_MOULDS._
TWENTY years since, and some of these little pests were altogether
unknown, whilst others were only recognized and partly understood by a
few scientific men. During the period to which we have alluded more than
half the present species contained in the genus _Peronospora_ had never
been observed, and amongst these the most devastating of its tribe, the
associate and undoubted cause of the potato disease.
Parasitic fungi are far more numerous, both in individuals and species,
than most persons are aware, and cultivated plants of all kinds are more
or less subject to their ravages. Some are more susceptible than others,
of which the corn and grass tribe, or _Graminaceæ_, as they are termed
by botanists, is an example. Not less than thirty species have been
recorded upon plants of this natural order, and of these nearly one-half
are found upon the living plants. Upon the potato plant, again, no less
than ten different kinds of fungi have been described; whilst upon other
and more fortunate plants only one or two parasites of this nature
establish themselves.
Plate XIII.
[Illustration:
262.—TURNIP MOULD.
_Peronospora parasitica._
]
[Illustration:
263.—ONION MOULD.
_Peronospora Schleideniana._
]
It will be sufficient for our present purpose to state that one of the
six families into which fungi are divided for scientific purposes is
called _Hyphomycetes_, a name compounded of two Greek words signifying
“thread” and “mould,” or “fungus,” and is applied to this group because
the thread-like filaments of which they are largely composed are the
most prominent feature. In this family there are again a number of
smaller groups called orders, having an equal value to the natural
orders of flowering plants; and one of these orders, called _Mucedines_,
has the fertile threads perfectly distinct from the mycelium or spawn.
These threads are sometimes simple and sometimes branched; they may be
articulated or without articulations or septa, short or long, erect or
creeping, hyaline or whitish, mostly free from colour, and are not
coated with a distinct membrane. The spores are generally simple,
sometimes solitary, at others in pairs, or strung together like beads
for a necklace. Amongst all this variety of arrangement there is order,
for these are but features, or partly the features, of the different
genera of which the _Mucedines_ are composed. One of the genera is
termed _Peronospora_, and to this the parasitic fungus of the potato,
and some others to which we shall have occasion to refer, belong. In
this genus the threads are generally branched, but without
articulations. The spores, or seed-like bodies, are of two kinds; one
kind is borne on the tips of the branches; and the other kind, which is
larger and globose, is borne upon the creeping mycelium or spawn. All
the members of this genus with which we are acquainted are parasitic on
living plants, inducing in them speedy decay, but preceding that decay
of which they are themselves the cause. Hence we have deemed it the more
advantageous course both for writer and reader to associate together the
different species of this particular genus of parasitic moulds in one
chapter, rather than bring together the different kinds of fungi,
belonging perhaps to widely separated genera, but all associated with,
or parasitic upon, the same plant. The botanical student will thank us
for following this plan, and the general reader will labour under no
disadvantage, in this instance at least, from the similarity of the
diseases produced in the plants infested.
It has been recently proposed to associate the genera _Peronospora_ and
_Cystopus_ together in one group, under the name of _Peronosporei_; but
with the discussion of this subject we have nothing to do in this
volume. Having announced this fact, we shall continue to notice them in
the several positions heretofore accorded to them.
The mycelium (root-like fibres) in this genus greatly resembles that
described for the “white rusts,” though neither so thick, nor is the
membrane so gelatinous as in that genus. In some instances the mycelium
is confined to the inter-cellulary passages; but in most cases they also
perforate the cells of the plant which nourishes them. The sucker-like
bodies already described in _Cystopus_ are often found produced on the
mycelium of _Peronospora_, but occasionally they appear to be absent,
especially in the mould causing the potato disease.
From the mycelium erect threads are produced, upon which one form of
fruit, which may be termed “acrospores,” is borne. These filaments are
sometimes single, and sometimes in small tufts or fascicles. In some
instances they are considerably branched, so as to present a dendroidal
or tree-like appearance; in others they are nearly simple, being only
surmounted by short spicules; or, in one instance, quite simple, and
only surmounted by a single acrospore. The branching habit is by far the
most common.
