CHAPTER XXXV.

POTATO DISEASE, I.

Peronospora infestans, Mont.

ITS ACTIVE STATE.

THE question is often asked, When did the potato disease first appear? No one is able to answer this question. The fungus which causes the disease is, like the potato itself, of exotic origin. Peronospora infestans, Mont., grows on the wild potato plants of Peru. The strong probabilities are, that ever since the potato plant has existed, there has also been the putrescent fungus to prey upon it. The family of parasites to which the potato fungus belongs existed in geological times, long prior to the potato plant or any of its relatives.

It is important to remember, in the consideration of this subject, that the potato and its immediate allies are not the only plants destroyed by the potato fungus, for various members of the family to which the potato belongs also fall before the parasite. Of late years, in some districts the out-of-door cultivation of the tomato, Lycopersicum esculentum, Mill., has been quite stopped by the ravages made upon it by the potato fungus. All the species of Lycopersicum, of which we have at least eight forms in our gardens, are commonly attacked by the fungus of the potato disease. Sometimes the pest may be seen growing upon the henbane, Hyoscyamus niger, L., or common bitter-sweet, Solanum Dulacamera, L., of our hedges; at other times it may be observed upon various other species of Solanum, as S. demissum, Lind., and S. cardiophyllum,

Sometimes the

Lind., or on the Petunias of our gardens. parasite may be seen upon an entirely different natural order of plants from that of the potato; it may leave the Solanaceae and prey upon members of the Scrophulariaceæ, as the New Holland plant named Anthocercis viscosa, R.Br., or, as pointed out by Professor de Bary, the Chilian Schizanthus Grahami, Gill. We have both these plants in our gardens. It is worthy of note that there is a second species of Peronospora met with on Solanaceous plants, named Peronospora Hyoscyami, P., and peculiar, or nearly so, to the common henbane, Hyoscyamus niger, L. Hyoscyamus is by no means common in Britain, but its parasite has been recorded from a single locality near Market Deeping.

Circumstances might have been, and probably were, adverse in this country to a rapid spread of the potato fungus soon after the first introduction of the potato. At length the time arrived when circumstances changed, and something-itis impossible to say what-greatly accelerated the growth and vigour of the fungus. When the potato and its parasite were transferred from South America to Northern Europe, the climatic conditions were changed, and the potatoes were grown in a new, artificial, and unnatural manner. The constitution and habit of the potato also became changed,—we do not say weakened, although this may be the case, but altered. At the time of the alteration or modification of the nature of the potato plant, the potato fungus acquired greater potency over it. The same phenomenon has occurred with the tomato: as the plant has been gradually altered in habit by cultivation, so the habit of the assailing fungus has varied. There is no evidence to show that the Peronospora has altered in the least in the potency or non-potency of its attacks upon our neglected wild plants.

The first accounts of the potato disease in Europe are very obscure, but as our business is less to detail the history of the fungus than to describe it with its effects,

we shall dismiss this part of the subject briefly. Mr. Berkeley, writing in vol. i. p. 9 of the Journal of the Royal Horticultural Society, stated that at that time, 1846, a "very serious disease" had existed for more than half a century under the name of "Curl," which committed "immense ravages" in the north. At the present time we know that the "Curl" was, and still is caused by the fungus of the potato disease, Peronospora infestans, Mont.

