_WHITE MILDEWS OR BLIGHTS._
NOTWITHSTANDING the inconvenience to ourselves of calling very different
fungi by the same common name of “mildew,” the popular mind does not
recognize the inconvenience, since it scarcely troubles itself to
inquire whether they are not all the same thing. In obedience to this
custom, we again write of “mildew,” or “blight,” as it is called in some
districts, but of a very different kind to that which is so detrimental
to growing crops of corn. In the present instance it is our intention to
illustrate a group of fungi which are exceedingly common, and which
differ greatly in appearance and structure from any to which we have had
occasion to allude. To obtain a general knowledge of these forms let our
reader proceed at once to a clump of rank grass; if it is his fortune to
dwell in the country, the walk of a few yards will suffice. Let him
examine this clump more carefully, perhaps, than he has been accustomed
to do, and we venture to predict that he will find some of the leaves
covered with what appears to be a dirty white mould, or mildew (Plate
XI. fig. 235). One of these leaves should be collected as carefully and
conveyed to the microscope as speedily as possible, taking care not to
touch, or brush it against any other object so as to disturb the
arrangement of the delicate little threads upon its surface. If a small
portion, say about an inch, is cut from this leaf with a sharp pair of
scissors, and laid upon a slide, or pinned down upon a strip of sheet
cork, so as to keep it flat, and then submitted to examination under the
microscope, with an inch power, a beautiful forest of crystalline
vegetation will be observed. If the examiner on this occasion should not
possess a binocular microscope we are sorry for him, because in that
case he will not see all that is to be seen under the greatest
advantages. If we ever truly enjoy looking through such an instrument,
it is on an occasion like this, when a low power is all that is needed,
and the object is required to be seen in relief. It is scarcely possible
to convey an adequate idea of the beauty of such a scene as the
microscope reveals upon this fragment of grass-leaf. Little bundles of
delicate threads, clear and crystalline, are seated upon a slender
branching mycelium. These threads, sometimes erect, sometimes drooping,
flexuous, or prostrate, are composed of numerous roundish or spherical
cells attached to each other in a moniliform or bead-like manner (fig.
236). These easily separate from each other. Let a portion of the
threads be removed from the leaf on the point of a lancet and laid upon
a glass slide, with a thin cover over them. Submit this object to a
quarter-inch power, as a drop of water is let fall at the edge of the
cover and insinuates itself, by capillary attraction, between the two
plates of glass. So soon as it touches the moniliform threads, the
disunion commences, and almost before they are enveloped in the fluid,
two spherules will scarce remain attached to each other. This delicate
little mould on the grass leaf at one time bore the name of _Oidium
monilioides_. It is now regarded only as a condition of another minute
fungus, to which attention will shortly be directed.
The vine disease, so fearfully destructive on the Continent, and not
altogether unknown in this country, is another of these incomplete
fungi. From an individual who at the time of its first discovery in the
south of England took considerable interest in the subject, it was
called _Oidium Tuckeri_, which name it continued to bear, both here and
abroad, until, with many others, probably nearly all of the same genus,
it was found to be only a barren state of what is called by mycologists
an _Erysiphe_. The real discoverer of this mildew was undoubtedly the
Rev. M. J. Berkeley, who has successfully devoted a long life to the
study of these minute organisms, through evil and through good report,
and when that study was beset with more difficulties, and received less
encouragement than at present. If, towards the autumn, we should again
collect some whitened, mouldy, or mildewed grass-leaves, similar in
appearance to those mentioned above, and carefully look at them with a
pocket lens, little black points, almost as small as a pin-point, or
more resembling the full stop with which, this sentence closes, will be
found scattered over the white threads. The aid of the microscope must
be again sought to make out the structure of the little black dots.
Closely nestling upon the mycelium, the little points will prove to be
spherical brownish, conceptacles, surrounded with transparent floccose
appendages. Many other species are far more beautiful than that of the
grass-leaf, as will be seen by reference to our plate. The variation
consists chiefly in the form of the appendages which spring from the
conceptacle and surround it in a radiating (as in figs. 219, 222, 225,
and 230), or in a more or less confused and entangled manner (as in
figs. 216, 240, 245, and 251). The surface of the conceptacle is
minutely reticulated, and its base is attached to the mycelium. When
first formed, these globose conceptacles are almost colourless; they
afterwards acquire a yellow colour, and are ultimately of a deep brown.
