In 1845 the potato crop in various parts of Europe, and in the United States and British provinces, was attacked by a most destructive disease, which was called the potato murrain or potato rot. It has occurred in other years, both before and after 1845, on both continents, and to some extent in 1874; but at no time has it been so severe as in 1845. So sudden and general was the disease in that year that it caused heavy losses to farmers, and in Ireland, where the rural population depend largely upon potatoes for food, their sudden failure brought with it severe famine and distress. At first various causes were assigned for the disease, notable among which was the theory that the potato, having been multiplied for many years by the subdivision of its tubers, had become debilitated, and that the disease was due to a depraved constitution. Though this view led to the general improvement of our varieties, especially through the labors of Mr. Goodrich (see Potato), its fallacy was long ago demonstrated, and it is known that the disease occurs in the potato in its wild state in South America. The presence of aphides or plant lice upon the tubers, and electricity and other meteorological influences, have been among the causes assigned.
When the disease exists in a mild form, brown spots appear upon the herbage; after a while these spread over the plant until it dies, and the tubers will be found more or less affected; the trouble may be confined to a few plants or extend over all in a field. When it comes in its worst form the destruction is complete in a few hours, and a field that was in the morning green and flourishing will be at night a mass of rotten herbage and decaying tubers. The disease appears usually early in August and in damp weather. In 1846 the Rev. J. M. Berkeley, the eminent cryptogamic botanist of England, published his observations on the rot, and found it due to a minute fungus which, though it has been studied with great care by mycologists, still keeps a part of its history concealed, and though generally called perono-spora infestans, there is some doubt whether it belongs to this genus. The mycelium or vegetative portion of this fungus (see Fungi) is so small that it can find abundant room to live and spread within the tissues of the leaf, stem, or tuber, where it grows at the expense of the contents of the cells; it may exist to some extent within the tissues of the potato without seriously injuring the plant, and be quite unsuspected, as it can only be detected by careful microscopic observation.
When from any cause the mycelium of the fungus increases so rapidly that its demand upon the potato plant for nourishment is greater than it can supply, then the plant assumes a sickly aspect, black spots appear on the leaves, and death soon follows. When the weather is cool and dry the fungus vegetates very slowly within the tissues of the potato, but warm weather accompanied by dampness favors its rapid and destructive development. The mycelium can only spread through the tissues of the particular plant it inhabits; but it produces something corresponding to seeds, by which it may be propagated from plant to plant. When fructification, as it may be called, takes place, the fungus seeks the open air; some of the threads make their way through the stomata or breathing pores of the leaf, and appear like a slight frost on its under surface; the threads often branch, and their tips and those of the branches swell up to form the egg-shaped bodies called spores, which when perfected fall away and are ready to reproduce the fungus.
They germinate either by pushing out a germinal tube, or sprouting at once, or their contents undergo a change and form several small bodies (zoospores), which to the number of 6 to 15 escape, and have the power of moving over a moist surface by means of two cilia or vibrating hairs; when these zoospores come to rest they produce at one end a germinal tube, the same as formed by the spore; these spores or zoospores may fall upon other plants, or may be carried there by the wind or other means, and when they germinate are able to push their germinal tube directly into their tissues and there grow to form a mycelium. These spores are asexual, being produced by the subdivision of an individual without any cooperation of two distinct plants, or two distinct organs of the same plant. They retain their germinating power for several weeks, but their vitality is destroyed by a winter's exposure. This is a brief outline of the life of this fungus so far as known, but its history is not complete; other similar fungi at some time in their career produce sexual spores or oogonia; and this may and does take place upon quite another plant.
Another peronospora, which is very destructive to lettuce, produces on that plant only asexual spores as described for the potato fungus; but these' asexual spores when they germinate on the groundsel produce a mycelium which has organs corresponding in function to stamen and pistil, and these by their union form an oogonium or sexual spore, which differs from the others in being much tougher, is not killed by freezing, and may even pass through the alimentary canal of animals and retain its vitality, and through the medium of manure be brought around to the lettuce again, when it can go on with its destructive work. This alternation of generations is very common among fungi, and botanists are now seeking for the sexual spore, or resting spore, of the potato fungus. Statements that it has already been discovered are without foundation in fact. There is some evidence pointing to clover as the plant upon which the sexual spore is produced. Prof. W. G. Farlow of the Bussey institution, Harvard university, has proposed a series of questions to cultivators to call out such evidence as may be afforded by the effect of certain rotations of crops upon the rot in potatoes. - There is no cure for the potato rot; as it does not appear until August, early varieties which ripen before that time escape it; while some more vigorous kinds seem to be better able to resist the disease than others, none are quite free from it.
One of the most obvious methods of avoiding the disease is to plant only tubers that are apparently sound, although, as stated, the fungus may be within the tissues in so small quantities as to produce no rot or other manifestation of its presence. Should the suspicion that the oospore is formed in clover be confirmed, then the avoidance of animal manures will be one of the principal means of preventing the spread of the fungus.
Fig. l. - a. Asexual spore. b. The same germinating, c. Ejection of zoospores, d. A zoospore with cilia.
Fig. 2. - The Mycelium within the tissues of the leaf; the aerial portion bearing asexual spores. (After Farlow).