CHAPTER XIV.

SURFACE MILDEW OF TURNIPS.

Oidium Balsamii, Mont.

THE mildew of turnips, named Oidium Balsamii, Mont., is often confounded with the true putrefactive mildew of the cabbage tribe, named Peronospora parasitica, Pers. The two are indeed so much alike to the unaided eye that it is often impossible for even an experienced observer, without a lens, to distinguish one from the other. As if to make the subject still more involved, it frequently happens that the two fungi grow in company on the same host plant. They are, however, wholly distinct from each other, both in habit and structure.

The name Oidium is derived from the Greek oon, an egg, and eidos, resemblance, and refers to the usual eggshaped form of the spores or conidia. In the present

instance the generic name is not very appropriate, for the conidia are somewhat barrel-shaped. The specific name Balsamii was given in honour of Balsamo, a Milanese gentleman, who first noticed the species. When first detected the fungus was growing on a Continental species of Mullein, named Verbascum montanum, Schrad. We have this plant in our gardens, but the fungus is more common here on the Black Mullein, Verbascum nigrum, L.; it also grows on cultivated strawberries. In the latter case the fungus makes its first attack on the leaves, and then speedily invades with increased vigour the flowers and footstalks, ultimately inducing the wretched appearance so well known in connection with grape vines when attacked by the allied fungus named

Oidium Tuckeri, B. Although several species of Oidium have been recorded from Scotland, Oidium Balsamii, Mont., has not yet been detected there. If it really grows so far north, it could hardly have been overlooked, as it is a remarkable species.

Oidium Balsamii, Mont., first attracted attention as a pest of turnips in September 1880, when Prof. James Buckman, F.L.S., of Bradford Abbas, Dorsetshire, saw the fungus growing in such profusion over hundreds of acres of Swede turnips that the boots and clothes of persons walking through the turnip fields were whitened with the spores. Until 1880 the fungus was not supposed to be common in Britain; and it is remarkable that the same fungus should be found growing upon three different natural orders of plants, viz. the Scrophulariacea, the Rosacea, and the Crucifera. Some farmers say the plants produced from early sown seeds are the most subject to this mildew. It first attacks the lowermost leaves, and then quickly covers every part of the affected plant. The presence of this pest, which is now known to be a common and injurious mildew of turnips, generally foreshadows a deficiency of roots.

To the unaided eye the foliage of affected Swedes is white on both sides when attacked by the mildew; but when seen under a low power of the microscope this white coating resolves itself into a dense felted mass of spider-web-like threads, dotted all over with innumerable barrel-shaped spores.

The higher powers of the microscope are required to show the exact nature of the Oidium of turnips. A minute fragment must be cut from an infected place on a turnip leaf, and from this fragment an exceedingly thin transparent slice should be cut. When placed in a dry state under the microscope it must be specially noticed that the fungus growth is wholly superficial, and that no spawn threads belonging to the Oidium occur within the leaf. In this respect the Oidium essentially differs from

When a

the Peronospora, next described in this work. minute slice, as just mentioned, is examined under the microscope and enlarged 400 diameters, it will be seen as illustrated at Fig. 27. The fungus grows on both surfaces of the leaf, and springs in both positions from a

FIG. 27.-SURFACE MILDEW OF TURNIPS.

Oidium Balsamii, Mont. Enlarged 400 diameters.

dense stratum of matted and jointed spawn, as shown at АА. From these horizontal spawn-threads arise innumerable vertical club-shaped growths, each club being furnished with three joints and surmounted by a barrelshaped spore or conidium, as shown at BB. The cells at

CC, represent the lower cuticle of the leaf, whilst the openings at D D D, show the stomata or organs of transpiration. It will be observed that the spawn of the invading fungus does not enter the stomata or traverse the intercellular spaces of the leaf, such as are shown at E E E. The barrel-shaped spores or conidia of this fungus are so numerous that more than 10,000 are produced on every square inch of leaf surface, and every

Oidium Balsamii, Mont.

Germinating Spore enlarged 1000 diameters.

turnip leaf will carry on its two surfaces a million or more of these reproductive bodies.

The spores germinate very readily, for they have only to be dusted on to clean glass and kept moist, under a bell-glass, when they will be seen to germinate at once as illustrated at Fig. 28, enlarged 1000 diameters. The germinating conidium bursts at one corner as at A, and from this corner the contained protoplasm or vital

material streams in threadlike form, and from this thread new clubs immediately arise. Four of these bodies, in different stages of growth, are shown at B, C, D, and E. The surface of one of the organs of transpiration is shown at F, over the opening of which a thread of mycelium has passed.

This Oidium chiefly injures the turnip by weaving a thick web of mycelium over the organs of transpiration. The spawn effectually stops the passage of watery vapour from the interior of the affected plant, and so puts an end to one of its chief vital functions. The general result is an arrest of growth, and ultimately a poor crop of roots.

Oidium Balsamii, Mont., is supposed to be an early condition of some more perfect fungus, probably an Erysiphe, such as one sees on the hop, on roses, on mildewed grass, and on peas. The two latter species of Erysiphe are referred to in detail farther on in this work. Important as this Oidium is to agriculturists, no one at present has worked out its life history or knows whence it comes, where it goes, what other form it takes, or how it hibernates through the winter. The fungus is more prevalent when a humid September follows on a dry August.