THE FUNGUS PESTS OF CERTAIN GARDEN PLANTS
Many of our garden plants are liable to the attacks of fungi. Cures are in most instances unknown, but in some cases preventives—which are better—have been adopted with partial or entire success. Plants raised from robust stocks, grown in suitable soil and under favourable conditions, are known to be less liable to disease than seedlings from feeble parents, or those which have been rendered weakly by deficiencies in the soil or faulty cultivation. Whether weakness is hereditary, or is attributable to a bad system, the fact remains that disease generally begins with unhealthy specimens, and these form centres of contamination from which the mischief spreads. It is, therefore, important that seed from healthy stocks should be sown, and that a vigorous constitution should be developed by good cultivation.
Anbury, Club, or Finger-and-toe.—The disease known by these various names is common in the roots of cultivated cruciferous plants such as Cabbages, Kohl Rabi, Radishes, Swedes, Turnips, &c., and also in many cruciferous weeds, including Charlock and Shepherd’s Purse. The cause of this disease is an extremely minute fungus, which may lie dormant in the soil for several years for want of a comfortable home, and when a cruciferous plant becomes available the fungus fastens on the fine roots, multiplies rapidly in the tissues, and produces malformation and decay. After the disease has made some progress insect agency frequently augments the mischief, so that on cutting open a large decaying root it is not unusual to find the interior packed with millipedes, weevils, wireworms, and other ground vermin.
Unlike the Potato disease, which spreads from plant to plant through the atmosphere, the fungus of Finger-and-toe infects the ground, and from the first spot attacked the disease spreads rapidly in all directions and in various ways. It may be carried by the soil adhering to implements or the boots of labourers. And each patch becomes a new centre of infection which is spread by digging or raking. Every scrap of infected soil, or of diseased fibre which may be added to the manure-heap, distributes the virus over a wider area, so that Finger-and-toe may suddenly appear in parts of the garden which have hitherto been free from this troublesome pest. A very simple experiment will prove the certainty and ease with which the spores may be introduced to fresh land. Macerate the tissue of old Finger-and-toe in water; use this on young isolated plants of Cabbage or Turnip and in a short time the plants will be infected.
The fungus which produces Finger-and-toe is known as Plasmodiophora brassicæ, and it belongs to the Myxomycetes, or ‘slime-fungi,’ which, as a rule, live upon decaying vegetable material. The protoplasm of the fungus ramifies among and within the tissues of the roots of attacked plants, and eventually produces an amazing number of spores so small that more than thirty millions would be required to cover a superficial inch. A microscope of great power is necessary to reveal them to human vision.
FUNGUS OF FINGER-AND-TOE DISEASE
Plasmodiophora brassicæ
The spores are capable of resting in a state of vitality for a long time, and can easily withstand the frosts of winter. The illustration shows at A the fungus in its protoplasmic condition, and at B its ultimate sporiferous or ‘seed’-producing stage, after the protoplasm has changed to a mass of minute spores (enlarged five hundred and twenty diameters). When a spore in due course germinates, its protoplasmic contents escape through a small aperture in its wall and begin moving about of their own accord in a slow writhing manner. The movement is so much like that of the microscopic animal organism found in ponds, and called Amœba, that this tiny mass of moving protoplasm is called Myxamœba, to denote that it is an amœba-like form produced by one of the Myxomycetes. Each myxamœba is drawn out at one spot into a fine delicate tail or cilium, as at C, D, E, and is capable of a creeping motion in moisture. When quite free from the spores, transparent expansions or limbs extend from the bodies of the myxamœbæ, as at F, G, and when these organisms, after existing in the soil for a longer or shorter time, reach the roots of cruciferous plants, which they apparently enter through the root-hairs, they again assume the protoplasmic condition shown at A, and live within the cells, at the expense of the nurse-plant. Other cruciferous plants are less seriously damaged by the pest than are Turnips and Cabbages; but it is evident that if diseased Charlock is near Turnips, the latter are very likely to fall a prey to the disease. We advise the sowing of the best seeds, the eradication of cruciferous weeds, and the destruction by fire of all decaying Finger-and-toe material, for it is in this material that the spores of the disease rest ready for continuing the disease in the following season. It is also desirable that cruciferous plants should not be continuously grown in the same quarter—in other words, it would be prudent after an attack of Anbury not to repeat a cruciferous crop on the same ground, but to follow on with a crop of some other class.
Numerous experiments have shown that slaked lime can be relied on to destroy the spores of Finger-and-toe in infested land. An application of from fourteen to twenty-eight pounds per pole may suffice in the case of light soils, but fifty-six pounds per pole will not be too much on heavy land, and the dressing should be given either six or eighteen months before a Cabbage or Turnip crop is sown; the longer period is the more certain in its effect. Preference should be given to stone or rock lime over chalk lime. The former is much more powerful and efficient. It may be necessary to repeat the dressing twelve months after the first application. As regards the occurrence of Anbury in seed-beds, frequent transplantation is a very effectual mode of stopping its progress, for the little galls can be pinched off by the workman, and burned as he proceeds; and the plant, being invigorated by change of soil, will soon grow away from the affection. In transplanting Cabbages it is a good plan to discard and burn such plants as are obviously affected with Anbury. It is worthy of remark that in market-gardens this disease is by no means so prevalent as to interfere with the routine of cultivation, although the Cabbages, Broccoli, and Cauliflowers grown in these grounds are, under other circumstances, especially liable to attack. By ‘other circumstances’ we mean that market-gardens are generally kept under high cultivation, the land being perpetually turned and heavily manured; and these measures appear to be a preventive of Anbury, while they result in heavy crops. But on land less energetically tilled Anbury may prevail to such an extent as to interfere seriously with the order of cropping. Another very important mode of keeping down the pest consists in burning instead of burying the stumps and all other refuse of the crop that cannot be turned to account.