Rust, Smut, Mildew, & Mould: An Introduction to the Study of Microscopic Fungi

by M. C. Cooke

With a view to the clearer understanding of these parasites in the phases of their development, let us select one, and we cannot do better than adhere to that of the wheat and other graminaceous plants. A fine day in May or June dawns upon our preparations for a stroll, far enough into the country to find a wheat-field. Even now, with the area of the metropolis constantly widening, and banishing farmers and wheat fields farther and farther from the sound of Bow-bells, a corn field may be reached by a good stiff walk from Charing-Cross, or a six-penny ride at the most, in nearly any direction. Having reached the field, it may be premised that a walk into it of less than twenty yards will be sure to reward you with the fungus we are in quest of. Look down at the green leaves, especially the lower ones, and you will soon find one apparently grown rusty. The surface seems to be sprinkled with powdered red ochre, and grown sickly under the operation. Pluck it carefully, and examine it with a pocket lens. Already the structure of a healthy leaf is familiar to you, but in the present instance the cuticle is traversed with numerous longitudinal cracks or fissures, within which, and about their margins, you discern an orange powder, to which the rusty appearance of the leaf is due. Further examination reveals also portions in which the cuticle is distended into yellowish elongated pustules, not yet ruptured, and which is an earlier stage of the same disease. This is the “rust” of the agriculturist, the Trichobasis rubigo-vera of botanists, the first phase of the corn mildew.

To know more of this parasite, we must have recourse to the microscope; having therefore collected a few leaves for this purpose, we return homewards to follow up the investigation. We will not stay to detail the processes of manipulation, since these will not offer any deviation from the ordinary modes of preparation and examination of delicate vegetable tissues.

The vegetative system of the “rust,” and similar fungi, consists of a number of delicate, simple, or branched threads, often intertwining and anastomosing, or uniting one to the other by means of lateral branchlets. These threads, termed the mycelium, penetrate the intercellular spaces, and insinuate themselves in a complete network, amongst the cells of which the leaf, or other diseased portion of the plant, is composed. High powers of the microscope, and equally high powers of patience and perseverance, are necessary to make out this part of the structure. We may regard the whole mycelium of one pustule, or spore-spot, as the vegetative system of one fungal plant. At first this mycelium might have originated in a number of individuals, which afterwards became confluent and combined into one for the production of fruit, that is to say, an indefinite number of points in the vicinity of the future mycelium developed threads; and these, in the process of growth, interlaced each other, and ultimately, by means of transverse processes, became united into one vegetative system, in which the individuality of each of the elementary threads became absorbed, and by one combined effort a spore-spot, or cluster of fruit, was produced. In the first instance a number of minute, transparent, colourless cellules are developed from the mycelium: these enlarge, become filled with an orange-coloured endochrome, and appear beneath the cuticle of the leaf as yellowish spots. As a consequence of this increase in bulk, the cuticle becomes distended in the form of a pustule over the yellow cellules, and at length, unable longer to withstand the pressure from beneath, ruptures in irregular, more or less elongated fissures ([Plate VII.] fig. 141), and the yellow bodies, now termed spores (whether correctly so, we do not at present inquire), break from their short pedicels and escape, to the naked eye presenting the appearance of an orange or rust-coloured powder. In this stage the spores are globose, or nearly so, and consist of but one cell [Plate VII.] figs. 142, 144). It will afford much instructive amusement to examine one of these ruptured pustules as an opaque object under a low power, and afterwards the spores may be viewed with a higher power as a transparent object. The difference in depth of tint, the nearly colourless and smaller immature spores, and the tendency in some of the fully matured ones to elongate, are all facts worthy of notice, as will be seen hereafter.

A month or two later in the season, and we will make another trip to the cornfield. Rusty leaves, and leaf-sheaths, have become even more common than before. A little careful examination, and, here and there, we shall find a leaf or two with decidedly brown pustules intermixed with the rusty ones, or, as we have observed several times during the past autumn, the pustules towards the base of the leaf orange, and those towards the apex reddish-brown. If we remove from the browner spots a little of the powder, by means of a sharp-pointed knife, and place it in a drop of water or alcohol on a glass slide, and after covering with a square of thin glass, submit it to examination under a quarter-inch objective, a different series of forms will be observed. There will still be a proportion of subglobose, one-celled, yellow spores; but the majority will be elongated, most with pedicels or stalks, if they have been carefully removed from the leaf, and either decidedly two-celled, or with an evident tendency to become so. The two cells are separated by a partition or dissepiment, which divides the original cell transversely into an upper and lower cell, with an external constriction in the plane of the dissepiment ([Plate IV.] fig. 59). These bilocular or two-celled spores are those of the “corn mildew” (Puccinia graminis), which may be produced in the same pustules, and from the same mycelium, as the “corn rust,” but which some mycologists consider to be a distinct fungus, others only a modification or stage of the same fungus. After an examination of the different forms in the allied genera to which these chapters are devoted, we shall be able with less of explanation and circumlocution to canvass these two conflicting opinions.