Each ultimate branch in the ramification of the fertile filaments
engenders a single acrospore. Its extremity, at first thin and pointed,
swells in the form of a globular vesicle, which soon takes the
elliptical or ovate shape of the perfected acrospore, and at length
separates itself from the branch that supports it.
In all instances the acrospores have a similar structure, but with
minute differences in form, &c., which have their importance in the
determination of species. In most cases the apex of the acrospore is
obtuse, and the entire body has a violet tint, more or less deep; in
some it is completely colourless. These acrospores, when placed in
favourable conditions, will germinate, and, in fact, comport themselves
in the manner of true spores.
During the year 1861, Dr. de Bary published an account[8] of the
discovery by him of zoospores, similar to those already described in
connection with the conidia of the “white rusts,” produced from the
acrospores of the mould which originates the potato disease. In the same
author’s memoir of 1864, already quoted, the observations there made are
confirmed. When the acrospores of the potato mould and the parsnip mould
are sown in water upon a glass slide, their contents become divided, and
vacuoles are formed, as already described in _Cystopus_; these parts are
expelled through an apical orifice, and, when free, take the form of
perfect zoospores, and commence swimming about in the fluid surrounding
them.
Footnote 8:
Die gegenwartig herrschende Kartoffelkrankheit, ihre Ursache und ihre
Verhütung. Von Dr. A. de Bary. Leipsig: 1861.
These zoospores are oval, or semi-oval, with a structure resembling that
of the “white rusts,” save that the two ciliæ, or vibratile hairs,
proceed from the same point. The number of zoospores from each acrospore
of the potato mould is stated to be from six to sixteen, and in the
parsnip mould from six to fourteen. Their ultimate development is the
same as has already been described. In the potato mould, the production
of the zoospores is much favoured by the exclusion of light.
Another mode of germination in the acrospores of the potato mould has
been observed by the same eminent mycologist. This results when the
spores are sown upon a humid body, or on the _surface_ of a drop of
water. The acrospore emits from its summit a simple tube, the extremity
of which swells into an oval vesicle; into this the contents pass, and
it isolates itself by a partition from the germ-tube. Thus it becomes a
duplicate of the acrospore from whence it was derived. This secondary
body has also the power of producing a tertiary cellule in a similar
manner. Both the second and third cellule, when _immersed_ in water,
produce zoospores in the ordinary manner, as above described.
Yet another and a third mode of germination is described by the same
author, in which the acrospore emits from its apex a germ-tube, which
elongates considerably; and into this long and tortuous tube the
contents of the acrospore pass, and accumulate at the opposite
extremity.
The germ-tubes, produced in the manner last described, when developed on
the surface of a favourable plant, perforate the cells of the epidermis,
or enter by the stomata. In the case of the potato mould, the germ-tubes
enter by the stomata; but in the majority of species the germ-tubes do
not enter by the natural pores of the leaves upon which the acrospores
are sown, but perforate the cellules, and thus effect admission into the
tissues of the plant, where they extend, ramify themselves, and become a
mycelium. This mycelium originates branched threads, bearing acrospores
at the tips of their branchlets, and in many species of _Peronospora_
another kind of reproductive body upon the threads of the mycelium
itself. To these bodies we must briefly address ourselves.
This last kind of reproductive organs (not yet positively found in the
potato mould) appear to be wholly analogous to the oogonia of the “white
rusts” already described, producing oospores in like manner. Dr. de Bary
avows with regret that his numerous efforts to observe the germination
of these oospores were unsuccessful. Nevertheless, he considers that the
perfect resemblance between them and the oospores of the “white rusts”
will justify him in concluding that the germination in both is very
similar. It will be unnecessary to repeat here the observations already
made on the growth and development of oogones and zoospores. What has
been advanced respecting these organs in _Cystopus_ will apply also to
_Peronospora_.
Plate XIV.
[Illustration:
264.—POTATO MOULD.
_Peronospora infestans._
]
[Illustration:
265.—LETTUCE MOULD.
_Peronospora gangliformis._
]
POTATO MOULD.—Towards the close of the summer of 1845, in the course of
a few weeks, every one became aware of the fact that a new disease had
appeared which threatened the entire destruction of the potato crop.
