_SMUTS._
ONE of the fungal diseases of corn long and widely known has obtained
amongst agriculturists different appellations in different localities.
In some it is the “smut,” in others it is respectively “dust-brand,”
“bunt-ear,” “black-ball,” and “chimney-sweeper,” all referring, more or
less, to the blackish soot-like dust with which the infected and
abortive ears are covered. This fungus does not generally excite so much
concern amongst farmers as the other affections to which their
corn-crops are liable. Perhaps it is not really so extensively
injurious, although it entirely destroys every ear of corn upon which it
establishes itself. Wheat, barley, oats, rye, and many grasses are
subject to its attacks, and farmers have been heard to declare that they
like to see a little of it, because its presence proves the general
excellence of the whole crop. No one who has passed through a field of
standing corn, after its greenness has passed away, but before it is
fully ripe, can have failed to notice, here and there, a spare,
lean-looking ear, completely blackened with a coating of minute dust
(Plate V. fig. 98). If he has been guilty of brushing in amongst the
corn, it will still be remembered how his hands and clothing became
dusted with this powder; and if at the time he should have been clad in
sombre black, evidence will have been afforded—in the rusty-looking tint
of the powder when sprinkled upon his black continuations—that, however
sooty this powder might appear whilst still adhering to the ears of
corn, it has an evident brown tint when in contact with one’s clothes.
This powder, minute as it is, every granule of it constitutes a spore or
protospore capable of germination, and ultimately, after several
intermediate stages, of reproducing a fungus like the parent of which it
formed a part. During the growth of the plant its virulent contents flow
like a poison through the innermost tissues, and at length attack the
peduncle or axis of the spikelets of the ear, raising up the essential
organs and reducing them to a rudimentary state. Brongniart, who made
this species the special subject of observation, states that the fleshy
mass which is occupied by the fungus consists entirely of uniform
tissue, presenting large, almost quadrilateral cavities, separated by
walls, composed of one or two layers of very small cells filled with a
compact homogeneous mass of very minute granules, perfectly spherical
and equal, slightly adhering to each other, and at first green,
afterwards free or simply conglomerate towards the centre of each mass,
and of a pale rufous hue; at length the cellular walls disappear, the
globules become completely insulated, and the whole mass is changed into
a heap of powder, consisting of very regular globules, perfectly alike,
black, and just like the reproductive bodies of other fungi (Plate V.
fig. 99). A scientific botanist of some repute, M. Unger, published a
work in Vienna during the year 1823, in which he sought to prove that
this, and allied species of fungi, were not fungi at all, but merely
broken up cells, or disruptured and altered conditions of certain
portions of the diseased plants. The most satisfactory refutation of
this theory may be found in the fact that the spores of the smut can be
seen to germinate under favourable conditions, and produce fruit,
whereas, if they were only the ordinary cells of the plant broken up by
disease, fructification would not take place.
The spores in this species are exceedingly minute. It has been
ascertained that forty-nine of them would be contained within a space
the one-hundred-and-sixty-thousandth part of a square inch; hence one
square inch of surface would contain little less than eight millions.
These myriads of spores are shed from the ears, and nothing remains but
the barren matrix in which they were borne when the farmer proceeds to
gather in his crops. At that time he sees no more of the “smut,” all
remembrance of it for the time is gone, his only thought is to stack his
corn in good condition. But the millions of spores are dispersed, ten
millions at least for every ear that has been “smutted,”—and will they
not many of them reappear next year, and thus year after year, with as
much certainty as the grain upon which they are parasitic?
Like many of the parasitic fungi, so destructive in the farm and the
garden, this species belongs to the family in which the spores are the
distinctive feature. After many botanical changes, the “smut” is at
length regarded as a fixed resident in the genus _Ustilago_; with the
specific name of _segetum_, which latter signifies “standing corn;” it
is therefore the _Ustilago_, or _smut of the standing corn_. The
characters of the genus are, chiefly, that the spores are simple and
deeply seated, springing from delicate threads, or in closely-packed
cells, ultimately breaking up into a powdery mass. Fifteen members of
this genus have been described as British. One of these (_U. maydis_)
attacks the maize or Indian corn grown in this country in a similar
manner as the common smut attacks wheat or barley; but as maize is not
an established crop with us, a more minute description of this species
is unnecessary; the spores are figured in Plate V. fig. 108. Another
species (_U. hypodytes_) makes its appearance at first beneath the
sheaths of the leaves surrounding the stems of grasses (fig. 100), and
ultimately appears above and around them as a purplish-black dust (fig.