The appendages are seldom at all coloured. Within the conceptacle are
contained from one to several transparent obovoid sacs, or spore-cases,
called _sporangia_, enclosing a definite number of spores (figs. 218,
224, 228, &c.), which vary in different species. In the hazel mildew,
for instance, there are two spores in each _sporangium_; in the willow
mildew four; in the maple mildew eight; in the grass mildew, and some
others, numerous. The tips of the appendages are variable, and often
elegant (figs. 227, 231, 233, 234, and 247), sometimes simple and at
others symmetrically branched. All the species occur on the still living
and green parts of plants, especially the leaves, and are therefore
truly parasitic. A pocket lens will show whether any conceptacles are
present on any suspicious leaf which may be collected, but high powers
of the microscope are essential for their complete examination. It is
during autumn, when vegetation begins to languish, that we shall be most
successful in searching for specimens. They will then be found almost
everywhere, and the white mycelium forms an object too conspicuous for
them to be readily overlooked. Botanically, nearly all the species were
at one period included in one genus, under the name of _Erysiphe_, a
name derived from the Greek, and signifying “mildew;” at the present
time they are distributed through several genera, the chief distinctions
of which are based upon the form of the appendages. Though personally
disposed to question the generic value of such distinctions, it would be
imprudent to adopt any other names here than those to be found in recent
English works on fungi.
The first species in our enumeration is found on cultivated roses. What
a deplorable picture does a favourite rose-bush present when attacked by
this mildew! The leaves blistered, puckered, and contorted; their
petioles and the peduncles and calyces of the flowers swollen,
distorted, and grey with mould; and the whole plant looking so diseased
and leprous that it needs no mycologist to tell that the rose is
mildewed. The conceptacle in this species is minute, and contains but
one sporangium, which is one of the characters of the genus in which it
is now included, and a more justifiable distinction than the
ramifications of the appendages. The mycelium is rather profuse, and the
threads or appendages which spring from the conceptacle are simple and
floccose (fig. 216). The sporangium contains eight ovate spores. This
species (_Sphærotheca pannosa_, Lev.), in its oidioid or conidiiferous
form, was for some time known under the name of _Oidium leucoconium_.
An allied species constitutes the hop-mildew, a visitation with which
some of our Kentish friends are too familiar. This is not a prejudiced
species in the choice of its habitation, since it is found on many other
plants, where it flourishes with equal vigour. The meadow-sweet, burnet,
scabious, teasle, dandelion, and other composite plants, plantain, and
plants of the cucumber family, all suffer more or less from its roving
disposition. The mycelium of whitish threads is even more conspicuous
than in the last species, but the conceptacles are often not to be found
at all. These are also very minute and most common on the under surface
of the leaves. The appendages, or fulcra, are simple, floccose (fig.
217), and coloured. The sporangia are found singly in each conceptacle,
and each, sporangium contains eight spores.
An autumnal stroll amongst hazel-bushes, when the nuts are ripe, will
lead, if the nuts are not a greater attraction, to the discovery of
whitish, patches on the under surface of the leaves, caused by the
mycelium of the hazel mildew (_Phyllactinia guttata_, Lev.). These
patches are less distinct and conspicuous than in many other species,
but the little blackish dots of the conceptacles may be distinguished by
sharp eyes without the use of the lens. Though possessing a decided
preference for the hazel, this species is also found on the green leaves
of the hawthorn, ash, elm, birch, sallow, beech, oak, and hornbeam. The
conceptacles are larger than in the two preceding species, and somewhat
depressed above. The appendages are few (fig. 219), radiating, rigid,
and acicular, or like needles. Each conceptacle contains eight or more
sporangia, and each sporangium has from two to four spores (fig. 220).
This species being very common, its conceptacles large, and produced
copiously, and its appendages distinct, it will prove a good type with
which the student of these fungi may commence his examinations. This is
the only representative which we possess of the genus established by M.
