NOTWITHSTANDING the inconvenience to ourselves of calling very different fungi by the same common name of “mildew,” the popular mind does not recognize the inconvenience, since it scarcely troubles itself to inquire whether they are not all the same thing. In obedience to this custom, we again write of “mildew,” or “blight,” as it is called in some districts, but of a very different kind to that which is so detrimental to growing crops of corn. In the present instance it is our intention to illustrate a group of fungi which are exceedingly common, and which differ greatly in appearance and structure from any to which we have had occasion to allude. To obtain a general knowledge of these forms let our reader proceed at once to a clump of rank grass; if it is his fortune to dwell in the country, the walk of a few yards will suffice. Let him examine this clump more carefully, perhaps, than he has been accustomed to do, and we venture to predict that he will find some of the leaves covered with what appears to be a dirty white mould, or mildew ([Plate XI.] fig. 235). One of these leaves should be collected as carefully and conveyed to the microscope as speedily as possible, taking care not to touch, or brush it against any other object so as to disturb the arrangement of the delicate little threads upon its surface. If a small portion, say about an inch, is cut from this leaf with a sharp pair of scissors, and laid upon a slide, or pinned down upon a strip of sheet cork, so as to keep it flat, and then submitted to examination under the microscope, with an inch power, a beautiful forest of crystalline vegetation will be observed. If the examiner on this occasion should not possess a binocular microscope we are sorry for him, because in that case he will not see all that is to be seen under the greatest advantages. If we ever truly enjoy looking through such an instrument, it is on an occasion like this, when a low power is all that is needed, and the object is required to be seen in relief. It is scarcely possible to convey an adequate idea of the beauty of such a scene as the microscope reveals upon this fragment of grass-leaf. Little bundles of delicate threads, clear and crystalline, are seated upon a slender branching mycelium. These threads, sometimes erect, sometimes drooping, flexuous, or prostrate, are composed of numerous roundish or spherical cells attached to each other in a moniliform or bead-like manner (fig. 236). These easily separate from each other. Let a portion of the threads be removed from the leaf on the point of a lancet and laid upon a glass slide, with a thin cover over them. Submit this object to a quarter-inch power, as a drop of water is let fall at the edge of the cover and insinuates itself, by capillary attraction, between the two plates of glass. So soon as it touches the moniliform threads, the disunion commences, and almost before they are enveloped in the fluid, two spherules will scarce remain attached to each other. This delicate little mould on the grass leaf at one time bore the name of Oidium monilioides. It is now regarded only as a condition of another minute fungus, to which attention will shortly be directed.
The vine disease, so fearfully destructive on the Continent, and not altogether unknown in this country, is another of these incomplete fungi. From an individual who at the time of its first discovery in the south of England took considerable interest in the subject, it was called Oidium Tuckeri, which name it continued to bear, both here and abroad, until, with many others, probably nearly all of the same genus, it was found to be only a barren state of what is called by mycologists an Erysiphe. The real discoverer of this mildew was undoubtedly the Rev. M. J. Berkeley, who has successfully devoted a long life to the study of these minute organisms, through evil and through good report, and when that study was beset with more difficulties, and received less encouragement than at present. If, towards the autumn, we should again collect some whitened, mouldy, or mildewed grass-leaves, similar in appearance to those mentioned above, and carefully look at them with a pocket lens, little black points, almost as small as a pin-point, or more resembling the full stop with which, this sentence closes, will be found scattered over the white threads. The aid of the microscope must be again sought to make out the structure of the little black dots. Closely nestling upon the mycelium, the little points will prove to be spherical brownish, conceptacles, surrounded with transparent floccose appendages. Many other species are far more beautiful than that of the grass-leaf, as will be seen by reference to our plate. The variation consists chiefly in the form of the appendages which spring from the conceptacle and surround it in a radiating (as in figs. 219, 222, 225, and 230), or in a more or less confused and entangled manner (as in figs. 216, 240, 245, and 251). The surface of the conceptacle is minutely reticulated, and its base is attached to the mycelium. When first formed, these globose conceptacles are almost colourless; they afterwards acquire a yellow colour, and are ultimately of a deep brown. The appendages are seldom at all coloured. Within the conceptacle are contained from one to several transparent obovoid sacs, or spore-cases, called sporangia, enclosing a definite number of spores (figs. 218, 224, 228, &c.), which vary in different species. In the hazel mildew, for instance, there are two spores in each sporangium; in the willow mildew four; in the maple mildew eight; in the grass mildew, and some others, numerous. The tips of the appendages are variable, and often elegant (figs. 227, 231, 233, 234, and 247), sometimes simple and at others symmetrically branched. All the species occur on the still living and green parts of plants, especially the leaves, and are therefore truly parasitic. A pocket lens will show whether any conceptacles are present on any suspicious leaf which may be collected, but high powers of the microscope are essential for their complete examination. It is during autumn, when vegetation begins to languish, that we shall be most successful in searching for specimens. They will then be found almost everywhere, and the white mycelium forms an object too conspicuous for them to be readily overlooked. Botanically, nearly all the species were at one period included in one genus, under the name of Erysiphe, a name derived from the Greek, and signifying “mildew;” at the present time they are distributed through several genera, the chief distinctions of which are based upon the form of the appendages. Though personally disposed to question the generic value of such distinctions, it would be imprudent to adopt any other names here than those to be found in recent English works on fungi.