101). The seeds of sedges, the leaves and stems of certain definite
species of grass, the flowers of scabious (Plate VI. figs. 123-125), the
receptacles of the goatsbeard, the anthers of the bladder campion, and
other allied plants, and the seeds of the Bistort family, are all
liable, more or less, to the attacks of one or other of the residue of
the fifteen species of _Ustilago_ already referred to as indigenous to
Britain.
[Illustration:
Plate V.
W. West imp.
]
Although we do not profess to teach practical men how to grow good corn,
or how they shall get rid of, or keep clear from, the many foes to which
their crops are exposed, yet a suggestion may be offered, based upon the
facts obtained in our botanical researches, supported by the analogy of
allied circumstances. In this instance the extreme minuteness and
profusion of the spores would evidently render all the corn liable to
the attachment of, perhaps only two or three, spores to the seed coat.
Some ears of corn in nearer proximity to the smutted ears may be covered
with spores which yet remain invisible to the naked eye, and when these
grains are mixed with others in the heap, the chances are not much in
favour of any handful not becoming charged with spores. If the majority
of these were not redeemed from destruction by the many changes,
shiftings, rubbings, and scrubbings to which the seed corn is liable
between the time of its reaping and the period of its sowing, we might
expect a very large crop of “smutted” corn. Under ordinary circumstances
we can scarcely imagine that the loss arising from infected ears would
repay much special labour to prevent it, only that to a large extent the
precautions taken to cleanse the seed corn from the spores of one fungus
will also avail for another, and while cleaning it of the spores of
“smut,” those of “bunt” will also be removed. The facts that we rely
upon chiefly as indicating the remedy are that the spores are only
superficially in contact with the seed corn, and that they are of less
specific gravity, causing them to float on the surface of any fluid in
which the corn may be immersed. Again, the spores of many species of
fungi will not germinate after saturation with certain chemical
solutions. One of the most successful and easy of application is a
strong solution of Glauber’s salts, in which the seed corn is to be
washed, and afterwards, whilst still moist, dusted over with quick-lime.
The rationale of this process consists in the setting free of caustic
soda by the sulphuric acid of the Glauber’s salt combining with the
lime, and converting it into sulphate of lime. The caustic soda is fatal
to the germination of the spores of “bunt,” and probably also of “smut;”
although, as already intimated, except in cases where these affections
of the corn are very prevalent, we shall be informed by the
agriculturist that the cost of labour in the prevention will not be
compensated in the cure.
Experience has also taught us that many fungi flourish in proportion to
the wetness of the season, or dampness of the locality. A wet year is
always exceedingly prolific in fungi, and a dry season correspondingly
barren, at least in many kinds, whilst others, as the experience of 1864
has convinced us, are exceedingly common. In a field or wood the
mycologist reaps his richest harvest of mycological specimens in the
lowest and dampest spots, in swamps, ditches, and ill-drained nooks.
This is a fact worth knowing as much by the farmer as the amateur
botanist in search of specimens for his herbarium.
One of the most unmistakable species of “smut” is that which infests the
goatsbeard, on which we have already described an _Æcidium_. Generally
about the same time as the cluster-cups make their appearance on the
leaves, some of the unopened flower-heads of this plant will be found
considerably altered in appearance by the shortening of the segments of
the involucre, and at length by the whole inflorescence being invested
with a copious purplish-black dust. If, by any means, the lobes of the
involucre are any of them separated, the enclosed dust escapes,
blackening the fingers and clothing of the collector, as if it were soot
(Plate V. fig. 92). A little of this dust submitted to the microscope
will be found to consist of myriads of small globose spores, nearly
uniform in size and shape; and if a higher power be employed, each of
these will appear to have a papillose or minutely granulated surface.