Leveille for such of the _Erysiphei_ as have the conceptacle depressed,
and the appendages rigid and simple; by which features it is
distinguished from genuine species of _Erysiphe_.
Two species, also common, having many features agreeing with each other,
are found on the leaves of the maple and the willow. The willow blight
(_Uncinula adunca_, Lev.) is found irrespectively on various species of
poplar and willow (fig. 221). In size and external appearances, to the
unaided eye, it seems scarcely to differ from the preceding, but more
minute examination will show that in the appendages there is an
appreciable difference. Still rigid, but no longer aciculate, the tips
bent or curved like a little hook, or curled upon themselves (fig. 223),
radiating and numerous (fig. 222), and at length tending upwards. Many
sporangia are contained within each conceptacle, each of which is
furnished with four spores. The amateur must not be disappointed, if, on
examining mature conceptacles with a view to the discovery of the
sporangia, he finds only free spores. The investing membrane is very
delicate, and disappears generally as the spores are matured.
The “blight” or “mildew” which occurs on the common hedge-maple, as well
as on sycamore leaves, is exceedingly conspicuous when occurring on the
former plant. The whole bush often presents a hoary appearance as if
sprinkled with powdered chalk. In the spring, the under surface of the
leaves of the same plant are liable to become hoary from another cause.
The whiteness occurs in patches, has often a pinkish or violaceous tint,
and glistens like hoar-frost. This affection of the leaves was, at one
time, believed to be produced by a fungus which was called _Erineum
acerinum_, but now it is regarded as a diseased state of the tissues. In
the maple mildew, both surfaces of the leaves are alike affected, and
the little, dark, point-like conceptacles will be found studded over
both. It is not uncommon to meet with very white leaves, caused by the
mycelium, but which bear no fruit. The appendages in this species are
shorter than in the last (fig. 225), and the tips are bifid (fig. 226),
or divided into two short branches, each of which is bifid, and uncinate
or hook-shaped (fig. 227). The conceptacles contain not less than eight
sporangia, each of which encloses eight spores.
Amongst the parasites that prey upon the much abused berberry (which has
been charged in turn with producing the mildew in corn), is one which
causes the green leaves to assume a chalky appearance (fig. 229), though
less conspicuously than in the maple blight. This parasite is the
berberry mildew (_Microsphæria berberidis_, Lev.). In such localities as
the writer has met with the berberry suffering from mildew, he has
invariably found a larger proportion of leaves with the barren mycelium
than of those on which the conceptacles were developed. Perhaps in other
localities this may not be the case. The appendages, as will be seen on
reference to our plate, differ materially from any of those to which we
have referred; indeed, this genus (or sub-genus) has the most elaborate
and beautiful forms in these appendages of any of the Erysiphei. A
figure is given of the tip of a fulcrum from a continental species (_M.
Ehrenbergii_, Lev.), not yet found in this country (fig. 233). In the
berberry blight the appendages are straight at the base, but afterwards
become forked, each fork being again forked, and these yet again
branched in a similar manner (fig. 230); so that a complex dichotomous
tip is formed to each of the appendages (fig. 231). Each conceptacle
contains about six sporangia, and each sporangium contains from six to
eight spores (fig. 232).
The common gooseberry is also liable to a visitation from an allied
species, in many respects closely similar, but differing in having the
tips of the appendages more branched, and the extremities of the
ultimate branchlets are not entire and attenuated, as in the berberry
mildew; but divided into two toothlike processes. The conceptacles in
this species contain from four to eight sporangia, each of which has
four or five spores.
In England, the leaves of the guelder-rose, and in France (perhaps also
in this country) those of the alder, nourish a parasite belonging to
this division. This “blight” possesses so much in common with others to
which allusion has been made, that it will scarcely be necessary to
describe it in detail. A figure of the tip of one of the appendages of
the variety found on the alder is given in the Plate XI. fig. 234.
