CHAPTER XXVIII.
EPIPHYTICAL PARASITES (VEGETABLE BLIGHTS) OF CORN CROPS.
These forms of parasite are so numerous, that nearly every species of flowering plant may become the nidus even of several named genera, with many species, or, at least, varieties of them. We here say attacked, because the advent of many of their forms passes under the name of “blight;” a term which at once recognises their injurious tendency.
Whether these epiphytes are the causes of the so-called blighted conditions, or merely their effects, is a subject upon which no little discussion has been expended. We do not, however, mean to re-open the question here; we will only remark, that in all probability this very wide range of the lower tribes of the vegetable kingdom is very variable in these respects.
Again: it will be impossible to enter into details of the different species of epiphytes. We shall hope, therefore, to elucidate their natural history, in so far as the farmer is concerned, by pointing out the more general facts connected with the following forms:—
| 1. | Uredo segetum—Smut or dust-brand of wheat, barley, and grasses. | ||||||
| 2. | Uredo caries (Tilletia)—Bunt | - | of wheat. | ||||
| 3. | Uredo rubigo—Red gum or red robin | ||||||
| 4. | Uredo linearis | - | —Straw-rust, or “mildew” | ||||
| 5. | Puccinia graminis | ||||||
| 6.[181] | Puccinia fabæ—Bean-rust. | ||||||
| 7. | Æcidium berberidis—Barberry-rust. | ||||||
| 8. | Cladosporium herbarum—Corn-ear mould. | ||||||
| 9. | Botrytis infestans—Potato-mould and mildew. | ||||||
| 10. | Botrytis—Turnip-mildew. | ||||||
| 11. | Oïdium erysiphioides | - | Hop-mildew. | ||||
| 12. | Erysiphe macularis | ||||||
| 13. | Oïdium abortifaciens—Ergot of grasses. | ||||||
1. Uredo segetum, Smut or Dust-brand, is common to barley, and not unfrequent in wheat; in both of which crops it is easily recognised from the affected ears of corn appearing as though they had been powdered over from the sweep’s soot-bag. On closely examining these blackened ears, we find that the whole flower has, as it were, effloresced into a black powder, which, on being placed under the microscope, is shown to be composed of myriads of granules, called by the fungologist spores, in which latter are contained still smaller grains, or sporidia.
These black spores are all washed away by the time the crop is ripe, leaving the stalks bare and grainless, so that the sample suffers no injury from this blight, which, even if present after threshing, would only tend to a slight discoloration of the sample, which is remediable by the smutter. Its chief effect, however, consists in causing the loss of much grain. We have observed it to the extent of as much as an eighth, but usually the diminution is about equal to the amount of seed sown; though it is not improbable that the whole crop may in many cases be greater when the smut is present. Sheep-folding previous to barley, special manuring for this crop, and other causes of increased fertility, are constant causes of the increase of the dust-brand.
2. Uredo caries—Bunt, Pepper-brand, Smut-balls.—This blight differs from the preceding in the fact that in the grain no flower is formed, but its interior becomes filled with a dark powder, which, when viewed under a high magnifying power, is found to consist of granules, with a surface which is rough, and not smooth as in the dust-brand.
In most cases, the whole grains of the ear will be so affected; in others, only a portion of them. They will be gathered in the harvest, and as the diseased grain is readily crushed, the black powder materially damages the appearance of the sample. Nor is this all: this blight has a most disagreeable odour and flavour, both of which are communicated to the sample, and so, besides diminishing the amount of produce, it greatly deteriorates it. Its specific name of caries of course refers to this fact, as also does that of U. fœtida, adopted by Baur, an author to whom we are greatly indebted for information upon these curious productions.
Before considering the remedy for this evil, it will be well to distinguish it from the “purples, ear-cockle, or peppercorn” (vibrio tritici)—a name expressive of its animal origin, and frequently rendered “wheat-eels.” In the purples, the grain is shorter than a healthy wheat grain, irregular in shape (cockled), and purple externally; but its interior is filled with what, to the naked eye, is like very short white cotton-wool. On placing a bit of this woolly substance with the point of a needle on a slip of glass, just touching it with water and submitting it to a high magnifying power, the term “wheat-eel” will at once be seen to be justified; for, if alive, thousands of eel-like creatures will be seen writhing in the fluid.
The differences of these two affections of wheat may be expressed as follows:—
| Bunt. | Ear-cockle.. |
| Grain smooth externally, sometimes appearing black from blackened interior grains showing through the thin epidermis (bran). These corns easily crush beneath the finger, emitting the black fungi. | Grain cockled and irregular in shape, purple externally, skin thickened, interior of the grains stuffed with a white cottony substance, not compressible by the finger; but being opened, and the interior magnified, exhibits the living wheat-eels. |
As regards the ear-cockle, we incline to the belief that a damp atmosphere and cold soil are chiefly concerned in its spread, if not in its production. As we have shown the difference between it and bunt, we now proceed to offer a few remarks upon the production of the latter, and its remedies.
