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CHAPTER XV.

PUTREFACTIVE MILDEW OF TURNIPS AND CABBAGES.

Peronospora parasitica, Pers.

THE fungus which causes this disease frequently accompanies Oidium Balsamii, Mont., already described, but it differs entirely from it both in anatomy and nature. When a Peronospora infected leaf is examined with the unaided eye, the thick white bloom on both sides of the leaves, as in Oidium Balsamii, Mont., is never seen. The Peronospora appears as a thinner, more scattered bloom on the under side of the leaves only, and generally borders pallid, discoloured, and decomposed patches on the leaf. A profuse growth of Peronospora is not to be distinguished from a slight growth of Oidium, without the aid of a lens.

If a small piece of the leaf of a turnip infected with Peronospora parasitica, Pers., corresponding in size and thinness with the Oidium infected slice already described, is placed under the microscope and examined, it will be seen, if enlarged to a scale one-half that of the last, viz. 200 diameters, like the drawing at Fig. 29. The first point to be especially noticed is, that the spawn which gives rise to the fruiting threads of the Peronospora mildew is inside the leaf, as shown between the letters A and B. These letters indicate the upper and lower surface of the leaf. The spawn threads are stout as compared with many other mycelia, and have very few septa or stops; they are notably furnished with numerous haustoria,—from haustor, a drawer,—or little suckers, as shown at C. The suckers attach themselves to the

[graphic][subsumed][subsumed][merged small][subsumed]

FIG. 29.-PUTREFACTIVE MILDEW OF TURNIPS AND CABBAGES.
Peronospora parasitica, Pers. Enlarged 200 diameters.

constituent cells of the leaf, and the mycelium sets up decomposition in every part of the leaf with which it comes into contact.

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The upper surface of a turnip leaf is shown at A, and the lower surface at B, and it will be seen at once that the spawn threads within the leaf are of too great a diameter to emerge through the little lancet-shaped orifices of the stomata. When the fungus threads from within approach the little openings of transpiration in their growth outwards, they slightly contract in diameter, form a stop or joint, and then, instead of emerging through the stomata with a pointed end, they present a chisel edge to the mouth of the pore, which exactly suits the shape of the little lanceolate opening. This mode of emergence is shown, enlarged 1000 diameters, on the left hand lower illustration of Fig. 30. The stems of the fruiting threads are therefore not truly cylindrical, but, when seen in section, present a flattened oval form as illustrated. As the stem now gradually grows upwards it usually twists round once upon itself. This twisting

habit is slightly retained by all the numerous branches and branchlets of the fungus. The upper part of each stem of Peronospora parasitica, Pers., is very much branched and rebranched, and each little branchlet carries a comparatively large ovate, almost globular spore, as shown, enlarged 1000 diameters, in Fig. 30. The spores or conidia usually germinate by bursting at the side, and the protruded vital material or germ tube has the power of piercing the cuticle of cruciferous plants. Although represented in the drawing as growing in an upright fashion, the real growth of every Peronospora is of course downwards from the under surface of the leaf. Now, if the illustrative drawing is turned upside down to present the Peronospora in a really natural manner, the resemblance of the fungus to a minute bunch of grapes is a striking one. From this resemblance the genus now termed Peronospora was for many years known as Botrytis—from botrys, a bunch, in reference to the resemblance to a bunch of grapes.

No zoospores have been detected in this species, but,

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FIG. 30.

Peronospora parasitica, Pers.

Spore and Conidiophore emerging through an organ of transpiration. Enlarged 1000 diameters.

like the Peronospora of the potato, it produces oospores or resting-spores; the resting-spores act as seeds, and carry on the life of the fungus in a hibernating state through

the winter. The resting-spores, which were first detected in turnips in 1849 by Mr. C. Edmund Broome, M.A., F.L.S., of Batheaston, Bath, were figured by Dr. Montagne, and named by him (like the similar bodies found in diseased potatoes) Artotrogus. The restingspores or oospores of Peronospora parasitica, Pers., are often extremely common in rotten turnips and mangelwurzels, as found in the fields in autumn. These roots are often destroyed by a combined attack of the putrefactive mildew of turnips and the fungus of club-root, described further on in this work. In order to see the resting-spores, portions of the brown decayed substance of the diseased root should be looked over till the oospores are found. The less ripe examples are smooth outside or slightly granular, and the more mature specimens are beautifully echinulate, as illustrated, enlarged 400 dia

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FIG. 31.

Resting-spores or Oospores of Peronospora parasitica, Pers.
Enlarged 400 diameters.

meters, at Fig. 31. Dr. Montagne and Mr. C. E. Broome first observed this fact in 1849, for on the original drawing in the possession of the Rev. M. J. Berkeley the smooth form is labelled "sp. jun.," and the nodulose form 66 matur." In colour the resting-spores are yellowishbrown. The examples found by Mr. Broome in 1849; those illustrated by Professor de Bary in 1863, Ann. Sc. Nat,, 4 ser., vol. xx.—again by him in the Beitrage zur Morphologie und Physiologie der Pilze, 1881, pl. 1, under the name of Artotrogus hydnosporus, Mont. (see also Gardener's Chronicle, April 26, 1884, p. 544); and the specimens shown at Fig. 31, agree in size, character, and colour.

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