The curl is a well-known disease amongst cultivators of potatoes, and is supposed to arise, in many cases, from using over-ripe seed stock, or seed that has been improperly kept during the winter and exposed to the light and air, instead of having been covered with earth, sand, or straw, so as to preserve their juices; or from the want of lime or salt upon the land, and the over-application of strong manures, and successively planting in the same ground. All these evils can be remedied. The disease which has justly excited so much attention is the same as that which has been known in South America from time immemorial, under the name of "Casaque" (great coat), which has been shown at annual periods, and which took, at one time, the form of a true calamity. Considerable losses have been sustained in Germany, where the epidemic caused very great ravages. The worst and most general change showed itself first, in 1843, in the United States of America and in Canada, and re-appeared there in 1844.

The following year it came over to Europe, and there increased rapidly from the 20th of July till the month of October. It showed itself successively in Germany, Belgium, Holland, France, England, and Ireland, in which latter country it increased with great virulence. From Westphalia it was propagated to Mecklenburg, Hanover, Denmark, and Russia. The disease of the potato appears generally in the months of July, August, September, and October; very few cases are observed in June, and scarcely any in May. A mild damp temperature always provokes the development and favours the progress of the disease. The early potatoes generally escape, when we can cultivate them before the time the malady invades us. Towards the time of maturity, this special infection strikes the leaves, passes through the stalk, and generally penetrates to the root, where the destruction is either partial or total. The external sign is the withering of the leaf; it presents a yellowish hue, and, under the microscope, small drops of moisture are visible upon it; minute brown spots then show themselves, the stalks become yellow, and soon after spotted with brown. These changes are observable in a single day.

We can more easily distinguish the disease by dividing the potato; when, in the section, may be seen numerous spots of a reddish hue, more or less distinct towards the centre, scattered or in lines. The malady will be made more apparent by cooking in water or steaming the potato which is attacked, and when done, all that part not tainted by the red matter will break easily between the fingers, whilst that affected, or marbled, will resist the pressure and remain solid; or if a slice of potato is put in water and remains there for twelve or fifteen days, the soundest part will be the first to decay, whilst that which is affected will remain unchanged. The first attributed cause was the inclemency of the weather; a second, the degeneracy of the root; a third, a fungous insect; and another, that the disease is sown by atmospheric agency. This is the prevailing opinion, and that it is a kind of floating blight, which the wind wafts from place to place; this has been observed to be checked when coming in contact with a wall or hedge. If a fog or light rain comes on during its flight, it instantly settles on the ground and destroys whatever it comes in contact with. Learned men have stated that this blight also attacks insects: and give for example the silkworm.

The tomato and beetroot, as well as other plants, are liable to the same disease.