CHAPTER XII.

_WHITE MILDEWS OR BLIGHTS._

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.

We have found another species which had not been before noticed in this country (_M. Hedwigii_, Lev.), on the leaves of the mealy guelder-rose in the vicinity of Darenth Wood, near Dartford, in Kent. The mealy character of the leaves of this plant, and the minute size of the conceptacles of the parasite, render it difficult to find; indeed, it could not be noticed unless it were sought for, as we sought it, lens in hand. It only occurs on the under surface of the leaves: the mycelium is very web-like and fugacious, the conceptacles minute, globose, and scattered (fig. 243). Four sporangia, each containing but four spores (fig. 244), are enclosed in each conceptacle, which is surrounded by a few appendages (about six) thrice dichotomous, and thickened at the tips of the ultimate branches, which are incurved (fig. 247).

The species of true _Erysiphe_ are distinguished botanically from the foregoing by the floccose character of the appendages, in which feature they accord with the species found on the rose and the hop, but from which they differ in the conceptacles containing numerous sporangia instead of only one, as in those species.

One of the most common and conspicuous of these is found on the leaves and leaf-like stipules of the garden pea. Every leaf in a crop will sometimes suffer, and the gardener, to his great mortification, finds that the mildew is more prolific than his peas. The leaves become sickly and yellow as the mycelium of the fungus spreads over them, when they present a peculiar appearance, as if growing beside a chalky road in dry dusty weather, and had become covered with comminuted chalk. Soon the conceptacles appear, profusely scattered over the white threads, like grains of gunpowder (fig. 237), and after a brief struggle for existence the pea and its parasite die together. In this species (_Erysiphe Martii_, Lev.), the appendages are nearly transparent, short, and much interwoven with the mycelium (fig. 238), the globose sporangia containing from four to eight spores (fig. 239). It is not confined to peas, although that habitat has been here given for it, because it is so common upon them. Beans, melilot, St. John’s-wort, some umbelliferous plants, and the meadow-sweet, have all been found affected.

The species found on grasses, especially the cocksfoot, has been already alluded to. The conceptacles contain from twenty to twenty-four ovate sporangia, each enclosing eight spores. The appendages and mycelium are much interwoven.

Another of these “white mildews,” not only on account of its frequency of occurrence on certain plants, but also from the numerous species of phanerogamous plants on which it is found (fig. 240), may be truly designated “common” (_Erysiphe communis_, Lk.); many kinds of crow-foot, especially _Ranunculus acris_, are subject to its parasitism. It is found also on other plants of the same natural order, on the rest-harrow, trefoils, enchanter’s nightshade, bindweed, and knotgrass. There are from four to eight sporangia in each conceptacle, containing from four to eight spores (fig. 241). In this species, more especially, M. Tulasne found curious sucker-like processes developed on the threads of the mycelium (fig. 242): their office may probably be only that of attachment.

Of the other species found in Britain an enumeration will suffice, since they contain no feature of interest to the microscopist; and all the members of this section are far less beautiful than those in other genera (especially _Microsphæria_).

The leaves of the dogwood or cornel (figs. 245, 246) are the home of one species (_E. tortilis_, Lk.), and the burdock of another (_E. Montagnei_, Lev.). Both of these, in addition to the above, have sporangia which contain more than two spores. There are also two species in which only two spores are contained in each sporidium. One of these (_E. Linkii_, Lev.) is found on both surfaces of the leaves of the mugwort (figs. 248, 249); the other (_E. lamprocarpa_, Lev.) occurs on salsafy, scorzonera, weasel-snout, and plantain (figs. 250, 251).

These complete the _Erysiphei_; but there are allied species of too much interest not to be noticed in connection with them. Three very singular fungi are found on damp straw and paper; two on the former and one on the latter. Of the species occurring on straw, the most common one is figured, natural size, in our plate (fig. 257); but from this no idea can be formed of its structure, which in some points resembles an _Erysiphe_. The conceptacles are thin and brittle, and are clothed externally with long dark-coloured branched hairs (figs. 258, 259). The conceptacle contains long narrow sporangia, each enclosing dark, almost black, lemon-shaped sporidia. For low powers this is a very interesting object. The minute structure affords no feature of popular interest. This fungus (which bears the name of _Chætomium elatum_) is common on old straw, thatch, reeds, matting, &c., resembling small brown tufts of hair, visible to the naked eye.

