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Plasmodiophora Brassicae

This is the cause of the disease known as "Clubroot," sometimes called also "Finger-and-Toe," in Turnips, Cabbages, Charlock, and other species of the genus Brassica, and also in Raphanus Raphanistrum, or Wild Mustard. It is a Fungus of very simple structure, and belongs to the curious group called Myxomycetes, the species in which, while vegetating, consist of minute, naked masses of protoplasm, endowed with a power of movement, by changing their forms, like the low microscopic animals called Amaebae. These small, naked masses tend to unite when they meet, and thus they increas in size, and form what are called plasmodia. In some of the species, the plasmodia may reach a size of 2in. or more in breadth and resemble a mass of clear or muddy-looking jelly. After a time, the plasmodia become covered with a firm coat, and break up into myriads of small, round cells, or spores, each inclosed in a cell-wall. These spores may remain for a time unchanged; but, under favourable circumstances, they burst, throw off the cell-wall, and appear as very minute plasmodia, which go through the same development as before. Almost all the Myxomycetes live on decaying organic matter, but P. Brassicae does not. Its plasmodia are always minute, and they penetrate into the tissues of its hosts. The roots of a plant attacked by "Finger-and-Toe" vary considerably in appearance, according to the stage of the attack. The branch-roots very often bear swellings much exceeding the proper thickness of these roots. The swellings are usually spindle-shaped or roundish; at first, they are nearly smooth, but after some time they become rough. The main root is also often injured, as shown by its enlarged size, and, frequently, by its roughly-fingered appearance. In the autumn, and as winter advances, the diseased portion becomes more and more pulpy and decayed, and is also overgrown with other Fungi, which find their food in the rotting mass. Towards the end of winter, only the woody bundles of the roots remain moderately firm, the cellular tissues having dried up, and resembling coarse dust.

Microscopic inspection of a thin slice from a diseased portion of a root, shows numerous cells of the tissue of the host-plant hardly altered in any way; but, inter-mixed with these, there are many others, from four to six times their diameter, which are occupied by the parasite; and it is these enlarged cells that alter the colour of the root, and make its surface in a transverse section, when a little magnified, look mottled. In the newly-diseased roots, these enlarged cells inclose a slimy, yellowish, granular plasm, in which are usually numerous cavities, filled with cell sap or with air. If examined at a later period, these enlarged cells are found filled with myriads of the minute, globular, thin-walled, transparent spores of P. Brassicae. These remain uninjured during the keenest frosts of winter, and form a large part of the powdery material that fills up the decayed root between the woody bundles, and they become widely scattered through the soil by the action of wind and rain. When the weather becomes warmer, in the following spring, they begin to germinate, the cell-wall splits, and the proto-plasmic contents pass out, and creep about through the damp soil like Amaebae, and also by the help of a slender hair or cilium at one part. When they come into contact with others like themselves, they unite completely, and form larger plasmodia. If they meet with young roots of Cabbages, Turnips, Charlock, or Wild Mustard, they make their way into them, and produce the disease in them also.

This disease has, of late years, been spreading far and wide, and has proved very destructive in Turnip fields; so much so, that, in some districts, Turnips are no longer a profitable crop. It also attacks Turnips and Cabbages in vegetable gardens, though the less extended diffusion of the host-plants renders the attacks less fatal, and remedies more readily applicable. Frequent experiments have shown that, in soil in which diseased roots have been allowed to rot, a new crop of Cabbages, or other host-plants, will certainly suffer from disease if sown in that soil in the ensuing year. Even after a year has elapsed, the crop is liable to suffer from this cause, though to a less extent. Yet farmers almost always, and gardeners frequently, leave the diseased roots in the soil, as not repaying the labour of pulling them up. Experiments have been conducted by Mr. T. Jamieson, to ascertain the result of different manures on this disease, and have led him to the conclusion that the use of superphosphates, or soluble phosphates, is followed by a great increase of disease; and that ground coprolites, and ground or steamed bone-flour, are the manures that best enable the plants to resist it; but these conclusions are opposed by other experimenters. If the effects on Charlock, Wild Mustard, and Cabbages, are compared with those on cultivated Turnips, it is at once seen that the last plant suffers far more severely. The cause seems to be that the Fungus attacks the cellular tissue only, and this in Turnips is very largely developed, as it is this that renders the roots useful for food, and, accordingly, it has been promoted by the selection of those varieties for propagation that produce the largest roots with least woody fibre in them. The former plants are crippled, but are generally able to produce some seeds; but the Turnips are, in general, destroyed before they reach the stage to form seeds.

Remedies. All diseased roots ought to be collected at as early a stage as possible, and destroyed by burning, if this is practicable. Ground where Turnips have become diseased should not have Turnips or Cabbages grown in it for at least two years, to permit any spores in the soil to germinate, and thus to starve and kill the plasmodia. With the same object, all Charlock and Wild Mustard should be carefully eradicated, both from the ground and from its neighbourhood. Further experiments upon the effect of different manures are desirable, but, in the meantime, they point to ground coprolites, and to ground or steamed bone-flour, as the best artificial manure for Turnips. Unfortunately, no means are known for the cure of plants already attacked, prevention alone being practicable in any way.