The florets, dwarfed in size and contorted, or the remains of them, are
embedded in the mass of spores (fig. 93), and if one or two of these are
removed and placed under a good one-inch objective, every part will be
found covered with adhering spores, to the apparent exhaustion of its
substance. Of course, the florets are never developed when subjected to
the attack of “smut.” The whole plant assumes a faded, sickly
appearance, even before the spores are fully ripened. We would recommend
our readers, if they collect one of the infected flower-heads, to put it
into a box or paper by itself, for if placed in the box with other
specimens it will so sprinkle them with its black powder as to render
them nearly useless for microscopic examination: everywhere the
microscope will detect, where the unaided eye failed to recognize a
trace, the ubiquitous spores of _Ustilago receptaculorum_ (Plate V. fig.
94).
In the fenny districts of the eastern counties a species of “smut”
called _Ustilago typhoides_ attacks the stems of reeds, forming thick
swollen patches of several inches in length (fig. 128), sometimes
occupying the whole space between two joints or nodes, and lying beneath
the sheath of the leaves. The spores in this species are larger than in
the species which attacks the culms of grasses in a similar manner
(Plate VI. fig. 129).
There are not many features in the rest of the species of this genus of
sufficient interest to the general reader or microscopist to render it
advisable to furnish any detailed account of them. We may, however, note
that in a species found on the leaves of the common cock’s-foot grass
the spores are large, obovate, and rough, with minute granules (figs.
117, 118). This species is known botanically as _Ustilago_ _salveii_,
and externally bears considerable resemblance, except in the size and
colour of the spores (fig. 119), to another species much more common,
and which occurs on the leaves of _Poa aquatica_ and _P. fluitans_. The
last-named species forms long parallel sori, extending often for several
inches along the leaves of the aquatic grasses just named, giving them a
very singular appearance (Plate V. figs. 105, 106). The spores are small
(fig. 107), not being more than one-fourth the length of the last
species, and smooth, whilst those are minutely granulated.
An interesting species occurs, very rarely, on the stems of such grasses
as _Aira cæspitosa_ and _A. aquatica_. The sori are in bands at regular
distances apart (Plate VI. fig. 120), each band being composed of a
number of short parallel sori (fig. 121). The spores are not more than
one-third of the size of those in _U. longissima_.
Sedges are also subject to attack from other species of smut; one of
these (_U. olivacea_) appears to convert the seeds into a fine
olive-coloured dust (Plate VI. fig. 126), which gives to the fruit a
similar appearance to that presented by corn when attacked by _Ustilago
segetum_. Another species, which also absorbs the seeds, becomes
hardened and consolidated more than in any other species, and, though
larger than the normal seeds, still retaining their form (figs. 109,
110). This is _Ustilago urceolorum_, the spores of which are also
figured (fig. 111).
The beaksedge (_Rhyncospora alba_) suffers from an allied species which
affects it in a similar manner (Plate V. figs. 96, 97), but is not
equally common. The spores of _Ustilago utriculosa_, found on different
species of _Polygonum_, instead of being granulated, are reticulated on
the surface (Plate VI. figs. 114, 116). The chief interest attaching to
_Ustilago antherarum_ consists in its habitat, for it is developed in
the anthers of the flowers of the bladder campion, and other plants of
the same natural order. The anthers are much swollen and distorted by
this parasite, which is not uncommon, though easily overlooked unless
specially sought after (Plate V. figs. 102-104). A list of all the
British species will be found at the close of this volume. It will be
noted that as in the genus _Æcidium_ the prevailing colour of the spores
is orange, so in the genus _Ustilago_ it is black, with a purplish or
violaceous tinge.
Four diseases in wheat of fungal origin are known and recognized in the
popular language of the farm as “mildew,” “rust,” “smut,” and “bunt.”