The first species in our enumeration is found on cultivated roses. What a deplorable picture does a favourite rose-bush present when attacked by this mildew! The leaves blistered, puckered, and contorted; their petioles and the peduncles and calyces of the flowers swollen, distorted, and grey with mould; and the whole plant looking so diseased and leprous that it needs no mycologist to tell that the rose is mildewed. The conceptacle in this species is minute, and contains but one sporangium, which is one of the characters of the genus in which it is now included, and a more justifiable distinction than the ramifications of the appendages. The mycelium is rather profuse, and the threads or appendages which spring from the conceptacle are simple and floccose (fig. 216). The sporangium contains eight ovate spores. This species (Sphærotheca pannosa, Lev.), in its oidioid or conidiiferous form, was for some time known under the name of Oidium leucoconium.
An allied species constitutes the hop-mildew, a visitation with which some of our Kentish friends are too familiar. This is not a prejudiced species in the choice of its habitation, since it is found on many other plants, where it flourishes with equal vigour. The meadow-sweet, burnet, scabious, teasle, dandelion, and other composite plants, plantain, and plants of the cucumber family, all suffer more or less from its roving disposition. The mycelium of whitish threads is even more conspicuous than in the last species, but the conceptacles are often not to be found at all. These are also very minute and most common on the under surface of the leaves. The appendages, or fulcra, are simple, floccose (fig. 217), and coloured. The sporangia are found singly in each conceptacle, and each, sporangium contains eight spores.
An autumnal stroll amongst hazel-bushes, when the nuts are ripe, will lead, if the nuts are not a greater attraction, to the discovery of whitish, patches on the under surface of the leaves, caused by the mycelium of the hazel mildew (Phyllactinia guttata, Lev.). These patches are less distinct and conspicuous than in many other species, but the little blackish dots of the conceptacles may be distinguished by sharp eyes without the use of the lens. Though possessing a decided preference for the hazel, this species is also found on the green leaves of the hawthorn, ash, elm, birch, sallow, beech, oak, and hornbeam. The conceptacles are larger than in the two preceding species, and somewhat depressed above. The appendages are few (fig. 219), radiating, rigid, and acicular, or like needles. Each conceptacle contains eight or more sporangia, and each sporangium has from two to four spores (fig. 220). This species being very common, its conceptacles large, and produced copiously, and its appendages distinct, it will prove a good type with which the student of these fungi may commence his examinations. This is the only representative which we possess of the genus established by M. Leveille for such of the Erysiphei as have the conceptacle depressed, and the appendages rigid and simple; by which features it is distinguished from genuine species of Erysiphe.
Two species, also common, having many features agreeing with each other, are found on the leaves of the maple and the willow. The willow blight (Uncinula adunca, Lev.) is found irrespectively on various species of poplar and willow (fig. 221). In size and external appearances, to the unaided eye, it seems scarcely to differ from the preceding, but more minute examination will show that in the appendages there is an appreciable difference. Still rigid, but no longer aciculate, the tips bent or curved like a little hook, or curled upon themselves (fig. 223), radiating and numerous (fig. 222), and at length tending upwards. Many sporangia are contained within each conceptacle, each of which is furnished with four spores. The amateur must not be disappointed, if, on examining mature conceptacles with a view to the discovery of the sporangia, he finds only free spores. The investing membrane is very delicate, and disappears generally as the spores are matured.
The “blight” or “mildew” which occurs on the common hedge-maple, as well as on sycamore leaves, is exceedingly conspicuous when occurring on the former plant. The whole bush often presents a hoary appearance as if sprinkled with powdered chalk. In the spring, the under surface of the leaves of the same plant are liable to become hoary from another cause. The whiteness occurs in patches, has often a pinkish or violaceous tint, and glistens like hoar-frost. This affection of the leaves was, at one time, believed to be produced by a fungus which was called Erineum acerinum, but now it is regarded as a diseased state of the tissues. In the maple mildew, both surfaces of the leaves are alike affected, and the little, dark, point-like conceptacles will be found studded over both. It is not uncommon to meet with very white leaves, caused by the mycelium, but which bear no fruit. The appendages in this species are shorter than in the last (fig. 225), and the tips are bifid (fig. 226), or divided into two short branches, each of which is bifid, and uncinate or hook-shaped (fig. 227). The conceptacles contain not less than eight sporangia, each of which encloses eight spores.
Amongst the parasites that prey upon the much abused berberry (which has been charged in turn with producing the mildew in corn), is one which causes the green leaves to assume a chalky appearance (fig. 229), though less conspicuously than in the maple blight. This parasite is the berberry mildew (Microsphæria berberidis, Lev.). In such localities as the writer has met with the berberry suffering from mildew, he has invariably found a larger proportion of leaves with the barren mycelium than of those on which the conceptacles were developed. Perhaps in other localities this may not be the case. The appendages, as will be seen on reference to our plate, differ materially from any of those to which we have referred; indeed, this genus (or sub-genus) has the most elaborate and beautiful forms in these appendages of any of the Erysiphei. A figure is given of the tip of a fulcrum from a continental species (M. Ehrenbergii, Lev.), not yet found in this country (fig. 233). In the berberry blight the appendages are straight at the base, but afterwards become forked, each fork being again forked, and these yet again branched in a similar manner (fig. 230); so that a complex dichotomous tip is formed to each of the appendages (fig. 231). Each conceptacle contains about six sporangia, and each sporangium contains from six to eight spores (fig. 232).
The common gooseberry is also liable to a visitation from an allied species, in many respects closely similar, but differing in having the tips of the appendages more branched, and the extremities of the ultimate branchlets are not entire and attenuated, as in the berberry mildew; but divided into two toothlike processes. The conceptacles in this species contain from four to eight sporangia, each of which has four or five spores.
In England, the leaves of the guelder-rose, and in France (perhaps also in this country) those of the alder, nourish a parasite belonging to this division. This “blight” possesses so much in common with others to which allusion has been made, that it will scarcely be necessary to describe it in detail. A figure of the tip of one of the appendages of the variety found on the alder is given in the [Plate XI.] fig. 234.