Bunt is mainly produced by defective seed. It occurs on all kinds of soils—sands, clays, and limestones—and is not peculiar to any climate. Professor Henslow believes the disease to be wholly propagated by the spores of the fungus adhering to the wheat-seed. He says, “It has been clearly proved that wheat plants may be easily infected, and the disease thus propagated, by simply rubbing the seeds before they are sown with the black powder or spores of the fungus. It is also clearly ascertained that if seeds thus tainted be thoroughly cleansed, the plants raised from them will not be infected;” and he deduces from this a proof in favour of steeping; for he says, “This fact is now so well established, that the practice of washing or steeping seed wheat in certain solutions almost universally prevails.”[17]
[17] See an essay on Diseases of Wheat, in the Journal of the Royal Agricultural Society for 1841, by the Rev. Professor Henslow.
Our own experiments, however, recorded in the “Journal of the Royal Agricultural Society” for 1856, led us to conclude that the success derived from pickling wheat in different caustic and corrosive solutions arose from the fact of diseased grain being destroyed in the process; and we extract the following record of experiments made in 1853, as explaining this view of the matter.
Four plots of wheat, all from the same sample, were sown in the following order:—
| 1. | 2. | 3. | 4. | |||
| Much diseased wheat, without pickle. | Much diseased; treated with sulphate of copper. | Perfect picked seed, without pickle. | Perfect picked seed, with sulphate of copper. |
The results of these were as under:—
Plot 1. Most of the seed germinated, but the crop was much blighted, both in straw and grain; in fact, scarcely a perfect ear of the latter.
Plot 2. A small quantity of the seed germinated; the few resulting ears were free from blight.
Plot 3. Germinated, with a good and clean resulting crop.
Plot 4. The same result as Plot 3.
These experiments seemed to show that the pickling of wheat destroys the seed, so as to prevent germination when the seed is diseased or ill-formed; but if perfect seed be always employed, no pickling at all is necessary, it being strictly true that a diseased progeny must result from an imperfect stock in plants no less than in animals.
We have said that bunt is not peculiar to any climate; we have, however, always observed that employing seed from a warm district on a cold one, or using the finer white wheats in cold, exposed, or ill-drained situations, is sure to produce a large quantity of this fungus. Autumn-sown wheat, too, is less liable to the infection than spring wheat, which we attribute to the fact that many of the weaker plants will succumb to the cold rain and frost.
3. Uredo rubigo (Red-rust, Red-rag, Red-robin) makes its appearance in the inside of the chaff-scales, and ultimately in the green epidermis of the growing grains of wheat. Its first appearance is that of oval pustules, caused by the raising of the skin, which, ultimately bursting, shows the orange-coloured spores of the epiphyte. This must not be confounded with Cecidomyia tritici (wheat-midge), the larvæ of which are of a bright orange-colour; in the latter, the living moving worms may be easily detected by any common pocket lens or magnifying glass. Both these pests, to which we would apply the distinctive terms of Uredo rubigo (red-rust) and Cecidomyia tritici (red-gum), are exceedingly common in some seasons, and not unfrequently in the same crop. Good deep cultivation is the best remedy for the rust; but the treatment of the fly is a different matter. We would suggest the burning of smother-heaps on calm days, just as the wheat is bursting into ear, as smoke is decidedly obnoxious to these small insects, which in some seasons may be seen in thousands about the bursting wheat.
4 and 5. Uredo linearis; Puccinia graminis (Straw-rust and Mildew).—We refer to these epiphytes under one heading, as there can be but little doubt that the latter is a more advanced state of the former. They both occur in oblong patches on the leaves and straw of wheats and other grasses: in the uredo stage, of a dull red colour; in the puccinia stage, of a blackish hue. They are both, as, indeed, are all these fungi, interesting microscopic objects; but our object now is to describe them popularly. Both will always be found in abundance in cold poor soils, and more especially if the finer wheats be grown in such situations. The application of a dressing of salt to the soil is said to be a preventive. Be this as it may, the disease is said to be rarer in Cheshire, where salt is so much used by the farmer, than in any other county, in as far as we have observed.
Here, again, we incline to think that these are morbid affections of the plant. They are, indeed, viewed as such by Unger, in his “Die Exantheme Pflanzen,” in which the very title classes them with eruptive diseases of animals. Berkeley and Henslow, the two great authorities, however, do not accord with this view: the former remarks in reference to it—“Surely these plants are too distinctly, too regularly, and too beautifully organized to be the products of disease like warts or purulent matter in animals.” As, however, the microscope demonstrates that warts and eruptive diseases have also their special and curiously formed organisms, such a mode of reasoning is not conclusive.
Weeds have a great influence in producing mildew, which perhaps may be accounted for from the fact that weeds are in active growth as the wheat-stalks decline in vigour; and hence the constant evaporation of moisture from the weeds to the wheat is continually re-moistening an ever-drying surface—a most fertile source of mildew and moulds of several descriptions.