Sometimes one and sometimes another is most prevalent, and he is an
exceedingly fortunate individual who can walk through his fields and
find only one of them, especially if that one should be sparingly
distributed. It has been our good fortune to dwell much amongst
cornfields, and the terror of the word “mildew” to a farmer’s ears is
not unfamiliar in our reminiscences of the past, ere we discarded the
much-loved country to become a dweller in town. The subject of our
present remarks inspired no such alarm in the districts of our
experience, but in some seasons and localities it is certainly one of
the “pests of the farm.” Under the different appellations of “bunt,”
“pepper brand,” “bladder brand,” and sometimes “smut,” this infection is
very generally known. Externally there is no appearance, except to the
practised eye, that anything is wrong. There is no black impalpable dust
about the ears as in the true “smut,” no red withered leaves or spotted
stem as in the “rust” and “mildew,” and no stunted growth or
malformation, evident to the casual glance, by which the insidious foe
can be recognized; but stealthily and secretly the work is accomplished,
and until the “bunted” grains make their appearance in the sample, the
disease may, perchance, be unchallenged.
Externally the “bunted” grain is plumper, and whilst the corn is still
green these will be of a brighter green than the rest (Plate V. fig.
84). When broken, the farinaceous interior will be found replaced by a
minute black dust of a very fœtid, unpleasant odour, and greasy to the
touch (fig. 85). This powder constitutes the spores of the “bunt” mixed
with myceloid threads. It may happen that much of the corn in a field is
“bunted,” and the discovery not made till the wheat is being ground for
flour; then the odour and colour will speedily decide the produce to be
unfit for human food. We have not the least doubt that “bunted” corn,
when ground with flour, is injurious in proportion to its extent, whilst
at the same time we can scarcely conceive an intelligent miller grinding
up a sample containing any large proportion of “bunted” grains in
ignorance of the fact.
If we break open a grain of wheat infested with the “stinking rust” or
“bunt,” and then place some of the powder in a drop of water on a glass
slide, and submit this to the microscope, first using the half-inch
power, then the quarter, or fifth, and finally an eighth or tenth, we
shall find that this minute dust consists of myriads of globose brown
bodies termed spores, which possess certain reproductive functions.
These spores will be found mixed with a number of delicate branched
threads, to which they are attached by a short stalk or pedicel, visible
with the higher powers (fig. 86). The surface of the spores you will
also observe to be beautifully reticulated. These features just
described as visible in the “bunt” are the characteristics of the genus
to which it belongs (_Tilletea_), and of which it is the only British
species. An allied species infests the Sorghum or durra, a grain but
little cultivated in Europe, but found extensively in Africa and Asia,
and also apparently found on the _Bajra_ of India.
The interesting experiments of the Rev. M. J. Berkeley on the
germination of “bunt” spores have been already alluded to. They were
undertaken shortly after the outbreak of the potato disease, to
ascertain, if possible, the mode by which the minute spores of fungi
inoculate growing plants; and although at that time only a bare
suspicion of the nature of the bodies resulting from the germination of
“bunt” spores was entertained, succeeding examinations in the same
direction have brought to light extraordinary facts, and manifested the
progress of the successive developments of four generations. The spores
of “bunt” are larger than those of the different species of “smut,” and
reticulated on the surface (Plate V. fig. 86). When these are made to
germinate a kind of stem is protruded (fig. 87), upon which small
clusters of elongated thread-like spores of the second generation, or
sporidia, are produced (fig. 88). After a time these spores conjugate,
or become united by short transverse processes in the same manner as has
been observed in some of the lower forms of Algæ (fig. 89). The
conjugated spores in the next stage germinate and produce a third kind
of fruit, different from either of the preceding, and constituting a
third generation (fig. 90). These in turn germinate and produce a fourth
order of reproductive organs (fig. 91), so that in the process of growth
the “bunt” spores evidently pass through four generations. Hence, as one
result, the number of germinating bodies is greatly increased, as well
as their power of inflicting injury in a corresponding diminution in
size. There are still many points in the history of the growth and
development through successive generations of the “bunt” spores, but
enough is known, on the one hand, to show that this is a true vegetative
parasite, and not merely a diseased condition of the tissues of the
wheat plant, and on the other that it is perfectly distinct from all the
phases of the other and similar parasitic fungi which affect the wheat
crop.
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