Paper much exposed to damp will occasionally develop a similar “bristle-mould,” surrounded by a yellowish spot (_Chætomium chartarum_, Ehrb.); but it is not so common as the last. In habit and structure it is very similar (figs. 252, 253).

In 1838, the Rev. M. J. Berkeley announced the discovery by him of a singular production, for which he was unable to find a fitting location in any genus then established, and for which he accordingly characterized a new one, under the name of _Ascotricha_. This new species of paper mildew was found by him on some printed paper in a box. It somewhat resembles the other species above alluded to, at a casual glance; but more minute examination will reveal its differences. The author to whom we are indebted for this species thus describes its development. At first it appears as a minute branched mould interspersed with globose brownish conidia. As it advances in growth, globose black peridia become visible amongst the threads, clothed with and supported by alternately branched obscurely-jointed filaments, the branches of which generally form an acute angle with the stem (fig. 254). The ramification of these is very peculiar, the stem and main shaft of each sub-division being almost constantly shortened and surmounted by the branches given off near its apex; this, again, is often abbreviated and another branchlet given off, which again surpasses it; and occasionally the same circumstance takes place a third time. The apices are clavate and colourless; the rest of the filaments, when viewed by transmitted light, brown, even, and pellucid: a few globose conidia are usually attached to them (fig. 255). The conceptacle is thin, black to the naked eye, of an olive-brown under the microscope, filled with a mass of linear extremely transparent asci (fig. 256), each containing a single row of broadly elliptic chocolate sporidia. These have a paler border; sometimes the colour entirely vanishes, either from age or abortion, and there is only a minute globose nucleus or more probably a vesicle of air, in the centre; occasionally they become so transparent that the globular bodies alone are visible. After the conceptacles burst, several are frequently collected together into an irregular linear body, which consists principally of the conglomerated sporidia.

One other very common and troublesome little fungus (_Eurotium herbariorum_) will for the present close our examples. This is found creeping over dried plants preserved in herbaria, on decaying fruit, preserves, and various other substances, sometimes animal as well as vegetable, but chiefly the latter. To the naked eye it appears as a myriad of little yellow spherical bodies, of the size of very small pins’ heads, resting upon fine cobweb-like threads (fig. 260). When magnified, the surface of the conceptacles is seen to be reticulated (fig. 261). In the interior the sporidia are borne, contained also, as in the former instance, in asci. It has been considered _probable_, but as yet not fully proved, that this mildew is a compound fruited (ascigerous) condition of an equally common mould (_Aspergillus_).

Dr. Shortt, of Chingleput, in a recent report on the growth and production of Indian Cotton, remarks that the plants are subject to the attacks of a kind of mildew. He writes:—“They appear in the form of rounded fibres or thallus, shooting up in the air, having the lamina of the leaf as a base, and feeling villous to the touch. The small fibrillæ that form the nap appear shooting up as sharp projections when seen by the naked eye; under the microscope they are found to consist of pointed tubes, interspersed here and there with minute granular cells. It first attacks either the upper surface of the petioles, or the margins of the leaf, gradually extending over the lamina, and matting together the whole leaf into a greyish-white, felty mass. At first it attacks the young shoots and tender leaves, preventing them from expanding. The extension of the parasite deprives the plant of its juices, and eventually either destroys or renders it sterile. The spores seem to be derived from the atmosphere, and finding the plant in a state fit to receive them, from either the results of excessive cultivation, or from the effects of heat and want of moisture rendering it unhealthy, and thus favouring the reception of the spores of the fungi. Another variety speckles the leaves with whitish dots. These remain separate, but the lamina is covered with them, and in time the leaf changes colour, becomes yellowish, and eventually dies away. This is evidently the disease called _Bunt_, or some variety of it, as it is seated beneath the epidermis, and eventually the spores escape. Under the microscope they seem to consist of small dark cells or spots attached to a thread-like mycelium.”

The writer seems certainly to have made a mistake in its affinities, and on the faith of the above quotation we should be more disposed to regard it as an _Erysiphe_. It is to be hoped that specimens of the affected leaves will be forwarded to this country for examination.