POTATO DISEASE, or POTATO MURRAIN. No subject connected with agriculture or with botany has given rise within so short a time to so extensive a literature as this. It has been treated in books and pamphlets, and in magazines and periodicals of every kind. The terrible famines caused by the failure of the potato crop in Ireland and other coun tries, particularly in 1846 and 1847, concentrated upon it the attention of the whole civ ilized world; and yet it remains very obscure.
The potato disease seems to have been at first confounded with dry rot and wet rot (see POTATO), which appeared a number of years before it to a formidable extent, although not to be compared with it in their ravages. This faet—and all the more if the potato disease is to be ascribed to the presence of a different and peculiar fungus—may perhaps be held as giving support to the opinion that its chief cause was really the weak ening of the plant through too constant cultivation on the same land, and continued propagation by tubers alone.
The potato disease was first observed in Germany, and first assumed a very serious character near Liexre, in 1842. In 1844 it broke out in Canada, and all at once proved very destructive. In 1845 it was first noticed in England, and first in the isle of Wight. But during that year, its ravages were considerable in the British islands; much more so in the year following, when the Irish famine was the consequence, and in the same year it prevailed very extensively over almost all parts of Europe. The summer was unusu ally cloudy and moist, a circumstance probably not without its effect. In 1847 the dis er.se was still prevalent, but to a smaller extent; and since that time its prevalence has gradually diminished, although it occasionally breaks out in particular localities. Mean while, it is to be observed, that almost all the varieties of potato cultivated to any con siderable extent before 1846 have disappeared, and been replaced by others. Lest too much, however, should be inferred from this in favor of a particular theory, it must also be stated, that potatoes newly raised from seed were sometimes severely attacked by the disease during the period of its greatest prevalence.
No fully satisfactory theory as to its cause or origin has been proposed. That it has long existed in the western parts of America, may probably be true, as has been alleged, although the distinction between it and other diseases of the potato might not perhaps be noted with sufficient care; but even this would not account for its sudden appearance and terrible devastations in other parts of firs world. Many observers ascribed it to
insects and aeari, some even to infusoria in the tissues, but the presence of none of these was found to be constant, and they appeared therefore rather to be the consequences than the cause of the disease. It is otherwise with the fungus, botrytis infestans (see BOTRY vs), which is alway's present, although, probably like other parasites, it generally attacks an already weakened plant.. The disease generally first appears in the leaves, and thence extends to the tubers, although it has been sometimes observed to appear in the tubers of some of the early kinds of which the leaves have perished before the season when it breaks out. It sometimes also lies dormant in the tubers for months, so that after being stored apparently sound in autumn, they become diseased in the following spring. When the disease appears in the growing plant, brown spots are first to be noticed on the margins of the leaves, corrugating the leaves as they spread. Very rapid extension of the disease, and decay of the leaves and stalks often ensue. It is on the under surface of the leaf that the botrytis is found; it abounds also in the diseased tubers, which, when cut, produce an abundant crop of it from the fresh surface, and it sometimes vegetates even from the natural surface. The same fungus has been found in the berries of the tomato (q.v.) when diseased, and on the leaves of other plants of the natural order solanea, but never on any plant not of that order. See COLORADO BEETLE.
The starch disease, which exist within the cells of potatoes seem not to he affected by the potato disease, but remain unaltered in quality, so that as good potato starch is made from unsound as from sound tubers. On occasion of the great ravages of the dis ease in 1846, however, advantage was but partially taken of this fact, partly from ignor ance of it; partly from an apprehension, apparently quite unfounded, that the starch might prove unwholesome; and partly from the want of machinery to grate down the diseased potatoes before rottenness had involved the whole.