NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
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base of the stem and the larger roots. The mycelium was of characteristically
aseptate type save in the immediate neighbourhood of the oospores where a
few partitions could occasionally be recognised. The fructification was of two
kinds ; one presenting the character of asexual sporangia (Plate I, Fig. 15),
occurred abundantly within the tissues of the pith, whilst the other, consisting
of oospores (Plate I, Figs. 13-14), was present in smaller numbers there, but in
excessive abundance within the cells of the cortex. When portions of tissue
containing oospores were immersed in water germination presently occurred
with the formation of a branched aseptate mycelium, so that there could be no
question of the capacity of the fungus for saprophytic existence for some time,
at all events, as the tissues of the host were completely dead and consequently
incapable of providing for the requirements of a pure parasite.
The oogonia (Plate I, Fig. 14) were colourless and measured about 18µ, in
diameter. The oospores were considerably smaller than the oogonia, so that
they lay quite free within the cavity of the latter. They had a pale ochre tint.
No unequivocal evidences of the presence of antheridia could be detected, but
this does not necessarily imply a parthenogenetic production of the oospores as
all the specimens were more or less completely dried up ere they were examined
and contained only fully matured fructification, so that previously existent
antheridia might quite possibly have become unrecognisable.
The occurrence of species of Pythium as parasites capable of affecting the
potato plant has already been recorded. Sadebeck found a potato-field, near
Coblenz, in which his Pythium Equiseti or autumnale replaced Phytophthora as
a cause of blight,* and De Bary has shown that Hesse's Pythium de Baryanum
attacks potatoes energetically†. As I have not had an opportunity of examining
specimens of these species, I am unable definitely to determine whether the
present one is identical with either of them, but it would appear probably to be
distinct. The oogonia can hardly be described as yellow, as Sadebeck states
that those of his species are, and the oospores do not require to pass through
a prolonged period of rest as those of Pythium de Baryanum do, according to
De Bary.
On the assumption that the blight is essentially dependent on the presence
of the Pythium, there can be little room to doubt that the proper measures to be
adopted in endeavouring to prevent its appearance and limit its extension consist,
in the first place, of an avoidance of all excessive irrigation, and, in the second,
of the careful removal and destruction by burning of all portions of blighted
haulms and roots. Any excessive saturation of the soil must necessarily favour
extension of the disease, as it provides the conditions for the saprophytic growth
of the mycelium external to the host, and therefore provides the means for
primary infection and for spread from plant to plant ; and from the abundance of
* Bot. Zeit., 1876, s. 268.
† Bot. Zeit., 1881, s. 520.
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days' duration, and were consequently hopelessly wilted. The disease was
assuredly not caused by Phytophthora, and may have been caused by the first
form of fungus described above. This is a question which can only be satis
-
factorily worked out in localities within which potatoes are cultivated and are
subject to the disease, but there the solution of the problem would be extremely
easily arrived at by means of a few careful experiments, and it has, therefore,
been deemed advisable to publish the results of the present observations, im
-
perfect though they be, in the hope that they may lead to the local study
of the subject.
d. " Bangle-blight. "—In the spring of the year 1892, Mr. Mollison, Superin
-
tendent of Government Farms, Bombay, forwarded some specimens of diseased
potato-plants to the Indian Museum for examination, and these were sub
-
sequently sent on to me. They had been grown in the neighbourhood of Poona
and were affected by a disease ordinarily termed " Bangle-blight " by the native
cultivators, as descriptive of the appearance of a characteristic annular brown or
blackish line which is present in the substance of the tubers of the affected plant
in an area corresponding with that in which the diffused fibro-vascular bundles
are situated (Plate I, Figs. 2-3). In the letter which accompanied the specimens,
Mr. Mollison stated that the disease did not make its appearance in one spot
or patch in a field and radiate thence, but in scattered isolated plants, which
wilted suddenly, as though they had been attacked by a cut-worm or grub beneath
the surface of the soil. " The haulm rapidly withers and dries up, still retaining
its green colour, and in the course of a few days gets brown as a ripe stem
would do. A plant which is perfectly fresh one day shows signs of distress on
the next one and will be quite wilted and drooping on the third day." He also
pointed out that the methods of cultivation universally adopted near Poona
appeared to him to be faulty, the crops being grown year after year on the same
land and irrigated every eight days to such an extent that the stems were sub
-
merged to a depth of an inch or an inch and a half. Judging from the pheno
-
mena presented by the disease, Mr. Mollison correctly concluded that it could
not be dependent on the presence of Phytophthora, and it was with a view to
having this question definitely settled that the specimens were forwarded to
Calcutta.
