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

by M. C. Cooke

From the numerous instances we have met with of persons wholly disinterested in the subject, collecting and making inquiries concerning the bright orange patches next to be referred to, we are led to conclude that most persons have, at least, seen them. These patches of orange-coloured powder burst through the cuticle, and appear on the young branches of wild roses, extending sometimes for two or three inches in length [Plate VII.] fig. 147), and form one of the most beautiful and obtrusive of the dust-like fungi. It is not confined to roses; but the meadow-sweet, on which it is also found, does not grow in localities where its parasite is so liable to meet the gaze of the ordinary wayfarer. The habit of this rust is more or less that of those which are associated with it. The pustules are not small and orbicular as in most instances in other groups (except Polycystis), but are large and irregular, and generally but few together or single. The spores are variously coloured, and have peduncles, or footstalks, of a greater or less length. In the rose rust these spores are profuse, but the peduncle is short ([Plate VII.] fig. 148).

Plants of the pea and bean tribe are liable to be attacked by one of these rusts, and in this instance the spores are so characteristic that no one could well confound them with any other. Externally it appears as an irregular brownish pustule, breaking through the epidermis and filled with an impalpable powder, not unlike a pinch of “brown rappee” snuff. The spores are ovoid, with a very long peduncle, whence its name (Uromyces appendiculata). It appears on a great variety of plants, but from the peculiarity of the spores [Plate VII.] fig. 150), is easily recognized. Our figure represents it on the leaf of a vetch ([Plate VII.] fig. 149). Almost at the commencement of this volume ([Chap. III.]) we had occasion to refer in detail to some experiments made by De Bary on the spores of this species. In the remarks then made occurs a recommendation of reperusal, which would obviate any repetition here.

An allied species, and a beautiful one, is to be found on the stinking iris (Iris fœtidissima), and another on the under surface of the leaves of primroses.

The pilewort (Ranunculus ficaria) we have already seen attacked by one species of microscopic fungi, and we have now to record the occurrence of another. Some plants appear to be destined both alive and dead to become the prey of others. The common nettle, for instance, as if in retribution for the annoyance it often occasions on account of its stinging propensities, has not less than twenty different species of minute fungi, to say nothing of coleopterous and lepidopterous insects, which make a home, sometimes upon its green leaves, and sometimes on its dead stems. We might almost state that it has a flora and a fauna of its own. The pilewort, too, has many foes; but these are fewer in number, and mostly attack the living plant. The cluster-cups have been already noticed; some do not fall within the limits of this volume, but one, which is found in May and June, belongs to the present genus. It appears like a purplish-brown powder bursting through blistered spots on the leaves and footstalks [Plate VII.] fig. 156). The spores are small, and are, of course, provided with pedicels [Plate VII.] fig. 157).

The under surface of the leaves of the white Dutch clover are often sprinkled with black spots, which are nearly round and very numerous. These are so many clusters of fungi belonging to a different section, in which the threads are the important feature. But another parasite is also found on leaves of the same plant, in which the pustules are far less numerous and regular, and are often found on the petiole as well as the leaf, distorting them and twisting them in various directions [Plate VII.] fig. 154). This is the clover rust (Uromyces apiculata, Lev.), which is a parasite on numerous plants, being found also on the great water-dock and other kinds of dock. The spores are ovoid and brown, with a short peduncle ([Plate VII.] fig. 155). A very beautiful species occurs on the leaves of the ladies-mantle (Alchemilla), but hitherto we have not been fortunate enough to collect it.

It can scarcely be too great an assumption to suppose that every one is acquainted with the goat-willow (Salix caprea), or that every schoolboy knows the birch (Betula alba). It may be proceeding a step too far to affirm that all who know these trees well enough to distinguish the one from the other, will have observed the under surfaces of the leaves of both sprinkled with a golden dust, during the summer months, and which are the spores of a parasitic fungus. So common is this orange-coloured powder on leaves of the trees above-named, that we can hardly believe any one to have had a branch of either in his hand and not observed it, provided any leaves adorned the branch in question. What this parasite is, and what its associates, it is our province to endeavour to explain. Our figure ([Plate VIII.] fig. 160) represents a leaf of the sallow or goat-willow, with the under surface exhibiting yellow patches, consisting of spores, which are magnified in the next figure ([Plate VIII.] fig. 161). This exceedingly common rust is termed Lecythea caprearum, Lev., when in the condition figured; but in reality this is only the summer stage, bearing the summer fruit of Melampsora salicina, which latter attains its mature development on the same leaves in the succeeding winter ([Plate IX.] fig. 191) or early spring. Of course this latter remark applies to the fallen leaves, for at this period all the willows and other deciduous trees are bare. But the leaves, before they fall, give evidence of the parasite at work; and if the collected decaying mass of rubbish at the base of sallow bushes be examined about March, these leaves will be found bearing upon them mature heaps of elongated, wedge-shaped spores, closely packed side by side ([Plate IX.] fig. 192), and which, whilst still adherent, may often be found in active germination, as represented in an allied species at the bottom of our plate ([Plate IX.] fig. 197). This phenomenon consists in the production of cylindrical tubes, more or less elongated, from the upper extremity (rarely from the base) of the prismatic spores. These tubes are straight or twisted, simple or forked, and each of them becomes divided by transverse septa into four unequal cells towards their apex, from each of which is produced a spicule bearing a sporidium, or spherical secondary fruit, in the same manner as in the genera Puccinia and Aregma. It should be observed, that the winter spores of this rust are borne on the opposite surface of the leaves to the summer spores: for whilst the latter are developed from the under surface, the former are found on the upper. This being also an instance of di-morphism, the summer condition, when spherical spores are produced, should not be regarded as a distinct plant, and the name of Lecythea caprearum does not merit retention in the list of fungi.

