Trans. of the Horticultural Society of London 2: 64-67 (1822)
XIV. On the Prevention of the Disease called the Curl in the Potatoe.
By Thomas Andrew Knight, Esq. F.R.S. &c. President.

Read Feb. 2, 1813.

* See Vol. i. page 191.

THE rough and uneven surface of the leaf, which in excess, indicates, and indeed constitutes, the disease called the Curl in the Potatoe, appears to exist in, and to form an essential characteristic of, every good variety of that plant; for I have never found a single variety, with perfectly smooth and polished leaves, which possessed any degree of excellence; and I have endeavoured to prove, in the Horticultural Transactions of 1810,* that the rough and crumpled state of the leaf probably originates in the preternaturally inspissated state of the fluid, in the firm and farinaceous Potatoe. Those varieties are, however, generally most productive and grow with the greatest luxuriance, of which the leaves are smooth and polished; and this point tends to prove, that the smooth leaf is a more perfect and efficient organ than the rough one; the latter indicating some degree of approximation to disease.

† See Vol. i. page 249.

I have stated, in the Horticultural Transactions of 1811,† that I obtained a second crop of Potatoes by planting those of an early variety in the same soil from which a crop of the same variety had been taken, in the month of July; and that I had employed, with success, the tops of those taken up, with green fern and nettles, as manure. But I found the tubers produced by those last planted to be much more soft and watery, when boiled, than others of the same variety, and consequently much inferior in value for every culinary purpose; and therefore, these were kept for the purpose of planting in the last spring. I inferred, consistently with the hypothesis I adduced in the Horticultural Transactions of 1811, that the organizable matter these contained, being in a less firm and concrete state, would prove more disposable, and that I might therefore expect, in the succeeding season, plants of stronger growth, and more smooth and perfect foliage. The result, in every respect, coincided with my expectations; the plants presented the appearance of a different variety, and afforded a more abundant crop and larger tubers than I had ever obtained from the same variety.

This experiment was confined to a single very early kind, which had previously produced partially curled leaves; but I imagine the same mode of management will prove equally advantageous with other varieties which shew similar indications of incipient disease: and, as every improvement in the culture of this plant, which can add to the produce without increasing the expense, is of importance to the public, I submit the preceding account to the Horticultural Society.

*Vol. i. p. 50.
*Page 174.

A very respectable writer, in the Memoirs of the Caledonian Horticultural Society* Mr. Dickson, has advanced an hypothesis, somewhat different from mine, respecting the Curl in the Potatoe: he conceives it to originate in debility arising from the too great ripeness of the tubers, and in the parent plant having too much expended itself in affording blossoms and seeds, as well as tubers. But I can scarcely accede to this hypothesis, because I do not think it probable that a plant, which is a native of Virginia, can be over-ripened in the climate of Scotland; and because those varieties, which never afford either blossoms or seeds, have, in my garden, been quite as subject to that disease as others. Mr. Dickson has stated the curious fact (and I do not entertain the slightest doubt of his perfect correctness), that a cutting taken from the extremity, which is most firm and farinaceous, of a long, or kidney-shaped Potatoe, will afford diseased plants, whilst another cutting, taken from the opposite end of the same Potatoe, will produce perfectly healthy plants; but I do not attribute this to the greater maturity of the buds at the extremity, than at the opposite end, for those nearest the parent plant are really the oldest, the tuber being formed by a branch, which has expanded itself laterally, instead of having extended itself longitudinally. Its buds are in consequence arranged as they would have been upon the elongated branch; and every tuber, in its incipient state of formation, will extend itself into a branch, as I have shown in the Philosophical Transactions for 1809,* provided the plant, to which it belongs, be cut off close to the ground, and the current of ascending sap be in consequence diverted into, and through the tubers. Mr. Dickson, and myself, however, perfectly agree that a tuber, or part of one, which is soft and aqueous, affords a better plant than one which is firm and farinaceous; and the trifling difference of opinion between us, being purely hypothetical, is of no importance.

I observed that the crops of Potatoes, which I raised from the late ripened tubers above-mentioned, were not quite so early as others of the same variety; but I attribute this variation in the periods of the maturity of the crops solely to different degrees of luxuriance in the plants, and to the encreased size of the tubers in the one. In quality, the produce of both was the same.