CHAPTER VIII.
RUSTS.
UNFORTUNATELY, this group of fungi contains species but too well known for their ravages amongst graminaceous plants, especially the cereals. “Corn rust,” as it is generally called, has a reputation little better than mildew, and it really deserves no better, for it is only another form of that pest of the farm, from the mycelium of which the corn-mildew is at length developed. There are two species very closely allied (doubtless only forms of the same species with different spores) which attack the leaves and culms of growing corn, and, bursting through the cuticle in the manner represented in [Plate VII.] fig. 141 (magnified), give a peculiar rusty appearance to the plant, as represented in [Plate VII.] fig. 140. One of these corn-rusts is botanically named Trichobasis rubigo-vera (Lev.), or the “true rust Trichobasis;” the latter, which is the generic name, being a compound of two Greek words (thrix, a hair, and basis, a foundation), on account of the spores being at first furnished at their base with a short, thread-like peduncle, which at length falls away ([Plate VII.] fig. 142). The other Corn rust is Trichobasis linearis, or “line-like Trichobasis,” because the sori or pustules are linear, or lengthened out like a line; the spores nearly double the length of those of the other Corn rust ([Plate VII.] fig. 144), and not so bright in colour. By intermediate forms these two rusts pass insensibly the one into the other, so that it is sometimes difficult to distinguish them. Both have the spores clustered together in the pustules, at first ([Plate VII.] fig. 143) attached by their peduncles, but they soon become free, and are scattered like rust-powder over the plant. Adverting to the attack of rust upon the wheat crops to an unusual extent a few years since, the Rev. M. J. Berkeley wrote as follows:—
We have seen rust more prevalent in white wheats, especially in the variety called Russian white, which has red smooth chaff, than we ever remember it. It is, however, confined to particular spots; and while in one field not a single leaf is free, insomuch that a person walking through the wheat is completely painted with the spores, of a fine rust-red; an adjoining field, separated only by a hedge, has not a rusty leaf. So long, however, as the rust is confined to the leaf, it is, we believe, perfectly harmless. The grain swells in spite of it, and the only effect is that the flag dies a little earlier, which is not undesirable when it is too luxuriant. If, however, it gets to any extent upon the chaff, much more if it attacks the seed itself, as is sometimes the case, it is very mischievous.
Plate VI.
W. West imp.
There is, indeed, a notion that rust is merely a form of mildew, and this may be true; but if so, it is a harmless form. The true rust (T. rubigo-vera), if a form of mildew, preserves its own characters to the end, and merely presents a second form of fruit, a circumstance which is exceedingly common amongst the lower fungi. It may exist in company with the mildew, but it also more frequently is developed without a trace of mildew intermixed. As far, then, as its immediate effects upon the plant go, it may be treated as a distinct parasite, though advanced botanists may look upon it as offering a case of what they call dualism, which may be easily understood on a comparison with such plants as marigold and orache, which produce seeds of two different forms. Its ultimate effects may, indeed, be injurious, as, if it be really a second form of mildew, it is capable of propagating that pest. The case is quite different with Trichobasis linearis, which is in fact merely the young state of the mildew. When once that supposed species makes its appearance, it is quite certain that there will be mildew to a greater or less extent. While the spores of the one remain unaltered, though intermixed with the true bipartite spores of the mildew, the other exhibits every intermediate state of form and colour.
M. Tulasne, to whom we have already alluded as a high authority on this subject, is of opinion that all the species enumerated in this group or genus, are mere conditions of other species, and therefore excludes it altogether. Another genus (Lecythea), for the same reason, has shared the same fate.
It must not be forgotten that twenty-eight species of rust belonging to this genus are now recorded as occurring in Great Britain. Some of these have yellow or yellowish spores; the rest have the spores of a decidedly brown colour. One of the most attractive of the yellow-spored species is that which appears on the groundsel (T. Senecionis), and a good notion of its external appearance may be gained from our figure ([Plate VII.] fig. 145). It is not always to be found without searching closely, for in some localities we have found it very sparingly, whilst elsewhere—as, for instance, on Wandsworth Common—we have met with it as soon or as late as there were groundsel leaves for it to grow upon. No spot or change in the appearance of the upper surface of the leaves indicates the presence of the parasite beneath; this is, however, often betrayed by the golden-yellow streaks which appear on the stem. Many a time and oft the boys in the semi-rural districts about town have marvelled and questioned each other concerning the reason why, in gathering “groundsel” for the little bird at home, we should be so particular, and pass so many promising plants, plucking here and there a leaf, and seldom collecting one which they consider eligible for the purpose. Collectors of minute fungi must expect to overhear occasionally even hints touching their sanity from those who, without the remotest idea of their mission, think they must be slightly “wrong in the head” to gaze so narrowly and intently, amongst nettles, groundsel, grass, or dry leaves, and only take an occasional fragment of a rotten stick, or two or three sickly leaves, carefully deposit them in their wallet, hat, or pocket, and then “move on.”
It will be unnecessary to repeat what has been stated in a former chapter on spermogones. It will be remembered that these organs are found developed in connection with some of the species of the present genus, as well as with the cluster-cups, or Æcidiacei. They consist externally of small conical elevations, pierced at the apex, which contain minute, cellular, linear bodies called spermatia, invested with a kind of mucous substance, that over-flows with them from the orifice of the spermogone, like lava from the crater of a miniature volcano. The rust in company with which they have been found most plentifully, is that which covers the under surface of the leaves of the commonest of all thistles (Cnicus arvensis). The external form of one of these spermogones is figured [Plate VII.] fig. 153. The rust possesses, when fresh, a peculiar odour, which is said to resemble that of orange-flowers; whence was derived its name of “sweet-smelling rust” (Trichobasis suaveolens, Lev.). M. Tulasne writes:—“With respect to this species, it is, in my opinion, but the first form of a Puccinia, analogous to P. Compositarum, D.C.: the spermogones with which it is mixed being very abundant, it ought to be placed with that Puccinia. I should, perhaps, retain some doubts upon the legitimacy of this relationship, if it had not been my fortune to meet on another species of the same kind (Puccinia Anemones, P.) spermogones perfectly characterized, and in which the colour, habit, and position would not allow me to hesitate an instant in attributing them to the Puccinia.” In this species of rust the whole under surface of the leaf is covered with a purplish-brown dusty coating of the spores, from the numerous pustules which are produced ([Plate VII.] fig. 151). The plants, when attacked, have a paler and more sickly appearance; the leaves have a tendency to fold backwards at their margins, and thenceforth their growth seems to be determined. Sowerby, in his “British Fungi,” says:—“Two or three sorts of flies are occasionally found dead on this plant at the time of the fungus being upon it, which is after wet weather in the summer, or early in autumn; being apparently tempted by its flavour, they over-eat themselves, or else are destroyed by some poison.” This rust has spores resembling, in general characters, those of the yellow-spored series [Plate VII.] fig. 152).