There are five species of this interesting group, or genus, found in Great Britain, to the residue of which we may only briefly allude. It has already been stated that, in summer, the yellow spores of a rust are found on the under surface of birch-leaves. These must be sought on the young twigs or suckers, proceeding from the stumps of trees which have been cut down: pale discoloured spots on the upper surface of the leaves indicate the presence of the rust beneath. This is the Uredo betulina of old authors (begging their pardons, for some of them still live), the Lecythea longicapsula of more recent times, and the summer spores of Melampsora betulina, according to M. Tulasne and his disciples. When fresh, it is reputed to exhale a faint odour, as of the primrose. During the winter and spring months the wedge-like spores of the second crop are matured on the fallen leaves ([Plate IX.] figs. 189, 190), and these are capable of a speedy germination, and the production of secondary reproductive bodies, as in the willow rust above alluded to.

Probably, also, the similar rust on the poplar ([Plate IX.] figs. 195, 196), or on the aspen, may be met with under like conditions; i.e., the summer spores, which are yellow and spherical, on the living leaves, and the brown permanent masses of winter spores on the fallen and decaying leaves. The sole remaining British species is not uncommon on leaves of the common spurge in gardens, and whilst the yellow pulverulent spores occur on the upper, it will not be improbable that black permanent spots will be found on the lower leaves ([Plate IX.] figs. 193, 194), enclosing closely-packed, rudimentary, elongated or wedge-shaped cellules of the winter spores.

Any one may make himself acquainted with the genus Coleosporium with but little trouble, which the acquisition will more than compensate. A summer stroll into any locality in which the common coltsfoot can be found, will be certain to prove sufficient. Let the spot selected be any station on the North Kent Railway, for those who reside in town, or even a trip to the Crystal Palace and a stroll in the grounds, and when the well-known leaves of the coltsfoot are descried, the under surface of the first leaf will doubtless give proof of the presence of the fungus in question, by the orange spores amongst its dense woolly hairs. Sometimes the leaf is almost covered beneath with the bright orange-coloured dust. This is the coltsfoot rust (Coleosporium tussilaginis, Lev., [Plate VIII.] fig. 180), which may serve as a type of the rest. It may be observed that a species of cluster-cup, or Æcidium, with spores of nearly the same colour, is also to be found on the leaves of the same plant; but in this case the upper surface of the leaf has also corresponding purplish spots, and, what is of still more importance, the spores are seated in small fringed cups. The rust is common till the wintry frosts have set in, and is far more conspicuous than the cluster-cups. A kind of di-morphism prevails in all the species of this genus. Some of the pustules resolve themselves into a kind of powder, whilst others remain entire and solid. Generally there is the largest proportion of globose, dust-like, free spores, produced in the earliest developed fungi, whilst they become more rare towards the close of the season. The permanent spore-spots consist of obovate cellules placed side by side, each of which is divided transversely by three or four septa, and is filled with an orange-red endochrome ([Plate VIII.] fig. 181); the exterior being enveloped in a kind of mucous layer. The arrangement of spores when packed together in the pustule is shown in [Plate VIII.] fig. 182, from an allied species. When these spores germinate, which they do with great readiness, each division emits a long tube, which generally remains simple and undivided, and from its extremity is produced a reproductive body of an obovate or nearly kidney-shape. These filaments are about 1/120th of an inch in length, of a colourless transparent membrane, along which the orange-red contents of the spores pass into the newly-formed sporidia, or reproductive bodies by which they are terminated. Most of these reniform sporidia disengage themselves from the filaments on which they are produced, and either elongate themselves into a simple and uniform filament, or swell at the extremity as if to reproduce a second spore. If the newly-formed sporidia do not become free, they increase the length of their primitive filament, which by a frequent repetition of the process becomes a tube swelling out at unequal distances.