They showed not a trace of the presence of either the fructification or
mycelium of Phytophthora. The leaves, although wilted, either retained their
green colour or merely showed signs of normal fading, and the haulms throughout
the greater part of their course appeared to be perfectly healthy. Just at the
base, however, they were somewhat discoloured (Plate I, Fig, 1), and transverse
and vertical sections showed the presence of points or streaks of brown within the
substance of the vacu1ar bundles (Plate I, Fig. 4), which on microscopic examina
-
tion were found to be owing to mycelium and minute sclerotia. The fungal
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upper surface, and, having done so, ramify in a complicated fashion so as ulti
-
mately to form dense masses of pseudo-parenchymatous tissue, about the size
of a pin's head, which are at first pure-white throughout, and subsequently become
first ochreous and then deep-brown superficially (Plate II, Fig. 6).
The sclerotia, when mature and dry, are very readily detached owing to the
rupture of their slender pedicels, and may be found lying loose in large numbers
among the dead leaves and on neighbouring surfaces. With the onset of
continuous relatively dry and cool weather the extension of the disease is arrested
the dead leaves fall off and a bare patch is left which is anew clothed with
leaves in the succeeding spring. But the sclerotia, where they escape the attacks
of insects and other destructive agents, do not lose their vitality, but, as may
readily be determined experimentally, are capable of lying dormant for many
months, and then serving as the source of renewed appearance of the disease
when exposed to favourable conditions. I have thus preserved some of them
for an entire annual period, and then employed them as the means of establish
-
ing patches of blight at an elevation of twenty feet or so above the ground,
thereby not merely demonstrating the prolonged vitality of the sclerotia, but
also that the fact that the disease normally originates at the level of the soil is
simply owing to the fact that the detached sclerotia naturally tend to
accumulate there.
The extreme abundance with which sclerotia are produced by this species,
and the fact that its parasitism is so feebly specialised that it is capable of
attacking very many distinct hosts, renders it most undesirable to employ Ficus
stipulata as a creeper to screen the sides of any plant-house in which orna
-
mental succulent foliage-plants are cultivated. So long as it was employed
thus in the Botanic Garden in Calcutta, the annual blighting of Begonias and
other succulents within the houses was enormous, but since it has been replaced
by Ficus repens and other creepers which are not so subject to attack, the
disease has practically disappeared. Sclerotia are produced on the leaves of
Begonias and various other plants, but by no means in such abundance as on
those of Ficus stipulate, and as the blighted leaves of plants within the houses are
naturally in greater part removed ere the sclerotia have matured, the numbers of
the latter which tend to accumulate as a source of renewed disease are trifling as
compared with those which are constantly liable to be present among the remains
of the dead leaves of the Ficus at the base of any surface covered by it.
The mycelium grows abundantly on the surface of sections of potato-tubers,
producing a densely filled, snowy-white mass of considerable thickness, and its
capacity for purely saprophytic existence is demonstrated by the fact that it
produces similar although somewhat more limited growths on the surface of
media consisting of starch-paste containing sugar and peptones. In either of
these cases, however, it fails to produce any sclerotia, so that apparently para-
Medical Officers of the Armyof India.
107
sitism, although relatively undifferentiated, has been evolved so far that the
saprophytism is facultative only.
