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

by M. C. Cooke

We received, for the first time, in July, 1864, from Mr. Gatty, student at Winchester, a portion of a plant of Thesium humifusum (which is by no means common in Britain), covered with beautiful cluster-cups of a species never before recorded as occurring in this country ([Plate III.] figs. 50, 51) named Æcidium Thesii, but which is far from uncommon on the Continent. It occurred in this instance on the Downs, in the vicinity of Winchester.

It is unnecessary here to refer to other allied species of Æcidium, except one to be presently noticed, since we have, at the end of the volume, enumerated and given descriptions of all the species hitherto found in Britain. Suffice it to say that the Buckthorn cluster-cups on the alder buckthorn (Rhamnus frangula), is usually very common in the Highgate and Hornsey woods, and on the common buckthorn (Rhamnus catharticus) in the neighbourhood of Dartford, in Kent. That on the honeysuckle we have found but very rarely. On the gooseberry and red-currant leaves, commonly in some years and rarely in others; whilst a few of those described we have never collected. The species on different composite plants is subject to great variation, and on most may be found in the autumn; one variety only, on the leaves of Lapsana communis, we have met with in the spring.

Very few years ago farmers generally believed that the cluster-cups of the berberry (Berberis vulgaris), were productive of mildew in corn grown near them; this opinion even received the support of Sir J. Banks, but no fungi can be much more distinct than those found on corn crops and this species on the leaves of the berberry. In this instance the cups are much elongated, and cylindrical, the clusters vary much in size, and the spots on the upper surface of the leaf are reddish, bright, and distinct. The teeth are white and brittle, and the orange spores copious ([Plate I.] figs. 7-9).

There are scarcely any of the epiphyllous fungi forming equally handsome or interesting objects for low powers of the microscope, than the genus to which attention has just been directed; and they possess the advantage of being readily found, for that locality must be poor indeed which cannot furnish six species during the year. We have found half of the number of described species within little more than walking distance of the metropolis, within a period of little more than three months, and should be glad to hear of the occurrence of any of the rest.

We have three species of fungi very similar in many respects to the foregoing, but differing in others to such an extent as to justify their association under a different genus and name. The hawthorn is a bush familiar to all who love the “merry month of May,” but it may be that its parasite has been unnoticed by thousands. If, for the future, our readers will bear this subject in their minds when they stand beneath a hawthorn hedge, they may become acquainted with clusters of singular brown pustules on the leaves, petioles, and fruit well worthy of more minute examination ([Plate II.] fig. 22). They scarcely claim the name of cups, and their lacerated and fringed margins rather resemble the pappus crowning the fruits of some composite plants than the cups of Æcidium. The peridia are very long, and split down throughout their length into thread-like filaments of attached cells; these gradually fall away and break up into their component parts till but short portions remain attached to the base of the peridia. These cells are elongated and marked on the surface with waved lines, forming in themselves pretty objects for a high power of the microscope ([Plate II.] figs. 23, 24). If the teeth of Æcidium resemble the peristome of some mosses, such as Splachnum; the threads of this species of Rœstelia, except in not being twisted, somewhat resemble the peristomes of other mosses of the genus Tortula. The spores in this species are less conspicuous, being of a light brown, and the whole plant, from its modest hue, may be readily passed over without attracting attention unless occurring in abundance.

The leaves of pear-trees afford a second species of this genus sufficiently distinct to commend it to our notice. Sometimes it is very common, at others but few examples are to be met with. The clusters occur on the under surface, and consist of half-a-dozen or less of large peridia, pointed at the apex and swelling in the middle so as to become urn-shaped ([Plate II.] figs. 20, 21). These vessels or thecæ split into numerous threads or laciniæ, which remain united together at the apex. Like the species already noticed, this is brown and inconspicuous except on account of its size, for it is the largest of all that we have had occasion to notice.

The third species occurs on the under surface of the leaves of the mountain-ash. The peridia are clustered on a rusty orange-coloured spot which is visible on the upper surface ([Plate II.] figs. 18, 19). They are long and cylindrical, with an evident tendency to curvature; the mouth is serrated, but not split up into threads, as in the species found on the hawthorn. There will often be found instead of well-developed peridia, what at one time were regarded as abortive peridia, forming a thickened orange or rust-coloured spot, studded with minute elevated points. These spots are clusters of spermogones, which organs are described in detail in our second chapter. The clusters and spores are of a brighter reddish-brown than in either of the other species. All are remarkably distinct, and perhaps the most curious and interesting of any that we have passed in review. To botanists, the species found on the hawthorn is known as Rœstelia lacerata, that on pear-leaves as Rœstelia cancellata, and the one on the leaves of the mountain-ash as Rœstelia cornuta.

Dr. Withering observed the spore-spots on the leaves of the mountain-ash, but was evidently puzzled to account for them. He writes (in his Arrangement of British Plants), “The spots on the leaves of Sorbus aucuparia consist of minute globules intermixed with wool-like fibres. On examining many of them in different states, I at length found a small maggot in some of the younger spots, so that the globules are probably its excrement, and the fibres, the woody fibres of the plant unfit for its food.” We now-a-days smile at such simple and singular conjectures. It affords evidence of the manner in which the speculations of one generation become follies in the next.

Only two species of cluster-cups are described in Withering’s Flora under the genus Lycoperdon: one of these is now called Æcidium compositarum, and is found on various composite plants; the other includes the species found on the wood-anemone and that on the moschatel, and also probably a species of Puccinia on the wood-betony.

To render this chapter more complete, though of less importance to the microscopist, we may allude to the other two genera comprised within this order. Peridermium is the name of one genus which contains two British species found on the leaves and young shoots of coniferous trees. In this genus the peridium bursts irregularly, and does not form cups, or horns, or fringed vessels. The most common species is found on the needle-shaped leaves of the Scotch fir ([Plate II.] fig. 27), and also on the young twigs, in the latter instance larger and more prominent than in the former. The elongated peridia burst irregularly at their apices without forming teeth (fig. 28).