Medical Officers of the Army of India.
103
elements appeared to be strictly confined to the wood ; the tissues of the bast,
cortex, and pith being seemingly entirely free from them. Within the wood,
they were present in the interior of the spiral vessels, the wood-cells, and the
great pitted ducts. In the latter they were specially abundant, and it was here
that the sclerotia giving rise to the streaks and points of brown discoloration,
previously alluded to, were present (Plate I, Figs. 5-11). The disease appeared
to be limited to the base of the haulm in its subterranean and aërial portions, and
a little farther up no traces of it could be detected. The roots appeared to be
entirely free of fungal elements, and, what is more remarkable, the tubers also
appeared to be entirely exempt even in the sites in which the brown discolora
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tion forming the so-called " bangle " was present.
The characters of the disease are, therefore, typical of those dependent on
the presence of a Sclerotinia or other closely allied form of Ascomycete parasite.
The distribution of the fungal elements within the host, the presence of
sclerotia, and the history of the conditions of cultivation under which the
blight originates, are all equally characteristic, and, under these circumstances,
there can be no doubt whatever in regard to the preventive measures which
ought to be adopted. The source of recurrent infection lies unequivocally in
the sclerotia which are situated within the substance of the vascular bundles,
and which, after lying dormant for a time in the dead tissues, are the origin of the
reproductive elements which invade the new crop of the host-plant, and the
germination of these elements and the growth and diffusion of the resultant
mycelium must assuredly be favoured by any conditions giving rise to saturation
of the soil. It is thus evident that the methods of cultivation described by Mr.
Mollison as prevailing near Poona are precisely those which are specially adapted
to secure the persistence and extension of the disease. The continuous cultivation
of the host-plant year after year within the same areas must tend effectually
to secure the constant presence in the soil of sclerotia, and the excessive irriga
-
tion must provide special facilities for their farther evolution and for the growth
and diffusion of the infective elements ultimately derived from them. But, if
this be so, it is equally evident that the proper preventive measures are the care
-
ful removal and destruction of all the diseased portions of blighted plants, the
abandonment of the system of continuous culture of potatoes within the same
areas, and the avoidance of all excessive irrigation.
As the group of blights, of which this is a. typical example, would appear
to be very prevalent in India, but at the same time to have received very little local
attention, it may, for practical purposes, be well to note briefly what are the
distinctive features of it. The fungi which are the specific causes of the diseases
included in it are, so far as is known, all Ascomycetes distinguished by the
fact that the common vegetative mycelium does not give direct origin to a car
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posporic fructification, but to dense pseudo-parenchymatous masses of felted