Among the several diseases of the potato doing great injury none are more troublesome than potato scab. The theories and reasons assigned for the disease are numerous, but scientists have settled down to the conclusion that this disease is caused by a minute parasitic plant, known as Oosporascabies, Thaxter.1 It will not be necessary in this connection to review the work of other investigators concerning the various theories. These briefly are chemical action, mechanical irritation, excessive moisture, and the insect theory, and the insect fungus theory of Hopkins. It will be well to briefly consider the nature of potato scab as shown by the investigations of mycologists.



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