In Zeitsih des Landwirthsch, Central Vereins der Provins Sachsen, January, 1889, Dr. J. Kuhn discusses the prevalent ideas respecting potato scab. The theories that the presence of a large amount of iron in the soil, or the use of manures or street sweepings may favor the scab received no support. The theory that the presence of much lime in the soil aids the growth of scab is denied by the author, who cites cases where potatoes have been grown free of scab upon land either naturally rich in lime or made so by the application of marl, and in one case where scabby seed had been used little scab resulted even upon highly limed land. Walroth believed the cause of the disease to be a parasitic fungus, which he named Erysibe subterranea, but the author shows that this claim was improbable, inasmuch as the fungus is not always found within the scabby spots. Later, Brung-horst found in the potato a slime fungus, which caused an affection similar to scab, but Kuhn is uncertain whether this is really the true scab, as generally known.

The whole result of the discussion is to show that none of the theories yet advanced are satisfactory, and the cause of the disease is a question yet open to investigators.

Potato Scab #1

It is now generally admitted that the scabs and cracks on potatoes are caused by some injury, mechanical or otherwise, to the tuber. They first appear at the so-called lenticels, which are small pimples on the surface of the potatoes and serve as breathing spots. If the cells at this point are injured, an attempt is made to repair the injury, and layers of corky cells are formed which produce the so-called scad. These spots often spread, and running together form large blotches. The cause of the injury to the lenticels has not been ascertained. It has frequently been attributed to insects and fungi. Wire worms are sometimes found feeding on scabby potatoes, and although we have no evidence that they attack perfect tubers, they may do so, and in that case, scab would undoubtedly be produced. Scab, however, is very prevalent in fields where wire worms cannot be found, and it must there be attributed to some other cause. It is not believed that fungi are in any way the cause of scab, although they are sometimes found on potatoes where scab has already developed.

Among the other causes that have been mentioned are continued drought, excessive moisture, or a drought followed by heavy rains, and the presence of various corrosive substances in the soil, as lime, iron or ammonia. From our present knowledge of the scab, any of these, or more likely a combination of two or more, may furnish the requisite conditions for its development. The use of scabby potatoes as seed was at one time said to lead to the development of scab in the resulting crop, but from various experiments this is not considered probable. It has been claimed that the use of sulphur would cause a great decrease in the amount of scab, but our experiments have not shown this to be the case, - L. R. Taft, Michigan Experiment Station.

Potato Scab #2

H. L. Bolly, of Purdue University, has been making a long and careful study of potato scab, and he is convinced that it is a bacterial disease.