It would lift a weight off the minds of vegetable growers if some good genius could give us the fine qualities of the best sorts in conjunction with such vigour of constitution as sets disease at defiance. It is a consummation which we shall continue to wish for devoutly, but shall probably never see. A few dry seasons may encourage the grower to believe that his arch-enemy, Phytophthora infestans, has gone for ever, but a wet one dispels the sweet dream.
Fig. 36. The Potato Disease.
A, infected leaf: a, leaflets attacked by the parasitic fungus, Phytophthora infestans, causing the disease in the early stages, the earliest indications of attack being the appearance of small brownish blotches on the leaves. These gradually increase in size, and are followed by a curling of the leaves. In case of a severe attack the leaves and stems become blackened and decayed in the course of a few days, and emit a disagreeable smell; b, brown spots on the under side of the leaves. These on the circumference bear, as seen by a pocket lens, numbers of delicate white threads - the fungal outgrowths; c, clean leaflet.
B, the fungus which causes the disease in fruit: d, conidiophores; e, conidia; f, stomata of Potato leaf.
C, a fallen conidium showing contents breaking up into zoospores.
D, the discharge of zoospores: g, conidium with four zoospores not yet discharged; h, zoospores with cilia, or hairs, by which they move more readily in a drop of water, and finally come to rest; i, zoospore become passive and geiminated; j, germinating or germ tube, which enters the Potato leaf, stem, or tuber, and gives rise to the disease.
E, a conidium not broken up into zoospores, but pushing a germ tube : k, conidium; l, germ tube.
Potatoes are subject to several enemies, and the crop is of such importance that a little attention may well be devoted to each.
This, Phytophthora, once called Peronospora, infestans, which first made itself formidable in the closing years of the first half of the nineteenth century, is essentially a child of moisture. It is rarely that one sees more than a casual trace or two of it in dry, warm summers, though it is not often completely absent from a large collection of varieties. I usually grow from fifty to seventy sorts, and I cannot remember the season when a plant here and there did not show signs of a slight infestation. This is a small matter, and really it serves a good purpose, for it reminds the grower that his enemy lives on, and that he must not be lulled into a false security. In wet seasons, or during cold, humid spells, the disease is on quite a different footing, and often spoils the crop. The brown patches under the leaf, the musty smell, the collapsing foliage, all tell their story. Potato growers now have a valuable remedy at their command, and if only they will look a little ahead, have their materials ready, and keep an intelligent watch on the weather, they need suffer little loss. I have proved the efficacy of Bordeaux Mixture, and now, with a small addition of my own, am always ready to take the field with confidence. Bordeaux Mixture as made by me consists of :-
1 lb. sulphate of copper (bluestone)
1 lb. freshly slacked lime
3/4 lb. soft soap in 10 gallons of water.
(1 lb. of cheap treacle is sometimes substituted for the soft soap.)
The bluestone, which may cost about 4d. per lb., is first dissolved in a little hot water in a wooden bucket, then the lime is stirred up into another vessel of water; finally the two are poured together, and the soft soap, churned up in a little hot water, is poured in. The stuff may be put on with a knapsack sprayer, or Abol syringe, and in order to get it well on to the under surface of the leaves, there should be two operators, one to hold back a line of tops with a Bean pole, the other to do the spraying. The grower should be on the watch in July and onwards. If continued heavy rains wash the deposit off the leaves, he must go to work again and repeat the dressing.
The following cultural points are worth observing: (1) Select well-drained soil if possible. (2) Ensure a strong, healthy plant by using good seed and practising sound culture. (3) Plant in wide ranks so as to allow of plenty of soil for earthing. (4) In moulding lip, do it thoroughly and finish the ridges sharply, dusting lime on them if the weather turn muggy in July and August. (5) Whatever show sorts may be grown, take care to include one or two tough disease resisters, such as Magnum Bonum and Main Crop.
Curl in the Leaf.-In the early part of 1900 Potato growers in most parts of Great Britain observed that their plants developed slowly, and that the foliage, which was very scanty, curled, without losing its colour. Large numbers of plants never made progress, but remained stunted. They yielded very poor crops, and it was noticeable that the seed tubers remained almost unchanged throughout the season. The same state of affairs was observable in 1901. Such a generally reliable variety as Beauty of Hebron was one of the worst affected. Premature ripening of the seed, owing to the hot, dry summers of 1899 and 1900 was suggested as the cause, not without reason, probably. I think, however, that the cold, late springs were contributory, and I am strengthened in this opinion by the fact that in one of my plots that was badly affected a piece of Up-to-Date planted very late (the end of May) was perfectly healthy. If growers have good seed they need be in no hurry to plant in exposed places.
I do not know if Potato scab has been exhaustively studied by micro-fungologists. When it does engage their attention, I shall not be surprised to hear that there are several distinct forms. One form of scab is directly due to excess of rank manure. Another (or it may be the same) is just as certainly due to contact with coarse, sharp particles, such as coal ashes or rough stones. Where scab is persistently troublesome in the absence of these conditions a little sulphur should be dusted in the drills, or the seed should be soaked in a solution of corrosive sublimate, 1 oz. to 15 gallons of water. Care should be exercised, as the sublimate is a dangerous poison.
Fig. 37. Scab In Potatoes.
A, Potato affected by shal low scab.
B, Potato affected by deep scab.
C, Scab fungus, Oospora scabies.
Fig. 38. Twitch In Potatoes.
Here is a curious instance of what twitch or couch, Triticum repens, will do if not cleared out of Potato ground before planting. It has gone right through the Potato.