long as external conditions are favourable, very
extensive blighting may be effected by means of mere ordinary vegetative growth
of mycelium, but, in the absence of sclerotia, there would be no tendency to any
regular annual recurrence of the disease. It is evident, then, that any species in
which parasitism has been so slightly specialised that sclerotia may be produced
in connection with very different hosts, is likely to be more mischievous than
one in which, although vegetative growth of mycelium may take place freely in
connection with various hosts, the production of sclerotia is limited to cases
where the growth occurs within one, or, at utmost, a limited number of hosts.
The importance of the production of sclerotia is strikingly demonstrated
by certain phenomena of annual recurrence in Calcutta. Every autumn large
unsightly patches of blighted foliage make their appearance on surfaces covered
by Ficus stipulate. They normally appear first at the level of the surface of the
soil, and gradually ascend thence in radiant fashion over areas whose magnitude
varies with the nature of the atmospheric conditions which characterise particular
seasons. When atmospheric humidity remains persistently excessive over
prolonged periods, the extension of the blight is correspondingly great ; when it
is relatively low and does not last long, the disease remains comparatively limited
and in any case, with the onset of the cool and relatively dry weather at the
beginning of winter, it is abruptly arrested. If the leaves within a blighted area
are examined, it will be found that towards the margins of it and where they are
only beginning to be discoloured, their under surfaces are coated with a web of
white mycelium, which, in some cases, extends directly from leaf to leaf, and, in
others, reaches successive leaves by means of connecting white strings which
run along the course of the axis on which they are situated. Where such a
string reaches the base of a petiole, it travels out along it and spreads out over
the under-surface of the lamina in a layer of divergent white filaments. These
adhere closely to the epidermis, and, after they have been in contact with it for
a short time, the substance of the lamina becomes first ochreous and ultimately
quite dead and brown. In the case, therefore, of leaves in which the disease is
still in progress, the under surface shows first areas already discoloured and
covered by mycelium ; secondly, and immediately adjoining these, others clothed
with a layer of mycelium, but still green ; and finally, others which are as yet quite
healthy. When the leaves have become completely killed and discoloured,
sclerotia begin to make their appearance. They, unlike the common vegetative
mycelium, are in most cases situated on the upper surfaces of the leaves, and occur
specially abundantly there towards the margins of the laminae (Plate II, Fig. 10).
They are developed from tufts of mycelium which break out at points in the
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