Until then it seemed to have been almost, although not entirely unknown.
It first appeared in the Isle of Wight about the middle of August, and a
week afterwards had become general in the South of England, and the next
week there were but few sound samples of potatoes in the London market.
Early in September the disease had commenced its ravages in Ireland, and
shortly afterwards it was discovered in Scotland. With the same rapidity
it seems to have spread throughout Europe and North America, or at least
the western portion of the former and the northern districts of the
latter. It must not be imagined, however, that the Isle of Wight was the
centre from which this disease spread over such an extended area and
with such alarming rapidity. From this spot it doubtless made its first
appearance that year amongst our own crops, but there is not the least
doubt of its existence both on the continent of Europe and in North
America in the previous year, and the farmers of Belgium had noted its
appearance in the province of Liège as far back as 1842 and 1843. Other
diseases had been observed affecting the potato crop before, and one
which was also associated with a parasitic fungus had made its
appearance in 1815. It is also exceedingly probable that, in a milder
form, the murrain was present with us a year or two before it broke out
to such an alarming extent. A correspondent to the _Gardeners’
Chronicle_, in 1844, notices it in the Isle of Thanet, and another
testifies to its occurrence in districts of Ireland for two or three
years previous to its general outbreak. The description of the disease
in Canada, in 1844, contained in a letter addressed to Dr. Bellingham,
and quoted by the Rev. M. J. Berkeley,[9] leaves no doubt of its
identity:—“During the months of July and August (1844), we had repeated
and heavy showers, with oppressive heat, and an atmosphere strongly
charged with electricity. Towards the close of the month of August I
observed the leaves to be marked with black spots, as if ink had been
sprinkled over them. They began to wither, emitting a peculiar,
offensive odour; and before a fortnight the field, which had been
singularly luxuriant, and almost rank, became arid and dried up, as if
by a severe frost. I had the potatoes dug out during the month of
September, when about two-thirds were either positively rotten,
partially decayed and swarming with worms, or spotted with
brownish-coloured patches, resembling flesh that had been frost-bitten.
These parts were soft to the touch, and upon the decayed potatoes I
observed a whitish substance like mould.”
Footnote 9:
Journal of Horticultural Society of London, vol. i. p. 11.
Although this disease made its first appearance, in the middle of
August, 1845, in the Isle of Wight, it had already appeared in Belgium
in the same year, a month previously; and although it may have been
noticed in other British localities in 1844, it was known in Canada and
in St. Helena in the same year to a far greater extent, and in Liège as
early as 1842. There are, therefore, good grounds for believing that the
European centre was Belgium; but if M. Boussingault was correct in
stating that “this malady is well known in rainy years at Bogota, where
the Indians live almost entirely on potatoes,” then it is not of
European but American origin, and is probably derived from districts not
far remote from those whence Europe first received the potato itself.
It would occupy too much space to detail the different theories and
opinions relative to the causes of this disease to which 1845 and
subsequent years gave birth. Suffice it to say, that the lapse of years
has silently proved the majority of these to have been fallacious. All
such as imputed to peculiar electric conditions, a wet season, or other
meteorological influences, the disease which has re-appeared under
different conditions and influences, and in seasons remarkable for
dryness, are manifestly refuted; whilst its mycological origin has
continued to gain adherents, and the gradual accumulation of fresh facts
has almost placed it beyond dispute not only that the potato disease is
accompanied by, but results from, fungal growth. Unfortunately, this
disease has been so prevalent, more or less, during the past eighteen
years, that few have been without the opportunity of making themselves
acquainted with its external appearance. To this may be added the minute
and exact account of its development, as recorded by that excellent
mycologist and careful observer, the Rev. M. J. Berkeley, in 1846, and
to which, even now, nothing of importance can be supplemented or
abstracted:—“The leaves began suddenly to assume a paler, and at length
a yellowish tint, exhibiting here and there discoloured spots. More or
less coinciding with these spots, on the reverse of the leaves, appeared
white mealy patches, consisting of a minute mould, proceeding, either
singly or in fascicles, from the stomata, and arising from an abundant
branched mycelium creeping in every direction through the loose tissue
beneath the cuticle. The upper surface rarely, if ever, exhibits the
mould, it being almost physically impossible for its delicate threads to
penetrate the closely-packed cells which, being arranged side by side,
leave scarcely any intercellular passages. The mould, in a few hours
from its first piercing the apertures of the stomata, perfects its
fruit, and in so doing completely exhausts the matrix, which in
consequence withers. No sooner have a number of the leaves been
attacked, than the stem itself is subject to change, becoming spotted
here and there with dark brown patches, in which the cells are mostly
filled with a dark grumous mass, without exhibiting any mucedinous
filaments; though, occasionally, I have ascertained their presence. Very
rarely fructifying but dwarfed specimens of the mould occur upon it. The
stem now rapidly putrefies, the cuticle and its subjacent tissue become
pulpy, and separate when touched from the woody parts beneath. The whole
soon dries up, and in many instances exhibits in the centre the black,
irregular fungoid masses which are known under the name of _Sclerotium
varium_, and which are believed to be the mycelium of certain moulds in
a high state of condensation.