A large number of the sclerotial blights that occur in India appear to be
characterised by a feature which renders them likely to be more extensively mis
-
chievous than those which ordinarily occur in Europe. This is that the extension
of disease from one part of a host-plant to another, and from one host to a
neighbouring one, is effected with great rapidity, in spite of the absence of readily
diffusible reproductive bodies such as conidia, owing to the fact that the
mycelium is not confined to the interior of the host, but also spreads with great
rapidity and luxuriance over the surface, giving off as it does so bundles of
invading filaments which penetrate the stomatic orifices and ramify throughout
the intercellular spaces and within the cavities of the cells of the tissues. The
presence of the blight is therefore rendered readily recognisable even within
areas in which the tissues yet retain their normal colour by the presence of strands
and webs of white mycelium extending over the surface. For example, in the
case of the blight affecting Ficus stipulata, the first conspicuous indication of the
presence of the disease appears in the form of yellowish or brownish patches of
discoloration on the upper surfaces of the leaves but if at this time we examine
the under surfaces we find that, extending far beyond the limits of the discoloured
areas, there are radiant strands of white mycelium spreading over the surface of
the epidermis. When the whole of a leaf has been blighted, similar strands
extend freely beyond its margins, and where they come into contact with other
foliar surfaces, adhere to them and serve as the source of further infection, whilst,
where they come in contact with the petiole, they creep along the course of it
and of the axis until they reach other leaves. This free superficial extension of
mycelium only occurs under conditions of high atmospheric humidity, and conse
-
quently we find blights whose extension is mainly determined by it, only present
-
ing themselves conspicuously during the rainy season and producing effects
directly proportionate to the degree and duration of excessive atmospheric
humidity which prevail in any particular year. Fortunately the foliage tissues
alone are attacked by most blights of this type, but very extensive defoliation is
yet in many cases effected owing to this capacity for superficial extension of
mycelium. Were it not for this, in the absence of readily diffusible repro
-
ductive bodies in the form of conidia, only those leaves would be affected
which formed the sites of primary infection, but the capacity for free superficial
extension serves to replace the presence of conidia and penetration of axial
tissues as a means of diffusion so long as atmospheric conditions remain favour
-
able.
The capacity which the mycelium has for invading the interior of the
tissues of host-plants varies in degree in various instances. In any case, entrance
is mainly effected by means of the stomatic orifices (Plate II, Fig 7), and in
P 2
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some cases it appears that it is only by means of the latter or of abnormal in
-
terruptions of the continuity of the epidermal surfaces that entrance is capable
of being achieved. When a blighted leaf of Ficus stipulata clothed with the
mycelium of the species of blight naturally affecting it is placed in contact with
the under surface of a Begonia leaf, rapid and indefinite superficial extension of
mycelium and spreading infection of the new host occurs (Plate II, Figs. 7.9)
; but, if the Ficus-leaf is applied to the intact upper surface of the leaf, a certain
amount of superficial extension of mycelium occurs, but this is very soon
arrested on the expenditure of the nutritive supply in the tissues of the primary
host, and no infection of the secondary one occurs. This, however, is not the
case with another species of sclerotial blight which occurs as a cause of destruc
-
tive disease in Evolvulus nummularius, Linn., and other plants forming common
constituents of turf in Calcutta. In this case, whilst infection of Begonia leaves
is induced more readily and extends more rapidly when portions of the diseased
tissues of the primary host are applied to the lower (Plate II, Fig. 11) than to the
upper surfaces, it also constantly occurs eventually in connection with the latter
also. The superficial extension of the mycelium is capable of being provided
for to a certain extent saprophytically, but it is only where penetration of host-
tissues provides nutritive supplies of parasitic origin that it becomes rapid and
indefinite. It is thus regulated by two distinct factors, the presence of atmo
-
spheric humidity of high degree and of facilities for penetration of host-tissues
and acquisition of nutritive supplies from them, and hence it is readily explicable
why, as a rule, it should occur so much less conspicuously on the upper than on
the under surfaces of leaves, as the latter not only afford greater protection from
desiccation, but, as a rule, much greater facilities for penetration than the
former do.
Blights of the above nature, which practically limit their attacks to the
foliar tissues, are naturally, as a rule, of less economic importance than those
which, like the so-called " Bangle-blight " of potatoes, specially attack the
tissues of the axis. They give rise to extensive defoliation, but do not neces
-
sarily cause the death of the whole or of great parts of their hosts. They may,
however, come to be of very considerable importance where they attack plants
which are cultivated on account of their foliage, whether for ornamental purposes
as is the case with Begonias and other so-called foliage-plants, or for economic
purposes as is the case with the tea-shrub, the " Thread-blight" of which would
appear, as will be subsequently shown, in all probability to belong to this class
of diseases.
e. Sclerotial blight affecting the tubers. —A sample of diseased tubers was
received from Poona in February 1894, in which the mischief was dependent
on the presence of a mycelium, which, after exhausting the nutritive properties
of the host, produced an abundance of ovoid sclerotia, closely resembling those
Medical Officers of the Army of India. 1 1 1
Taking the nature of these blights, whether of purely acarine or of mingled
acarine and fungal origin, into account, there can be no question that the means
to be adopted with a view to limiting their extension and preventing their
recurrence must lie in the first place in keeping the tubers as dry as possible, and
in the second in carefully removing and burning all those in which the disease
has already established itself. The first of these measures will tend to repress
the multiplication of both acarine and fungal elements, and the second to secure
the destruction of acari, ova and sclerotia, and thus to prevent the reproduction
of both the primary and secondary causes which provide for the recurrence of
the blight from year to year.