Let us proceed, for the third and last time, to our cornfield, when the corn is nearly or fully ripe, or let us look over any bundle of straw, and we shall find blackish spots, from the size of a pin’s head to an inch in length, mostly on the sheaths of the leaves, often on the culm itself. This is the fully developed mildew, and when once seen is not likely afterwards to be confounded with any other parasite on straw (fig. 57). The drawings of Bauer have already been alluded to. Bauer was botanical draughtsman to George III., and his exquisite drawings, both of the germination of wheat and the fungi which infest it, are marvels of artistic skill. A reduced figure from part of one of his drawings is given ([Plate IV.] fig. 58), exhibiting a tuft of the bilocular spores of Puccinia graminis bursting through a piece of wheat straw. These closely-packed tufts or masses of spores, when examined with a common lens, seem, at first, to resemble the minute sorus of some species of fern; but when seen with higher powers, the apparent resemblance gives place to something very different. The tufts consist of multitudes of stalked bodies, termed spores, which are constricted in the middle and narrowed towards either extremity. The partition, or septum, thrown across the spore at the constriction, separates it into two portions, each of which consists of a cell-wall enclosing an inner vesicle filled with the endochrome (fig. 59) or granular contents, in which a nucleus may often be made out. This species of Puccinia is very common on all the cereals cultivated in this country, and on many of the grasses. A variety found on the reed was at one time considered a distinct species; but the difference does not seem sufficient to warrant a separation. However near some other of the recognized species may seem to approximate in the form of the spores, a very embryo botanist will not fail to observe the distinctive features in the spores of the corn mildew, and speedily recognize them amongst a host of others; subject, as they may be, to slight deviations in form, resulting either from external pressure, checks in development, or other accidental circumstances, or the variations of age.

There is no doubt in the minds of agriculturists, botanists, savans, or farm-labourers, that the mildew is very injurious to the corn crop. Different opinions may exist as to how the plants become inoculated, or how infection may be prevented or cured. Some have professed to believe that the spores, such as we have seen produced in clusters on wheat straw, enter by the stomata, or pores, of the growing plant, “and at the bottom of the hollows to which they lead they germinate and push their minute roots into the cellular texture.” Such an explanation, however plausible at first sight, fails on examination, from the fact that the spores are too large to find ingress by such minute openings. It is improbable that the spores enter the growing plant at all. The granular contents of the spores may effect an entrance either through the roots or by the stomata, or the globose bodies produced upon the germination of the spores may be the primary cause of infection. We are not aware that this question has been satisfactorily determined. It is worthy of remembrance by all persons interested in the growth of corn, that the mildew is most common upon plants growing on the site of an old dunghill, or on very rich soil. As the same Puccinia is also to be found on numerous grasses, no prudent farmer will permit these to luxuriate around the borders of his fields, lest they should serve to introduce or increase the pest he so much dreads.

The germination of the spores of the corn mildew is a very interesting and instructive process, which may be observed with a very little trouble. If the spores be scraped from the sori of the preceding year (we are not sure that those of the current year will succeed), and kept for a short time in a damp atmosphere under a glass receiver, minute colourless threads will be seen to issue both from the upper and lower divisions of the spores. These will attain a length several times that of the spores from whence they spring. The extremities of these threads ultimately thicken, and two or three septæ are formed across each, dividing it into cells, in which a little orange-coloured endochrome accumulates. From the walls of each of these cells, or joints, a small pedicel, or spicule, is produced outwards, the tip of which gradually swells until a spherical head is formed, into which the orange-coloured fluid passes from the extremities of the threads.[^[4]] A quantity of such threads, bearing at their summits from one to four of these orange-coloured, spherical, secondary fruits, supply a beautiful as well as interesting object for the microscope. When matured, these globose bodies, which Tulasne has called sporidia, fall from the threads, and commence germinating on their own account. It is not impossible that the sporidia, in this and allied genera, may themselves produce a third and still more minute fruit, capable of diffusion through the tissues of growing plants, or gaining admission by their stomata. Nothing of the kind, however, has yet been of certainty discovered.

[4]. Similar in all essential particulars to the germination of Aregma ([Plate III.] fig. 45).

Forty other species of Puccinia have been recorded as occurring in Great Britain, to all of which many of the foregoing remarks will also apply—viz., such as relate to their two-celled spores being found associated with, and springing from, the same mycelium as certain orange-coloured one-celled spores; and also the main features of the germinating process.