“If the tubers are now examined, the greater part will often be found
smaller than usual, especially if the disease has commenced at an early
stage of growth; but in their natural condition, while here and there a
tuber, particularly if it has been partially exposed, exhibits traces of
disease. The surface is, however, soon marked with livid patches,
commencing generally about the eyes, or at the point of connection with
the fructifying shoots: these rapidly acquire a spotted appearance, the
spots being rather waved, and assuming often a more or less concentric
arrangement. Sometimes—especially on the smoother kinds of tuber—two or
more regular systems of concentric spots are exhibited on the same
tuber. The skin now withers, and is easily separated; the spots become
depressed and of a yellowish tinge; and if the tubers be laid in a moist
place, in a day or two—sometimes in a few hours—the same mould which
destroyed the leaves springs from them, piercing the cuticle from
within, yet not scattered, as on the leaves, but forming a conspicuous
white tuft. If a section of the diseased tuber be made on the first
symptoms of the disease, little brownish or rusty specks are found in
the cellular tissue, confined, with very rare exceptions, to the space
between the cuticle and the sac, if I may so call it, of spiral vessels
and their accompanying tissue, which, springing from the subterranean
branches, pass into the tuber, making their way to the several buds
disposed on the surface. These spots consist at first of a quantity of
discoloured cells, mixed more or less with others in a healthy
condition. The grains of fecula are for a long time perfectly healthy;
the cells themselves, so far from being looser, are more closely bound
together than in the more healthy portions. The rusty spots soon exhibit
a darker tint, spreading in every direction and becoming confluent; they
at length extend beyond the barrier of vascular tissue, and attack the
central mass. The tuber, meanwhile, assumes a disagreeable smell,
decomposes more or less rapidly, other Fungi establish themselves on the
surface, or in the decaying mass, which emits a highly fetid odour,
resembling that of decaying agarics; the union of the cells is
dissolved, animalcules or mites make their appearance, till at last the
whole becomes a loathsome mass of putrescence.”
The form of the mould itself is represented (fig. 264) as exhibited
under the microscope, with the nodose swellings of the branches, and
spores attached to the tips. These acrospores are filled with a granular
mass, from which, as hereafter described, zoospores are produced. The
branching dendroidal threads of this fungus proceed from a creeping
mycelium or spawn of entangled filaments which interpenetrates the
matrix, upon which it establishes itself. Upon these threads spherical
bodies were long since observed by Dr. Payen, and, under the name of
_Artotrogus_, described by Dr. Montagne as a new species of fungus. Dr.
de Bary failed to detect oospores on the mycelium of this mould,
although the organs found by Payen in some sort resemble them. De Bary
sought in vain, also, for the spherical bodies described by the author
above named. It has been calculated that one square line of the under
surface of the leaves is capable of producing 3,270 acrospores, each of
which yields at least six zoospores, sometimes double that number; thus
we have 19,620 reproductive bodies from that small space. The mycelium
from the zoospores is capable of penetrating the cellular tissue in
twelve hours, and, when established there, it bursts through the stomata
of the leaves, and fruit is perfected in from fifteen to eighteen hours.