6. Puccinia fabæ (Bean-rust).[18]—The brown pustular rust-looking spots on the foliage of beans, and, indeed, occasionally on the stems and pods of beans, are sometimes common to this crop. They are usually accompanied by a lessening both in quantity and quality of this pulse, both in the garden and in field culture, but certainly more generally in the latter. Too gross manuring without well mixing the dung with the soil would seem to be a constant source of the evil. In fact, highly nitrogenized manures appear to favour the development of all this class of epiphytes, just as too much meat might bring about different forms of rash or eruptions in the animal. Weeds, which are too much permitted in beans, here aid in perfecting the mischief; hence, then, we may perhaps take it for granted that the mention of the causes of mischief suggests the remedy.
[18] This blight is mentioned here on account of its affinity to the former.
7. Æcidium berberidis (Barberry-rust) is here referred to, from the opinion prevailing that it is the cause of rust and mildew in wheat. We can no more believe that the barberry-rust would produce rust in wheat, than the rust of any other plant would do so; for nearly all plants are affected with some kind or other of rust. This epiphyte, too, is very different in structure from wheat-rust. Still, that wheat growing under a barberry hedge may be more blighted than in the rest of the field is quite true; and so it is with wheat grown under any kind of hedge. High fences are known to favour wheat blights; open, exposed, well-cultivated positions, when not too elevated, and without trees or hedges, being those in which the best wheats are grown.
8. Cladosporium herbarum (Corn-ear Mould) is a brown-coloured mildew, mostly occurring on the exterior of the chaff-scales of wheat, but common to many plants in a state of decadence. It consists of greenish or blackish tufts, which appear on the outside of the chaff-scales of wheat under the two following conditions:—
On wet soils, where the ears appear to have been prematurely starved.
On dry sands, where long-continued drought has caused some ears to wither and die before the seed was fully formed.
In both these cases we see that the plant has been previously injured. The decay commences under alternations of moisture and drying, and hence the fungoid attack. Here, then, the conditions necessary for preventing will be deep cultivation and a due pulverization and mixture of the soils.
9-12. Botrytis, &c. (Mildew).—Under this head we include a multitude of epiphytes, to which the terms mildew, mealdew, mehlthau (Germ.) are applicable. They appear to the naked eye as patches of white dust or meal on the leaves and stems of the affected plants. With the microscope we see that they are beautifully-organized plants, having a kind of rootlet (mycelium) or spawn entering the tissues of the living plants on which they grow, and delicate pedicels supporting spores at the externally visible portion of the plant. The botrytis of the potato and turnip, the erysiphe or oïdium of the hop, vine, and other plants, are only different forms of mildew, which in some shape or another will be found on most plants. That these attack living tissues is quite certain; but in the case of the potato, the turnip, and the vine, there is reason to believe that they result, to a very considerable extent, from diseased action in their tissues. For example: the botrytis of the potato seems to attack a crop much over-cultivated, on the approach of wet and cold nights after a prosperous growth in warm sunshine. So, the oïdium seems to us to be most abundant on renewed growth after a season of dry weather. Again: mildew in turnips is sure to follow that check which a long season of dry weather brings after a prosperous and vigorous growth. All these circumstances at least show how these attacks are favoured by the conditions which bring disease. So much, indeed, is this the case, that we found, upon experimenting with some cucumbers in a warm stove, that as long as we regularly watered the plants and gave them the requisite air, they kept healthy; but, by neglecting these conditions for a few days, we obtained mildew with the greatest certainty.
The remedies against mildew are—to obtain as healthy a growth as possible, and to maintain this with as great regularity as circumstances will permit. Of late years, both the mildew of the vine and the hop have been treated with flowers of sulphur. Dusting the affected hop-leaves with sulphur certainly arrests the mildew in an incredibly short time; and we found that by dusting sulphur from a fine sieve on our cucumber plants, the disease was immediately arrested in its progress. We therefore look upon this as an invaluable remedy in these states of mildew, whether occurring on the vine, the hop, the turnip, the cucumber, or on other plants, as we have frequently seen it in hothouses—a circumstance which shows the near affinity of all those forms of epiphytes, which, perhaps, after all, only vary with the variations in the structure and economy of the different plants on which they occur.
13. Oïdium abortifaciens (Ergot); Secale cornutum (Ergot of Rye).—The black horn-looking spur which occurs in rye and other grasses was formerly looked upon as a distinct fungus; now, however, it is known to be a diseased or malformed condition of the grain or seed, resulting from an attack by an oïdium on the immature seed.
Most of the cereal and even the meadow grasses are liable to attacks of ergot, which is increased by cold damp fogs and a moist condition of the atmosphere, the difference of the size of the spur being in accordance with the size of the affected grass seed. Thus, in rye we have seen spurs more than an inch long, while in the cock’s-foot grass it is seldom a quarter of an inch.
The ergot, as it occurs in the rye, is much used by medical men in difficult cases of parturition; and we have had evidence before us, in some cases of abortion in cows, that the constant depasturing on grasses affected with ergot (and the Lolium perenne in aftermaths is often especially so) has been the predisposing cause.