We have found another species which had not been before noticed in this
country (_M. Hedwigii_, Lev.), on the leaves of the mealy guelder-rose
in the vicinity of Darenth Wood, near Dartford, in Kent. The mealy
character of the leaves of this plant, and the minute size of the
conceptacles of the parasite, render it difficult to find; indeed, it
could not be noticed unless it were sought for, as we sought it, lens in
hand. It only occurs on the under surface of the leaves: the mycelium is
very web-like and fugacious, the conceptacles minute, globose, and
scattered (fig. 243). Four sporangia, each containing but four spores
(fig. 244), are enclosed in each conceptacle, which is surrounded by a
few appendages (about six) thrice dichotomous, and thickened at the tips
of the ultimate branches, which are incurved (fig. 247).
The species of true _Erysiphe_ are distinguished botanically from the
foregoing by the floccose character of the appendages, in which feature
they accord with the species found on the rose and the hop, but from
which they differ in the conceptacles containing numerous sporangia
instead of only one, as in those species.
One of the most common and conspicuous of these is found on the leaves
and leaf-like stipules of the garden pea. Every leaf in a crop will
sometimes suffer, and the gardener, to his great mortification, finds
that the mildew is more prolific than his peas. The leaves become sickly
and yellow as the mycelium of the fungus spreads over them, when they
present a peculiar appearance, as if growing beside a chalky road in dry
dusty weather, and had become covered with comminuted chalk. Soon the
conceptacles appear, profusely scattered over the white threads, like
grains of gunpowder (fig. 237), and after a brief struggle for existence
the pea and its parasite die together. In this species (_Erysiphe
Martii_, Lev.), the appendages are nearly transparent, short, and much
interwoven with the mycelium (fig. 238), the globose sporangia
containing from four to eight spores (fig. 239). It is not confined to
peas, although that habitat has been here given for it, because it is so
common upon them. Beans, melilot, St. John’s-wort, some umbelliferous
plants, and the meadow-sweet, have all been found affected.
The species found on grasses, especially the cocksfoot, has been already
alluded to. The conceptacles contain from twenty to twenty-four ovate
sporangia, each enclosing eight spores. The appendages and mycelium are
much interwoven.
Another of these “white mildews,” not only on account of its frequency
of occurrence on certain plants, but also from the numerous species of
phanerogamous plants on which it is found (fig. 240), may be truly
designated “common” (_Erysiphe communis_, Lk.); many kinds of crow-foot,
especially _Ranunculus acris_, are subject to its parasitism. It is
found also on other plants of the same natural order, on the
rest-harrow, trefoils, enchanter’s nightshade, bindweed, and knotgrass.
There are from four to eight sporangia in each conceptacle, containing
from four to eight spores (fig. 241). In this species, more especially,
M. Tulasne found curious sucker-like processes developed on the threads
of the mycelium (fig. 242): their office may probably be only that of
attachment.
Of the other species found in Britain an enumeration will suffice, since
they contain no feature of interest to the microscopist; and all the
members of this section are far less beautiful than those in other
genera (especially _Microsphæria_).
The leaves of the dogwood or cornel (figs. 245, 246) are the home of one
species (_E. tortilis_, Lk.), and the burdock of another (_E.
Montagnei_, Lev.). Both of these, in addition to the above, have
sporangia which contain more than two spores. There are also two species
in which only two spores are contained in each sporidium. One of these
(_E. Linkii_, Lev.) is found on both surfaces of the leaves of the
mugwort (figs. 248, 249); the other (_E. lamprocarpa_, Lev.) occurs on
salsafy, scorzonera, weasel-snout, and plantain (figs. 250, 251).
These complete the _Erysiphei_; but there are allied species of too much
interest not to be noticed in connection with them. Three very singular
fungi are found on damp straw and paper; two on the former and one on
the latter. Of the species occurring on straw, the most common one is
figured, natural size, in our plate (fig. 257); but from this no idea
can be formed of its structure, which in some points resembles an
_Erysiphe_. The conceptacles are thin and brittle, and are clothed
externally with long dark-coloured branched hairs (figs. 258, 259). The
conceptacle contains long narrow sporangia, each enclosing dark, almost
black, lemon-shaped sporidia. For low powers this is a very interesting
object. The minute structure affords no feature of popular interest.
This fungus (which bears the name of _Chætomium elatum_) is common on
old straw, thatch, reeds, matting, &c., resembling small brown tufts of
hair, visible to the naked eye.
