In the fenny districts of the eastern counties a species of “smut” called Ustilago typhoides attacks the stems of reeds, forming thick swollen patches of several inches in length (fig. 128), sometimes occupying the whole space between two joints or nodes, and lying beneath the sheath of the leaves. The spores in this species are larger than in the species which attacks the culms of grasses in a similar manner ([Plate VI.] fig. 129).
There are not many features in the rest of the species of this genus of sufficient interest to the general reader or microscopist to render it advisable to furnish any detailed account of them. We may, however, note that in a species found on the leaves of the common cock’s-foot grass the spores are large, obovate, and rough, with minute granules (figs. 117, 118). This species is known botanically as Ustilago salveii, and externally bears considerable resemblance, except in the size and colour of the spores (fig. 119), to another species much more common, and which occurs on the leaves of Poa aquatica and P. fluitans. The last-named species forms long parallel sori, extending often for several inches along the leaves of the aquatic grasses just named, giving them a very singular appearance ([Plate V.] figs. 105, 106). The spores are small (fig. 107), not being more than one-fourth the length of the last species, and smooth, whilst those are minutely granulated.
An interesting species occurs, very rarely, on the stems of such grasses as Aira cæspitosa and A. aquatica. The sori are in bands at regular distances apart ([Plate VI.] fig. 120), each band being composed of a number of short parallel sori (fig. 121). The spores are not more than one-third of the size of those in U. longissima.
Sedges are also subject to attack from other species of smut; one of these (U. olivacea) appears to convert the seeds into a fine olive-coloured dust ([Plate VI.] fig. 126), which gives to the fruit a similar appearance to that presented by corn when attacked by Ustilago segetum. Another species, which also absorbs the seeds, becomes hardened and consolidated more than in any other species, and, though larger than the normal seeds, still retaining their form (figs. 109, 110). This is Ustilago urceolorum, the spores of which are also figured (fig. 111).
The beaksedge (Rhyncospora alba) suffers from an allied species which affects it in a similar manner ([Plate V.] figs. 96, 97), but is not equally common. The spores of Ustilago utriculosa, found on different species of Polygonum, instead of being granulated, are reticulated on the surface ([Plate VI.] figs. 114, 116). The chief interest attaching to Ustilago antherarum consists in its habitat, for it is developed in the anthers of the flowers of the bladder campion, and other plants of the same natural order. The anthers are much swollen and distorted by this parasite, which is not uncommon, though easily overlooked unless specially sought after ([Plate V.] figs. 102-104). A list of all the British species will be found at the close of this volume. It will be noted that as in the genus Æcidium the prevailing colour of the spores is orange, so in the genus Ustilago it is black, with a purplish or violaceous tinge.
Four diseases in wheat of fungal origin are known and recognized in the popular language of the farm as “mildew,” “rust,” “smut,” and “bunt.” Sometimes one and sometimes another is most prevalent, and he is an exceedingly fortunate individual who can walk through his fields and find only one of them, especially if that one should be sparingly distributed. It has been our good fortune to dwell much amongst cornfields, and the terror of the word “mildew” to a farmer’s ears is not unfamiliar in our reminiscences of the past, ere we discarded the much-loved country to become a dweller in town. The subject of our present remarks inspired no such alarm in the districts of our experience, but in some seasons and localities it is certainly one of the “pests of the farm.” Under the different appellations of “bunt,” “pepper brand,” “bladder brand,” and sometimes “smut,” this infection is very generally known. Externally there is no appearance, except to the practised eye, that anything is wrong. There is no black impalpable dust about the ears as in the true “smut,” no red withered leaves or spotted stem as in the “rust” and “mildew,” and no stunted growth or malformation, evident to the casual glance, by which the insidious foe can be recognized; but stealthily and secretly the work is accomplished, and until the “bunted” grains make their appearance in the sample, the disease may, perchance, be unchallenged.
Externally the “bunted” grain is plumper, and whilst the corn is still green these will be of a brighter green than the rest ([Plate V.] fig. 84). When broken, the farinaceous interior will be found replaced by a minute black dust of a very fœtid, unpleasant odour, and greasy to the touch (fig. 85). This powder constitutes the spores of the “bunt” mixed with myceloid threads. It may happen that much of the corn in a field is “bunted,” and the discovery not made till the wheat is being ground for flour; then the odour and colour will speedily decide the produce to be unfit for human food. We have not the least doubt that “bunted” corn, when ground with flour, is injurious in proportion to its extent, whilst at the same time we can scarcely conceive an intelligent miller grinding up a sample containing any large proportion of “bunted” grains in ignorance of the fact.
If we break open a grain of wheat infested with the “stinking rust” or “bunt,” and then place some of the powder in a drop of water on a glass slide, and submit this to the microscope, first using the half-inch power, then the quarter, or fifth, and finally an eighth or tenth, we shall find that this minute dust consists of myriads of globose brown bodies termed spores, which possess certain reproductive functions. These spores will be found mixed with a number of delicate branched threads, to which they are attached by a short stalk or pedicel, visible with the higher powers (fig. 86). The surface of the spores you will also observe to be beautifully reticulated. These features just described as visible in the “bunt” are the characteristics of the genus to which it belongs (Tilletea), and of which it is the only British species. An allied species infests the Sorghum or durra, a grain but little cultivated in Europe, but found extensively in Africa and Asia, and also apparently found on the Bajra of India.
The interesting experiments of the Rev. M. J. Berkeley on the germination of “bunt” spores have been already alluded to. They were undertaken shortly after the outbreak of the potato disease, to ascertain, if possible, the mode by which the minute spores of fungi inoculate growing plants; and although at that time only a bare suspicion of the nature of the bodies resulting from the germination of “bunt” spores was entertained, succeeding examinations in the same direction have brought to light extraordinary facts, and manifested the progress of the successive developments of four generations. The spores of “bunt” are larger than those of the different species of “smut,” and reticulated on the surface ([Plate V.] fig. 86). When these are made to germinate a kind of stem is protruded (fig. 87), upon which small clusters of elongated thread-like spores of the second generation, or sporidia, are produced (fig. 88). After a time these spores conjugate, or become united by short transverse processes in the same manner as has been observed in some of the lower forms of Algæ (fig. 89). The conjugated spores in the next stage germinate and produce a third kind of fruit, different from either of the preceding, and constituting a third generation (fig. 90). These in turn germinate and produce a fourth order of reproductive organs (fig. 91), so that in the process of growth the “bunt” spores evidently pass through four generations. Hence, as one result, the number of germinating bodies is greatly increased, as well as their power of inflicting injury in a corresponding diminution in size. There are still many points in the history of the growth and development through successive generations of the “bunt” spores, but enough is known, on the one hand, to show that this is a true vegetative parasite, and not merely a diseased condition of the tissues of the wheat plant, and on the other that it is perfectly distinct from all the phases of the other and similar parasitic fungi which affect the wheat crop.