In a communication to the French Academy, 17th November, 1845, M. Boussingault wrote, on the information of M. Joachim Acosta, that the malady was well known in rainy years at Bogota, where the Indians live almost entirely on potatoes. There was a disease of potatoes in 1815, and a second noticed under the name of "Dry Rot" in Germany in 1830; if this "Dry Rot" of potatoes was equally moist with the “Dry Rot" of timber, it would exactly agree with what we know of the potato disease now. It must be remembered here that Fusisporium Solani, Mart., often really dries up and destroys potato tubers. The year 1830 was not a year of daily newspapers, of sharp scientific observers, and students of the microscope. It may therefore be reasonably concluded that if potatoes were sufficiently diseased with Rot in 1830 to warrant a published account of the disease, they most probably were diseased to a less extent for several previous years, and probably before the year 1815 just mentioned. Many articles appeared in the newspapers and agricultural periodicals of 1833 regarding the "Rot" of potatoes in the northern counties of England (see Journal of the Royal Horticultural Society, vol. iii. p. 22). In 1840 the disease was widely spread in Germany and France, and in 1841 it again attracted great attention in Belgium. A sharp observer, Dr. Morren, at that time advised that the putrid stems should be immediately removed from diseased potato plants-a piece of advice which, under proper conditions of the growth of the potato tuber, might be followed with good results at the present day.

In the same year it was recorded in Vik, in Norway, by Mr. Westrem, the director of the Agricultural School. During the next year it had greatly extended itself and was recorded from Sogndal, as well as from Denmark in both years. In 1843 the disease was very destructive in Western Jutland, and in 1844 the potato disease was epidemical in St. Helena and Canada. From the published accounts of the periods mentioned, it may be seen that the potato murrain was then exactly as we see it now. It appeared at a similar period of the year, and during the typical moist warm weather so favourable to the growth of the fungus. The dark disease blotches were on the leaves and the tubers were murrain-stained and rotten. The offensive odour so familiar to us now was then specially noticed. The next year, 1845, was the ever memorable year of the great outburst of the potato disease over Western Europe, from Norway to Bordeaux, and the northern parts of the United States. In 1845 the fungus of the potato murrain acquired its greatest possible power for destruction. It was first noticed in the south of England in the middle of August, and in a fortnight it had spread over every part of the British Isles. So apparently sudden and destructive was this attack, that in the month of September it was hardly possible to procure potatoes unstricken by the murrain. From 1845 till now we have never been free from the assailing fungus; sometimes the attacks are extremely virulent-at other times slight; sometimes the fungus is common on various field and garden plants allied to the potato-at other times very little of the fungus is to be seen. Mr. Duncan Stuart has stated in the North British Agriculturist for 3d October 1883, that the fungus of the potato disease has never yet appeared in the Island of Rum, fifteen miles from the mainland, on the west coast of Scotland, and seven miles to the south of Skye. An exhaustive account, and the best ever written of the rise and spread of the potato disease in Europe, is given in the first volume

of the Journal of the Royal Horticultural Society, from the pen of the Rev. M. J. Berkeley. This paper also gives a complete description of the potato fungus,—a description so complete and admirable that, even now, very few new facts can be added to it. Since 1846, when that account was published, many fresh observers have written on the potato fungus, and some of Mr. Berkeley's original observations have been amplified, enlarged, and curiously confirmed.

It is

The potato disease is seldom seen in Europe before July or August, although its appearance has been noted on rare occasions in May and June. Mr. Jensen has stated that the fungus of the potato disease cannot exist in any country where the mean temperature exceeds 77° Fahr. for any length of time during the period when the fungus generally perfects itself, and that in a temperature of 34° it cannot produce either mycelium or spores. generally first distinctly seen in the midland and southern counties of England between the 20th and 31st of July. It generally appears during close humid weather, when there are mists in the fields in morning and evening, and the days are hot, damp, and possibly stormy. Many other fungi suddenly perfect themselves under exactly similar meteoric conditions. It will be pointed out later on why it is, as we think, that the potato fungus appears with apparent suddenness under these conditions and at this particular time of the year. In the meantime it may be noted that the fungus generally makes itself manifest to the less experienced observer as a fine white bloom on the leaves, accompanied by dark putrid spots. The bloom is sometimes more profuse on the lowermost leaves of potato plants, not because the fungus has travelled up the stem from the seed tuber, but because the air is more moist and stagnant near the ground. The bloom, with its accompanying black disease blotches, soon travels to the stems, and when at length the tubers are reached the exhausted seed tuber (the weakest part of the plant) is commonly