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.
Oospora scabies is one of the most common potato diseases.
In "Bulletin 71" of the Wyoming Experiment Station it is described as follows:
This fungous disease is too well known to need any description. All who use as well as all who grow potatoes know the familiar, irregular, sorelike blotches which sometimes are so numerous as nearly to cover the whole potato. Only the surface may be affected or the fungus may have penetrated and broken down the tissues almost to the centre. While probably not wholly preventable, yet it is the potato disease that is most readily held in check. With clean or properly disinfected seed, if one puts it into clean ground (free from the fungus) the crop should and will be essentially clean. It is well known that once the fungus gets into the soil it will live over winter and infect the next crop more completely than the former. Just how many years may be necessary to completely rid the soil of the fungus is not definitely known, but it is certain that another crop of potatoes should not follow scabby potatoes for two or three years and probably better not for four or five. Other crops on this land are not attacked, which points anew the safety and desirability of a scheme of crop rotation extending over several years.
Having decided upon the variety to be planted - and this choice must rest upon many characteristics, such as quality, shape, period of ripening, resistance to disease, marketableness, etc., - then select those that are the freest from scab of any that you can find. It is well to remember that the absence of the characteristic surface markings is not conclusive evidence that the potatoes are free from the fungus. They may have been in contact in the bin with scabby specimens, as a result of which they are infected abundantly with the scab spores. Unless you are sure of the condition of the seed it will pay as a precautionary measure to treat (disinfect) the seed.
The old method is quite largely in use in the state and must still be regarded with much favor.
The new method, however, has some very practical advantages. Both are given below, so that if the ingredients for one are not at hand, the other may be used.
Dissolve two ounces of corrosive sublimate (bichloride of mercury) in two gallons of hot water. When the corrosive sublimate is dissolved, add cold water until you have fourteen gallons in all. Having put the potatoes in a gunny sack, place the sack in the solution and leave it there for one and one half hours. Then empty the potatoes out upon the floor to dry before cutting and planting. If they can be left thus exposed to the light and air for a few days they will grow all the better.
If taken internally corrosive sublimate is a violent poison, hence all animals must be kept away from the solution and the treated seed. On account of its action on metals the solution must be prepared in wooden vessels, a barrel, for instance. See that the potatoes are clean. Put them into a coarse gunny sack and place it in the solution. The vessels and all objects in contact with this poisonous solution must be destroyed or thoroughly cleaned.
Formalin (or formaldehyde) may now be secured at moderate cost at any drug store, or can be secured from the larger drug firms (by express) at 50 cents (or less) per pound. Since this treatment is at least as effective as the other, most people will prefer to use it for the following reasons: (1) It is easily prepared; (2) any kind of vessel may be used; (3) it is not poisonous to handle.
Soak the seed potatoes for two hours in a solution of fifteen gallons of water and one half pint (half pound) of formalin. Smaller or larger quantities in the same proportion. Dry the soaked seed, cut and plant as usual.
It is well to remember that disinfected seed will be reinfected if it is put back into the dirty sacks or boxes from which it was taken. If to be used again, disinfect the sacks and boxes also."
It will be interesting to note how this subject is handled by the Board of Agriculture and Fisheries of Great Britain. Their "Leaflet 137" says:
At the present day Oospora scabies is one of the most widespread of diseases affecting the potato. The fungus usually attacks the tubers while young, forming scattered rough patches or scabs on the surface; these patches gradually increase in size and number, and not infrequently, when the tuber is full-grown, its surface is more or less completely covered with scab.
The injury is confined to the surface of the tuber, the skin being broken up into fragments over the diseased patches. Although the market value is much depreciated when scab is present in quantity, the quality of the potato is not in the least impaired for eating.
(1) If scabbed potatoes are used for ' seed' without having been sterilized, the resulting crop will almost certainly be diseased, and in addition the fungus will pass into the soil, where it is capable of living for several years. Scabbed potatoes may be used for 'seed' without the slightest danger of spreading the disease if they are immersed for two hours in a solution consisting of one pint of commercial formalin (formaldehyde 40 per cent.) mixed with thirty-six gallons of water. The potatoes should then be spread out to dry, when they may be cut and planted in the usual manner. Great care must be taken after potatoes have been treated as above that they are not placed in sacks or hampers that have contained scabbed potatoes.
(2) Land that has produced scabbed potatoes is certain to be infected with fungus, and should not be planted with potatoes for several years afterward; beets, Swedes, carrots, and cabbages are also attacked by the fungus. Cereals may be sown with safety on infected land.
(3) In the case of gardens and small allotments, where potatoes are of necessity grown every year, the trenches in which the potatoes are planted should be sprinkled with powdered sulphur.
(4) Lime favors the development of the fungus in the soil; the same is true of stable manure, night-soil, etc. Acid manures only should be applied to land that is infected.
(5) Peelings from infested potatoes, unless they have been boiled, should not be given to pigs. Burning is the safest, and in the end the most economical method of dealing with them."