Since the zoospores are perfected and ready to germinate in twenty-four
hours from their being placed in water, it becomes almost impossible to
calculate the myriads of fungi that may be produced from a single
centre. Dr. de Bary has also demonstrated that the brown spots so
characteristic of the disease are the result of the action of the spores
or zoospores. By placing a quantity of spores in a drop of water on the
leaves, stems, and tubers under a glass sufficiently air-tight to
prevent evaporation, he produced the brown spots, and traced their
progress from the earliest stages.
There are a few practical conclusions which may be drawn from these
discoveries. In the first place, it is clearly shown by the production
of the spots that the fungus is capable of _causing_ the disease, a fact
which has been disputed, but now placed beyond doubt. The inference is,
that not only is it capable of producing, but is really the cause of the
potato murrain. With bodies so minute and active as the zoospores, there
can no longer be difficulty in accounting for their penetrating the
tissues of the plant. They are most active and productive in wet
weather, especially when it is also warm. Moisture appears to be
essential, and a dry season the greatest enemy to the spread of the
disease. That bodies so minute and subtle should have baffled all
efforts to destroy or eradicate, is not now surprising. Whether any
method will be found to contend successfully with it, is now more
doubtful than ever. A careful reperusal of the old facts by the aid of
this new light will tend to the elucidation of much of the mystery in
which the subject has been involved. All who have hitherto been
sceptical of the mycological source of one of the greatest pests of
modern times should study M. de Bary’s pamphlet.
Plate XV.
[Illustration:
266.—PEA MOULD.
_Peronospora Viciæ._
]
[Illustration:
267.—ANEMONE MOULD.
_Peronospora pygmæa._
]
The potato mould has been judiciously named _Peronospora infestans_, or,
as it was at first called, _Botrytis infestans_; but on a revision of
the genera _Botrytis_ and _Peronospora_, it was transferred to the
latter genus, in which it remains. Three names were given to it, within
a short period of each other, by different mycologists, in ignorance of
its having already received a name. The one we have adopted appears to
have the priority, at least of publication, and was given by Dr.
Montagne. That of _Botrytis devastatrix_ was given by Madame Libert, and
_Botrytis fallax_ by M. Desmazières. The principal feature in this
species seems to consist in the branches becoming alternately thickened
and constricted, so as to resemble a moniliform string or necklace of
little bladders or vesicles. The branches are also more erect than in
the allied species, and the spores are solitary on the tips or from the
sides of the branches, and not in pairs or clusters, and the tips are
simple, and not bifid or trifid, as in most of its allies. It need
scarcely be remarked, that a high power of the microscope is necessary
to make out the distinctive features of the different members of this
genus, and that to the naked eye they only appear as a minute whitish
mould. As already stated, this little fungus makes its first appearance
on the under surface of the leaves, especially the lower ones, of the
potato plant, and afterwards attacks the stem, and ultimately the tuber.
For examination it is better to select the leaves soon after the fungus
makes its appearance.
TURNIP MOULD.—Since the advent of the potato murrain a similar disease
has been witnessed, though more limited in its extent, amongst Swedish
turnips, commencing in little waved irregular lines following the course
of the vessels, around which spots are formed by the deposition of dark
granules in the same manner as in the potato. In this instance, the
leaves apparently are first attacked in a similar manner by a species of
mould or _Peronospora_ allied to the one already described, but which
has been long known as parasitic upon cruciferous plants, to which the
turnip belongs. This species, termed _Peronospora parasitica_, is white
in all stages of its growth. It is much more branched, and the branches
are comparatively shorter than in the potato mould, and the tips of the
branches are bifid (fig. 262). The acrospores are very large and
globose, features also which distinguish this mould from the last. A
short time since we were called to witness a bed of splendid
cauliflowers, which had, up to that time, been the pride of their
cultivator; but, alas! their glory was threatened with speedy
annihilation, for in nearly every instance the lower leaves had become
more or less covered on their upper surface with yellow spots, and
beneath glaucous with the mould we have been describing. The diseased
leaves were all immediately removed, but we fear without success,
although no positive information has since reached us. The almost
unnatural vigorous green of the leaves, prior to the appearance of the
mould, is not at all an uncommon occurrence: this phenomenon has been
noticed in the ears of corn, in which every grain was soon afterwards
filled with spores of bunt.