Paper much exposed to damp will occasionally develop a similar
“bristle-mould,” surrounded by a yellowish spot (_Chætomium chartarum_,
Ehrb.); but it is not so common as the last. In habit and structure it
is very similar (figs. 252, 253).
In 1838, the Rev. M. J. Berkeley announced the discovery by him of a
singular production, for which he was unable to find a fitting location
in any genus then established, and for which he accordingly
characterized a new one, under the name of _Ascotricha_. This new
species of paper mildew was found by him on some printed paper in a box.
It somewhat resembles the other species above alluded to, at a casual
glance; but more minute examination will reveal its differences. The
author to whom we are indebted for this species thus describes its
development. At first it appears as a minute branched mould interspersed
with globose brownish conidia. As it advances in growth, globose black
peridia become visible amongst the threads, clothed with and supported
by alternately branched obscurely-jointed filaments, the branches of
which generally form an acute angle with the stem (fig. 254). The
ramification of these is very peculiar, the stem and main shaft of each
sub-division being almost constantly shortened and surmounted by the
branches given off near its apex; this, again, is often abbreviated and
another branchlet given off, which again surpasses it; and occasionally
the same circumstance takes place a third time. The apices are clavate
and colourless; the rest of the filaments, when viewed by transmitted
light, brown, even, and pellucid: a few globose conidia are usually
attached to them (fig. 255). The conceptacle is thin, black to the naked
eye, of an olive-brown under the microscope, filled with a mass of
linear extremely transparent asci (fig. 256), each containing a single
row of broadly elliptic chocolate sporidia. These have a paler border;
sometimes the colour entirely vanishes, either from age or abortion, and
there is only a minute globose nucleus or more probably a vesicle of
air, in the centre; occasionally they become so transparent that the
globular bodies alone are visible. After the conceptacles burst, several
are frequently collected together into an irregular linear body, which
consists principally of the conglomerated sporidia.
One other very common and troublesome little fungus (_Eurotium
herbariorum_) will for the present close our examples. This is found
creeping over dried plants preserved in herbaria, on decaying fruit,
preserves, and various other substances, sometimes animal as well as
vegetable, but chiefly the latter. To the naked eye it appears as a
myriad of little yellow spherical bodies, of the size of very small
pins’ heads, resting upon fine cobweb-like threads (fig. 260). When
magnified, the surface of the conceptacles is seen to be reticulated
(fig. 261). In the interior the sporidia are borne, contained also, as
in the former instance, in asci. It has been considered _probable_, but
as yet not fully proved, that this mildew is a compound fruited
(ascigerous) condition of an equally common mould (_Aspergillus_).
Dr. Shortt, of Chingleput, in a recent report on the growth and
production of Indian Cotton, remarks that the plants are subject to the
attacks of a kind of mildew. He writes:—“They appear in the form of
rounded fibres or thallus, shooting up in the air, having the lamina of
the leaf as a base, and feeling villous to the touch. The small fibrillæ
that form the nap appear shooting up as sharp projections when seen by
the naked eye; under the microscope they are found to consist of pointed
tubes, interspersed here and there with minute granular cells. It first
attacks either the upper surface of the petioles, or the margins of the
leaf, gradually extending over the lamina, and matting together the
whole leaf into a greyish-white, felty mass. At first it attacks the
young shoots and tender leaves, preventing them from expanding. The
extension of the parasite deprives the plant of its juices, and
eventually either destroys or renders it sterile. The spores seem to be
derived from the atmosphere, and finding the plant in a state fit to
receive them, from either the results of excessive cultivation, or from
the effects of heat and want of moisture rendering it unhealthy, and
thus favouring the reception of the spores of the fungi. Another variety
speckles the leaves with whitish dots. These remain separate, but the
lamina is covered with them, and in time the leaf changes colour,
becomes yellowish, and eventually dies away. This is evidently the
disease called _Bunt_, or some variety of it, as it is seated beneath
the epidermis, and eventually the spores escape. Under the microscope
they seem to consist of small dark cells or spots attached to a
thread-like mycelium.”
The writer seems certainly to have made a mistake in its affinities, and
on the faith of the above quotation we should be more disposed to regard
it as an _Erysiphe_. It is to be hoped that specimens of the affected
leaves will be forwarded to this country for examination.
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