This section is from the book "The Potato: A Compilation of Information from Every Available Source", by Eugene H. Grubb, W. S. Guilford. Also available from Amazon: The Potato: A Compilation Of Information From Every Available Source.
Preventive and remedial measures suggested are:
1. Potato disease is propagated and carried on from season to season in the sets. It is therefore of the utmost importance that sound crops only should be kept for seed, and that sets should be stored under the most favorable conditions. In an ordinary way seed potatoes should be carefully selected, all those being rejected that show the least sign of taint; they should be allowed to get thoroughly dry before clamping and should be stored separately.
2. Diseased haulm should be removed and burned before the potatoes are lifted. If the disease appears late in the season when the tubers have attained a fair size much benefit may be derived by pulling the haulm at once.
3. Potatoes are found to become less resistant to disease the longer they have been in cultivation, and therefore a good, new strain is to be preferred to an old stock; but most of the new varieties offered for sale are more susceptible to disease than old stocks of the best kinds, and growers are warned against relying on the disease-resisting power of a potato merely because it is a recent introduction.
A change of seed is desirable if sets from a sound crop can be obtained, because of the increase in the total crop of sound tubers which is likely to follow, but it must not be supposed that changing seed will enable the plant to withstand disease. On the contrary, it is often found that the luxuriant growth of haulm which may result from changing seed renders the crop more susceptible, and that the percentage, though not the actual weight, of sound tubers is reduced.
4. The rows of potatoes should be well 'earthed' or 'banked' up, as the thicker the layer of soil the less chance is there of the spores of the fungus reaching the young tubers.
5. Neither the planting of vigorous varieties nor cultivation can be trusted to ward off the disease in a wet season, but spraying with Bordeaux mixture has been found effective and is now a part of the regular routine of cultivation in humid districts. Even in dry seasons when no disease is apparent the treatment is found to be beneficial, producing a longer period of growth and an increased yield. This is so generally recognized that spraying has become general in several potato-growing districts, whatever the season promises to be. As the disease does not as a rule make much headway before the end of July, spraying is seldom wanted for the first early sorts, the leaves of which will be dying down before any great harm is done.
Bordeaux mixture may be prepared as follows:
Sulphate of copper or bluestone............. 12 lbs.
Freshly burnt quicklime..................... 8 lbs.
Water............................ 75 to 100 gals.
In order to obtain good results from Bordeaux mixture, careful attention must be given (1) to the materials employed, and (2) to the preparation.
1. Materials: Copper sulphate of 98 per cent, purity should be obtained, 'Agricultural' copper sulphate, which usually contains iron sulphate, must be avoided. Iron sulphate or copperas is valueless for this purpose.
An easy test for the presence of iron in the copper sulphate is to dissolve a little in water and add ammonia with constant stirring until a deep, blue liquid forms; any quantity of brown flocks floating about in this blue liquid indicates the presence of so much iron that the material should be subjected to a proper analysis previous to use.
The lime used should be white 'fat' lime from the mountain limestone or chalk, the kind of lime which is used by plasterers. 'It must be freshly burnt.' If of good quality eight pounds will be required to neutralize twelve pounds of copper sulphate, but the weight of lime required depends upon the quality, and while as little as six to seven pounds might be sufficient in one case, as much as ten to twelve pounds might be required in another.
2. Preparation: The copper sulphate and lime must always be diluted with a large quantity of water before being brought into contact, otherwise a very inferior mixture will result. When making a small quantity the best plan is to dissolve the copper sulphate in about one half of the water, mix the lime with the other half and then bring the two together; but when a large quantity of spray has to be prepared it is usually much more convenient to make a somewhat concentrated mixture and to dilute immediately before application to the crop. Under no circumstances, however, should the first mixture be made too strong, and when twelve pounds of copper sulphate and eight pounds of lime are to be employed, the first mixture should fill a forty-gallon cask. To make the mixture, proceed as follows: Run into cask about thirty gallons of water. Crush twelve pounds of copper sulphate, tie up in a piece of sacking and suspend just below the surface of the water; or, if preferred, dissolve the bluestone in boiling water and pour into the cask. Next moisten and slake eight pounds of lime; the lime must be allowed to swell and crumble slowly; when it has been well slaked, work it down first into a thick cream and gradually dilute to four or five gallons. The milk of lime must next be strained through a fine sieve or piece of sacking to remove grit; it should then be further diluted with water to about ten gallons and poured slowly into the cask containing the solution of copper sulphate. As the two fluids mix they must be thoroughly stirred. If the lime has been slaked slowly and the whole process has been carried out as indicated, a gelatinous precipitate forms in the cask - that is, the water becomes filled with starch-like flecks; these remain in suspension for a long time. On the other hand, if too little water has been employed, or if the lime has not been properly prepared, or if stirring has been neglected, a comparatively coarse powder forms in the mixture and soon settles, so that after standing for an hour or two the fluid in the upper part of the cask is quite clear. The starch-like precipitate, when once it dries on foliage, adheres closely for months, whereas the coarser powder, which results from careless preparation, washes off readily, so that the leaves lose much of their protection after the first heavy rain, and spraying does little or nothing to check disease.
When Bordeaux mixture has been made it should be diluted if necessary and used without delay. One or two days' supply only should be made at a time, for although well-made Bordeaux mixture will keep fairly well for several days, it is best used within forty-eight hours.
For spraying potatoes under favorable conditions the mixture, if made in a concentrated form, should be diluted to 100 gallons, but when spraying must be done in damp weather it should be diluted to from seventy-five to ninety gallons. Before pouring into the sprayer it should be stirred thoroughly. If possible, a sprayer provided with a dasher or other contrivance for keeping the mixture agitated should be used. If no mechanical contrivance is available, stirring should take place frequently while the work is in progress.
The amount to be applied per acre varies with the quantity of haulm in the crop to be treated, but is usually from 100 to 150 gallons where the foliage is fully developed. The plants must be sprayed from underneath as well as from above, so as to reach the fungus on the under side of the leaves. Machines can be obtained which spray the plants from below.
The cost of a single spraying need not exceed $2 per acre, and, with certain horse machines, thirty acres can easily be treated in a day. Bordeaux mixture does not begin to work until several days after it has been applied, and it must, therefore, be used some time before any symptoms of disease are to be expected, say, toward the end of June, or early in July, according to the locality and season. The crop should be sprayed twice at least. The first spraying should take place as soon as there is a good development of haulm, the treatment being repeated about three weeks later when the growth is complete. If only one spray ing is given it should take place about the middle of July."