This species was at one time believed to be parasitic on the “white
rust,” from which circumstance its specific name was derived. More
precise examination proves that it sometimes occurs where no white rust
is present, and therefore its parasitism is imaginary. The suckers in
this species are large and penetrate the cells, often entirely filling
them. This feature is sufficient to distinguish the mycelium of the
mould from that of the “rust.” The turnip mould occurs on many
cruciferous plants, and especially on the shepherd’s-purse, as well as
upon the different varieties of cabbage and its more immediate allies.
ONION MOULD.—Another disease, produced by fungi of the same genus, makes
its appearance upon young onion plants in the spring. The mould is
called _Peronospora Schleideniana_, and has many features in common with
those already described. In this instance the threads are greyish and
erect, with alternate branches, not divided by transverse septa, and the
spores are obovate, attenuated towards their base (fig. 263). This
mould, in some years, is very common and destructive, by preventing the
young plants which are attacked from coming to perfection. It is not
confined to the onion, but appears on other allied species of _Allium_
(to which the onion belongs). The threads form large patches or blotches
on the leaves, and sometimes cover them entirely. It very much resembles
the turnip mould, from which the form of the spores considerably
differs. This is the same species as that described by Caspary, and
afterwards by Berkeley, under the name of _P. destructor_.
_Lettuce Mould._—A very similar mould (_Peronospora gangliformis_) is
sometimes very common in spring on the under surface of the leaves of
the cultivated lettuce, appearing in definite white mouldy spots. By
reference to the figure of a portion of a thread magnified (fig. 265),
it will be seen that the peculiar form of the tips of the branchlets
evidences the distinctness of this species.
The oospores are small, globose, and of a yellowish-tawny colour. This
mould is by no means confined to lettuces, but has also been found on
species of ragwort, sow-thistle, nipplewort, endive, and other composite
plants; and has from time to time received numerous names, which it is
unnecessary to enumerate.
TARE MOULD.—The under surface of the leaves of tares, and sometimes also
of peas, is liable to attack from an allied species of mould
(_Peronospora Viciæ_). In the spring of 1846 it appeared amongst vetches
in some districts to such an extent as at one time to threaten the
destruction of the crops; but a succession of dry weather at once
abridged its power and limited its mischief. Mouldy vetches and mouldy
peas are, especially in moist seasons, evils to which the agriculturist
knows his crops to be subject; he may not know, however, that this kind
of mould (fig. 266) is of so near a kin to that which has acquired such
wide-spread fame in connection with the potato. Another species of
fungus attacks the garden pea in damp seasons, forming small depressed
brownish spots on the leaves and pods; but this is quite distinct from
the mould, though probably not less injurious.
The fertile threads are produced in dense clusters, each many times
branched, and bearing elliptic acrospores obtuse at their apices, and of
a violaceous tint (fig. 266). The oospores are beautifully reticulated
and of a yellowish-brown colour (Plate X. fig. 212).
Trefoil and some other allied plants are attacked by another species,
characterized by Dr. de Bary as _Peronospora trifoliorum_, which we have
found rather plentifully in some localities on lucern.
THE PARSNIP MOULD (_Peronospora nivea_, Ung.) is found on many
umbelliferous plants; but its attacks upon the parsnip are most to be
deplored, because it injures and ultimately destroys an article of human
food. The plants infested with this parasite are first attacked in the
leaves, but afterwards the roots become spotted and diseased in a
similar manner to the potatoes attacked by its congener. The disease has
not hitherto been so general with the former as the latter; but in some
districts it has been far from uncommon.
The fertile threads are collected in bundles, erect, and not so much
branched as in many other species. The acrospores are subglobose or
ovoid, and papillate at their apices. This species is sometimes called
_P. umbelliferarum_, and sometimes _P. macrospora_. Generally speaking
the average humidity of a season but little affects the production of
parasitic fungi. In a dry season, like that of 1864, we found as many
species, and these as flourishing and numerous in individuals, as in a
proverbially wet year. Such is not the case, however, with the moulds
under notice, or such fungi as are reproduced through the medium of
zoospores: these are undoubtedly less common in a very dry season; but
it must be remembered that a single shower is sufficient for the
development of zoospores, and occasional showers or heavy dews will
speed them on their course of destruction as readily almost as
continuous moisture. The large fungi, on the contrary, become very
limited in numbers when the weather is unusually dry.