III.—Vegetable blights affecting the Tea-shrub.
a. Bark-blight caused by Cephaleuros virescens, Kunze. —This blight
differs from any of the others dealt with in this paper in being dependent on an
algal, not a fungal, entophyte, and consequently belongs to a group of diseases
of very rare occurrence. Entophytic algae are by no means of uncommon
occurrence, but it is only very rarely that they give rise to any serious injury
to their hosts. So much so is this the rule that Frank when dealing with the
subject affirms that there is only a single exception to it.* In most cases the
intrusive elements play the part of pure entophytes and not of true parasites,
being dependent on their hosts for mere protection and water supply, and hence
interfering little with their nutrition unless they are present in excessive
numbers or are so situated as to interfere with the proper access of air to the
system of intercellular spaces. Their injurious effects are, as a rule, of purely
mechanical origin and dependent on the pressure which they exert on neigh
-
bouring tissue-elements. In many cases, too, the amount of mischief which
they thus induce is comparatively trifling, because the intrusive elements are
situated only in the substomatic or other larger intercellular spaces, and because
of the fact that they are in many cases of unicellular nature, so that even where
they are developed within the cells of the host the effect of their presence is
very greatly localised. When, however, the algal elements belong to multicellular
organisms capable of continuous growth, and are not confined to normal
intercellular spaces, they may, even if strictly entophytic in habit, give rise
to conspicuous mischief, both because of the abnormal drain on the water-
supply of the host which their presence occasions, and by the disruptive effects
which they produce on neighbouring tissues. I have already pointed this out
in a paper contained in Part III of this periodical, in which the phenomena
attending the growth of Cephaleuros on the leaves of Cinnamomum iners,
* Die Krankheiten der Pflanzen, s. 635.
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Reinwardt, were described,* but the injury which is produced in the host-plant
in that case is trifling in comparison to that occurring in connection with the
blight induced by the same alga when it attacks the cortical tissues of the
shoots of tea-plants.
Specimens of the disease were forwarded to me for examination by
Dr. King, of the Royal Botanic Garden, Shibpur, to whom they had been ori
-
ginally sent by the agents of a tea-garden in which it had been the cause of
considerable mischief. They consisted of branches bearing the remains of
numerous withered leaves on their twigs. The terminal portions of the twigs
where the bark was still young and green did not show any signs of disease,
but lower down the surface was either covered by dense felts of the fructifying
filaments of Cephaleuros, or the bark was entirely wanting and the outer surface
of the bast exposed to the air (Plate III, Fig. 1). Vertical and longitudinal
sections of portions of shoots in various stages of disease unequivocally demon
-
strated that the destruction of the bark was owing to the disruptive effects
produced by intrusive filaments, sheets and solid masses of the vegetative
cells of the alga whose fructification clothed the surface in those places where
any bark remained adherent (Plate III, Figs. 2-3). In the earlier stages
of the disease the cortex was still of considerable thickness and the algal
elements were situated comparatively superficially, spreading out in sheets
through the corky strata and breaking up at intervals through it to give
origin to erect, fertile filaments on the surface. In more advanced specimens
processes could be seen descending through the substance of the host-tissues from
such superficial layers, and in their turn spreading out at deeper levels to give
rise to new horizontally-disposed extensions which broke up the continuity of
the surrounding parts, and in still older cases the thickness of the cortex was
greatly reduced (Plate III, Figs. 3-4), and algal elements were visible throughout
its entire depth right down to the outer limits of the bast. Taking these appear
-
ances into account, there can be no difficulty in accounting for the decortication
and ultimate death of the affected shoots. It is evident that the disease origin
-
ates superficially, and is dependent on penetration of the outer layers of the
cortex by processes descending from the under surfaces of the normal discoid
expansions of Cephaleuros which are developed on any suitable surface on
which the zoospores may happen to come to rest and germinate. Such processes
find all the conditions for luxuriant growth in the constant water-supply and
protection from direct exposure to the sun's rays which the cortical tissues
provide. They grow freely and spreading out horizontally tend to break up the
continuity of the tissues and to cause the desquaniation of the layers which lie
above them, partly by mere increase in bulk, and partly because they give off
* On an Entophytic alga occurring in the leaves of Limnanthemum indicum, with notes on a
peculiarly parasitic variety of Mycoidea.