Of late years the diseases of Potatoes have attracted considerable attention, so much, indeed, that it has become quite usual to talk about various sprays and washes for eradicating or preventing them instead of adopting better cultural methods. Diseases, of course, are natural adjuncts to all living things, including Potatoes, but there is no need to invite their appearance by neglecting the dictates of common sense. It may be safely said that most of the Potato diseases are mainly due to three causes, viz.: (1) overcrowding; (2) lack of sufficient lime in the soil: and (3) absence of deep cultivation. Remedy these defects, and less will be heard in the future of Potato diseases. The air and sunshine, for which growers pay absolutely nothing, are the finest natural antidotes to disease in the leaves and stems of Potatoes, while a well-worked soil containing a fair supply of lime or chalk in one form or another will do much to prevent the diseases of the tubers.

The grower, however, must be always on his guard against attacks, and it is well that he should be acquainted with or be able to recognize the various Potato diseases here described.

The Common Potato Disease, or Potato Blight, is caused by a fungus called Peronospora infestans (fig. 485). The spores of this fungus are produced in enormous quantities, and, being very minute, are easily blown about by the wind from one place to another. The disease was discovered in Boston, U.S.A., and also in Denmark and Norway between 1840 and 1842, and by 1845 and 1847 it had ravaged the whole of Europe, and caused the great Irish Famine.

The first signs of this terrible disease are small brownish or yellowish blotches on the leaves; these blotches gradually increase in size, the leaves also begin to curl, and in severe cases the stems and leaves become blackened in the course of a day or two - especially in warm damp weather and when the Potatoes are planted too close together, as they usually are. Examination of the affected portions with a good lens or a microscope will show white and delicate threads. These are simple or branched stalks or conidiophores, which spring, through the leaf pores (stomata), from the mycelium of the fungus already feeding in the tissues of the leaf. At the tips of these delicate branches egg-shaped and colourless sacs, called conidia, are borne. From each of these conidia, when ripe, a number of zoospores are distributed. Each zoospore in due course germinates under the conditions mentioned, and sends a germ tube into the leaf tissues either through a stoma or the epidermis itself. Thus the disease spreads with great rapidity, and not only are the leaves and stems affected, but the tubers also, if so exposed that the spores fall upon them.

Peronospora (Phytophthora) infestans.

Fig. 485. - Peronospora (Phytophthora) infestans.

a, Fungus, with spore cases proceeding from stomata. b, Section of Potato leaf, showing the mode in which the mycelium creeps amongst the loose tissue of the leaf.

The usual remedy recommended for the Potato disease is Bordeaux mixture (a recipe for which is given by Mr. George Massee in Vol. Ill, p. 49), and there is no doubt as a preventive it is worthy of recommendation. The commercial aspect of spraying with Bordeaux mixture may be gleaned from the following statement of Mr. A. W. Sutton, who carried out an experiment on two plots of Magnum Bonum Potatoes - one being sprayed, the other not. One plot was sprayed three times, the other not at all. The effect was very marked. The growth of the sprayed plants continued some time after the unsprayed portion had died down. The weights of the two plots, when lifted, were: Sprayed, 3 cwt. 1 qr. 25 lb., Unsprayed, 3 cwt. 1 qr. 4 lb., a balance of 21 lb. in favour of the sprayed plot. Strange to say, the quantity of diseased tubers was the same in both plots, viz. 4 lb. "It is therefore a question", says Mr. Sutton, " whether the additional weight per acre would compensate the grower for the somewhat laborious task of spraying his crop three times during the growing period".

Somewhat similar results have been secured by the Department of Agriculture and Technical Instruction for Ireland. In its leaflet (No. 14, revised) is given the results of a series of experiments in spraying Potatoes with solutions of: (1) Sulphate of copper and lime, and (2) Sulphate of copper and washing soda. Here are the figures: -

No. of Tests.

Mixture Used.

Average total yield per statute acre.

Increase attributed to Spraying.

Sprayed.

Unsprayed.

tons.

cwt.

tons.

cwts.

tons.

cwt.

Average

15

Sulphate of copper and lime

11

15

10

1

1

14

,,

18

Sulphate of copper and washing soda (carbonate of soda

12

5

9

15

2

10

The grower must decide for himself whether the use of 20 lb. sulphate of copper and 25 lb. of washing soda, or 20 lb. sulphate of copper and 10 lb. lime (the quantities required per acre) will repay him for the trouble and expense involved if he is only to secure a benefit of 34 cwt. per acre in one case and 50 cwt. in the other, including, presumably, diseased tubers and chats. We feel sure better results will be secured at less expense and trouble by wider planting, as already advised at p. 144. Nevertheless, for the benefit of those who prefer to spend their money in washes and sprays rather than in better cultural methods, we give the recipes for making the mixtures from the Irish Agricultural Department's leaflet, copies of which, of course, can be obtained on application.