THE POTATO DISEASE: SPRAYING A PREVENTIVE.

Otago Witness, Issue 2690, 4 October 1905, Page 6

THE POTATO DISEASE : SPRAYING A PREVENTIVE.

The blight or disease which so discolours and blasts the haulm of potato fields is due to the growth of a low plant form or fungus -nithin. the plant tissues. How or where this fungus rests during 1 the autumn, winter, and spring has not yet been satisfactorily determined ; one thing is certain — that in esrly midsummer, ■especially on a dull, damp day, when the temperature is about 70 degrees, the seed or spores of this fungus is in one way or other conveyed on the surfaces of the potato leaves ; here it, fo to speak, germinates, and, putting out a little tube, sends it through one of the many breathing holes of the leaf, Once inside the tissues -the tended threads of the fungus, branching and ramifying through the cells which make" up the tissues of the leaf, 10b the cells of their starch content, and wax strong at the expense of the latter. Finally, they burst through the decayed surface and extrude tiny filaments, upon the points of which are borne small capsules, that burst and set free a number of minute .spores, that are in turn carried by the same agencies to other plants, and so spread the limits of the disease. For these minute spores to germinate and find an entrance into the leaves, a certain amount of dampness is necessary, and hence the suitability of a wet season for the spread of the disease will be readily appreciated. Again, it has been experienced that sultry weather, with occasional thunder showers, is very favourable to the development of the disease. Hence the efficacy of a slight coating of such a deadly poison to all low forms of vegetable'life as sulphate of copper. The result of spraying with solution of this is to cause this fungicide to be so distributed over, the leaf surface that the slight dampness, while it affords conditions suitable for the growth of the spores, at the same- time insures a continuous covering film of poisonous solution, in which the spores are at once killed, and tlieir entrance into the plant tissues effectually prevented. There is a further advantage in spraying the potato crop, and that is the fact that the sprayed crops continue growing much later in the season than the unsprayed so that the yield fiom the sprayed crops is appreciably heavier. In Ireland, where the area under potatoes is much greater than the whole area under potatoes in Great Britain, the Irish Department of Agriculture are exhausting their resources in providing facilities ?nd inducements for the growers to spray their crops. From the lecord of experiments published by the Department, it is shown that, year after year, the yield from the sprayed crops has b: eh fiom 1^ to 2^ tons per acre more than that from the' uns2)rayed crojjs, grown under conditions that were precisely similar. In confirmation of the efficacy of spraying, writing in May last, the Irish Farmers' Gazette says : — '' Although th* season for } spraying potato crops has not y_efc ar-

rived, it is well that timely attention should be drawn to its proximity, and that farmers should be forewarned of the necessity which exists for their carrying out this operation in due time. Too much stress cannot be laid on the fact that earl}- spraying is conducive to the best results. The very aim of spraying, it must be remembered, is prevention, not cure. Once the blight has-established itself upon the plants, spraying is of no effect in minimising tho attack. . . . Once the entrance has been effected, however, spraying is a useless and belated operation." Further than this, it will be readily seen that if spraying is done before the foliage has reached its full development the operation is from this fact much simpler, and can be performed much ; in addition, the application of the fine spray in which the solution is discharged to eveiy portion ot the leaf surface is much more thorough and I effectual. The doubters and scoffers ! when spraying was first recommended were many"; to-day it is, we are glad to say, practised generally all over Ireland 'as one of "the indispensable and ordinary processes m the cultivation of the tuber." The following are instructions in a leaflet issued by .the Irish Department of Agriculture : — *Et is necessary to indicate the strength of the solutions which are employed, and the proportions in which the various ingredients enter into their composition. Before doing so it is well to urge that care should in every case be taken that jjie copper sulphate smployed be of 98 per cent, purity; very "often this chemical is largely adulterated with sulphate of iron — a fact which makes it valueless for the purpose in question. The Department have announced their willingness to analyse, free of charge, any samples of copper sulphate submitted to them, so that those about to use samples which may betoken any appearances of adulteration can get tbair minds satisfied in this respect. Besides this, it is further advisable to impress on farmers that the soda employed in one of the mixtures known as the Burgundy Mixture should be pure washing soda, and no soda of any other name or description can take its place. Mistakes in this way have occurred before now, and serious loss resulted, so it may not be amiss to draw attention to this important point. The two mixtures employed* when' made up by the farmer liimself are known as the Bordeaux and the Burgundy Mixture : in the first mixture fresh slaked lime is employed in part to overcome the burningeffect of the sulphate solution, which, if not so treated, would kill the plants dressed Avith it; in the other, which has come into more general use of recent years owing to the claim made in its favour that it adheres better, and is notso readily washed off the leaves, the alkali employed is, as we have already stated, common washing soda. If lime be employed it is more important that it be freshly burnt unslaked lime ; but whichever of the two mixtures be used it is imperative, in order to safeguard the crop, to make sure that the acidity of the solution has been destroyed. To determine this the use of litmus paper, which can be had from every country chemist in penny packets, should be invariably resorted to. A small piece of blue litmus paper dipped into the mixture will indicate by its colour the state of the solution. If the blue colour remains, enough lime^oi soda, as the case may be. has been added to make the solution harmless to the crop ; if, however, it shows even a tinge of red, more lime or soda must be added until the blue colour of the litmus paper when immersed in the solution remains permanent. Whether blue or red litmus paper be employed in the test, there can be no confusion, if it be remembered that the red, as in the nature of things, signifies danger, whereas the blue connotes safety and harmles&ness. For convenience in reference we append the exact quantities of each constituent employed : — — Bordeaux Mixture.— 21b sulphate of copper. lib unslaoked lime. 10 gallons clean water. - — Burgundy Mixture.-^ ~]h sulphate of copper. 2?.1b puie washing sod*> 10 gallons clean water. ,