CHAPTER VI.
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.
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).