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

by M. C. Cooke

We have not thought it necessary to give figures of many species, partly on account of the uncertainty existing in many minds whether they ought to be regarded as species, and whether they will long claim a place in the British Flora; and partly on account of the similarity which exists between them, at least so far as they are of interest to the microscopist only.

During the autumn of last year, whilst on a botanical excursion through a portion of Epping Forest, the “great bog” became a centre of some interest. Bogs are generally attractive spots to those who are in search of microscopic organisms. On this occasion the chief objects of interest were the small brown pustules ([Plate VIII.] fig. 168) with which the upper surface of a large number of the leaves of the pennywort (Hydrocotyle vulgaris) were sprinkled. These pustules were brown, orbicular, regular, and in habit seemed to resemble rather those of most of the Pucciniæ than of a Trichobasis. The large, vigorous, and healthy leaves were less affected. Microscopical examination, at first incomplete, led us to the conclusion that it was a species of Uredo, which had been met with in France (Uredo Hydrocotyles, Mont.); but a re-examination, to which we were prompted by Mr. Currey, led to the conclusion that it belonged rather to the present genus; but it can scarcely be associated with any species already described, notwithstanding its apparent affinity with the brown rust found on umbelliferous plants, in which the pustules are invariably developed on the under surface. Under these circumstances, we have called it the Pennywort rust (Trichobasis Hydrocotyles), whilst still doubtful whether it is not the same fungus as that described by Montagne, with whose description it agrees in everything, except what is implied by the generic name. It should not be forgotten, that the work in which Montagne first described this species was published when the genus Uredo embraced the present genus, which was separated from it by Leveille twelve years later; and it is possible that this species was included in a more recent work by the same author, in error and without re-examination, under the old name. An examination of authentic specimens of Montagne’s plant would settle the point; but we know of no published collection which contains it. The characteristic difference between the two genera lies in the presence of a peduncle in the early stage of Trichobasis [Plate VIII.] fig. 169), and its absence in all stages of Uredo. Without wandering further into a subject which has not the merit of being very popular, let us away to some green lane in search of violets, and having found them, take a little of the brown dust from one of the small pustules on the leaves, upon the point of a penknife; place this, with a drop of water, upon a glass slide, and make a record of what we observe.

The field is covered with the myriad spores of a rust of a nearly spherical shape, brownish in colour, and here and there one with a short transparent colourless stalk or pedicel. This is the violet rust (Trichobasis Violarum, B.), very common all through the summer and autumn, generally on the under surface of the leaves of violets, in woods and hedgerows. Should it so happen that the spores when placed under the microscope are found to be two-celled, it will prove that instead of a rust, or Trichobasis, being under examination, a brand, or Puccinia, has been found, which is almost equally common, and which may, without such a test, be easily mistaken for a rust. According to the theory of di-morphism, this is the higher form or complete fruit of the same fungus, which in its simple-celled state is called Trichobasis Violarum.

A similar circumstance may befall the student in examining the rust of labiate plants (Trichobasis Labiatarum, Lev.), which occurs on different species of mint, especially the water-mint, about the month of August. We have found a few of the two-celled spores of the Puccinia imbedded in the pustules of this rust almost constantly, whilst the one-celled spores are not uncommon in the pustules of what is regarded as the true mint brand (Puccinia Menthæ, P.).

In spring the young leaves of the periwinkle (Vinca major) will, in some situations, become thickened considerably, and ultimately browned on both faces with the pustules of a rust (Trichobasis Vincæ), which though covered with a conidioid dust are often very tardy in bursting the epidermis. It is almost impossible, after the pustule is ruptured, to find one in which the bilocular spores of Puccinia (fig. 132) are not largely intermingled with the unilocular spores of the “rust” ([Plate VI.] fig. 131). Later in the season by two or three months, other leaves of the same plants will be found occupied by the smaller and more widely scattered pustules of Puccinia Vincæ, in which the unilocular spores of the “rust” will seldom be found. In this instance the leaves are scarcely thickened, and the colour of the spores is much darker. The under surface of the leaves is commonly alone occupied, and corresponding paler spots on the upper surface indicate the presence of the parasite beneath.

The garden and field bean is liable in some seasons to become quite rust-coloured in consequence of the profusion of spores with which the leaves and stems are covered, from the bean rust (Trichobasis Fabæ, Lev.), which in like manner is considered as the simple stage or form of the bean brand (Puccinia Fabæ, Lk.). If the legumes are also examined, a few pustules will sometimes be found on them. Beans thoroughly infected with this rust or brand are seldom of much service in either field or garden.

Beetroot, or mangold wurzel, is another example of garden and field produce which is subject to a similar visitation. This rust often has very red spores when produced on red varieties of beet. During September, 1863, it (Trichobasis Betæ, Lev.) was sent us on the leaves of the wild beet (Beta maritima).

All the species of Polygonum are exceedingly subject to the attacks of the Persicaria rust (Trichobasis Polygonorum, B.), which nearly covers the leaves, till the entire plants seem to be smothered in Scotch snuff. If it were allowable to affirm of any plants that they are martyrs to parasitism, such might be said of the Persicarias and their allies.

Plate VIII.
W. West imp.