This section is from the book "The Gardener's Monthly And Horticulturist V24", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
By J. L. Jensen, Director of Bureau Ceres, Copenhagen, Denmark. Translated from the Danish to English, and published by John Menzies & Co., Glasgow, Scotland. This is a treatise of sixty-five pages, and one which deserves more than usual consideration. It is written by a gentleman of high scientific attainments, and of vast practical experience, and a careful reading of his work brings the conviction that he is on the right track.
The potato disease is well known to be caused by a fungus, Perenospora infestans. It does not follow diseased or sickly vegetation, but feeds on the healthiest as well as the weakest. It is the only cause of the disease. This is well known to be beyond question. There are, of course, climatic reasons which will favor more vigorous growth and more vigorous destruction some seasons than others, just as there are some seasons when grass or grain grow better one year than another. Another fact is that some varieties seem to suffer more than others - that is to say while a plant of one kind, with the disease, has many bad tubers, another kind, with the disease, has the tubers comparatively sound. As the disease attacks all alike, and as the constitution of the potato in all varieties is alike, Mr. Jensen was led to examine the cause of this. He found that those varieties which rooted deepest - that is those which buried their tubers the best, were most free from disease. This he thinks reasonable from other considerations. The spores of the fungus are conveyed through the atmosphere to the plants. Earth is a protection.
The spores can only get to the tubers by being carried by rains through the earth, or by dew down the main stem ; the farther from the main stem and the deeper the tubers, the greater the protection.
On this reasoning many experiments were made, which are detailed in his work, proving that the deeper the potatoes the greater the protection.
So he earths up the potato. He finds no difference in the quantity per acre between flat culture and earthing up ; but he does find a remarkable freedom from disease by earthing up just as much as the potato plant will bear. The ordinary method of earthing up ridges does not do this well. The earth is made deeper about the stems where there are no potatoes, but it is left thin just where the potatoes are. This is the pith of the volume, one which it will profit every large potato grower to peruse.