SPINACH MOULD.—Spinach is likewise liable to suffer from the
establishment of a mould upon the under surface of the leaves:
unfortunately this is not unfrequent, and has been known in England
certainly for the last fifty or sixty years, since it was figured by
Sowerby in his “British Fungi” as many years since. We have lately seen
a bed of spinach, utterly destroyed by this fungus; whilst on another,
not twenty yards apart, not a spotted leaf could be found. This mould is
the _Peronospora effusa_ of botanists; it occurs also on some species of
goosefoot (Plate X. fig. 215), and probably on knotgrass. To the naked
eye it appears in pale purplish-grey patches, which, when examined
microscopically, are found to consist of dense bundles of branched
threads, bearing ellipsoid acrospores, the membranes of which have a
violaceous tint. The oogonia produced upon the mycelium vary
considerably in size. The oospores are of the character delineated in
our plate (Plate X. fig. 214).
Hitherto all the species of mould to which we have had occasion to refer
have been found infesting plants more or less employed as food; but
there remain one or two other species to which we must make special
reference. One of these affects the most universal of favourites amongst
flowers: this is the rose mould. Attention was directed to this mould,
and it was described for the first time under the name of _Peronospora
sparsa_, in the columns of the _Gardeners’ Chronicle_, in 1862. It
occurred on a quantity of potted rose-plants in a conservatory.
Irregular pale brownish discoloured spots appeared on the upper surface
of the leaves; these extended rapidly, and in a short time the leaves
withered and shrivelled up, and ultimately the whole plant perished. A
delicate greyish mould was to be seen by the aid of a lens, scattered
over the under surface of the leaves. By the microscope, the branched
threads, having the tips furnished with subelliptic spores, were
revealed, and an ally of the potato mould found revelling amongst the
roses.
During the winter of 1863-4, we found the leaves of several species of
dock occupied by a mould which appears to be a very low form of
_Peronospora_. Its presence was indicated by brownish orbicular spots,
on which the fertile threads occurred in small bundles. These threads
were generally simple, but occasionally forked, bearing rather large
elliptical acrospores _attached obliquely_ to the tips of the threads
(fig. 269). In consequence of this peculiarity, we have named the
species, which does not appear to have been noticed before, _Peronospora
obliqua_. It is clearly very distinct from another species found on dock
leaves by Corda.
Of the remaining British species, one (_P. Arenariæ_) is found on the
leaves of the three-veined sandwort (fig. 268); another attacks the red
corn-poppy, a third is found on the common nettle, one on the brooklime,
another on the wood-anemone (fig. 267), and another on the figwort.
Doubtless all the species in this genus are possessed of the third means
of reproduction, by zoospores, as discovered in the potato mould, not
only from the acrospores, but also from the oospores.
Plate XVI.
[Illustration:
268.—SANDWORT MOULD.
_Peronospora Arenariæ._
]
[Illustration:
269.—DOCK MOULD.
_Peronospora obliqua._
]
The fearful rapidity with which this method enables them to multiply
themselves may account for their widely spreading and devastating power.
No other genus of fungi can parallel this in the number of species
injurious to the field or the garden, or in which the injuries inflicted
are so great and irremediable.
* * * * *
N.B.—Since the foregoing chapter was in type, the Rev. M. J. Berkeley
informs us that both Mr. Broome and himself have examined the mould on
dock-leaves, to which we have given the name of _Peronospora obliqua_,
and have come to the conclusion that it is truly a member of that genus,
and not hitherto described; but they are also of opinion that it is the
same mould as one described by Dr. Montagne as _Ascomyces Rumicis_. We
concur with them in thinking it deficient in the important
characteristics of _Ascomyces_, and therefore retain its proposed name
of _P. obliqua_.
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