^ 



otatoes. 



POTATO SCAB. 



PKOF. S. B. GREEN, ST. ANTHONY PARK. 

 This disease and the losses from it are so well known, that no 

 description of it or enumeration of the loss it occasions is neces- 

 sary. It attacks any and all varieties of potatoes. 



The cause of this trouble has been variously attributed to wood 

 ashes, to insects, to stable manure, to heavy and light soil, to too 

 much and too little water in the soil and to many other causes. But 

 very lately it has been conclusively demonstrated that, while some 

 of these conditions may increase the amount and the rapidit3'^ of its 

 development, yet none of them, nor any of them combined, are the 

 first causes in producing- the disease. 



The real cause of the disease is a minute parasitic fungus plant, 

 that lives in the skin of the potato tuber. The growth of it there 

 produces an irritation which induces the healthy cells to start an 

 extra growth to repair the injury done by the fungus. This growth, 

 which takes place under the diseased surface skin, pushes it up, 

 thus rupturing the cells and forming the rough surface which we 

 call scab. In the same man- 

 ner an3^ growing healthy 

 plant over-grows the injur- 

 ies which it may receive. 



That this is so is well de- 

 monstrated bj' the follow- 

 ing experiments made by 

 Dr. R. Thaxter: A smooth 

 potato was marked with 

 water containing the pure 

 potato scab fungus, in the 

 form of a monogram of the 

 letters R. T. with the result 

 shown in figure 1. 



Fig. 1. Potato scab induced by inoculation in 

 form of monogram '-R. T." After Tliaxter. 



In another experiment, the result of which is illustrated in figure 

 2, one potato was marked with the pure scab fungus in the form of 

 the letter L. In the case of the small potato in figure 2, one end 

 of it was just touched with water containing germs of the scab fun- 

 gus. In each of these cases the fungus produced the rough- 

 ened surface on the skin of the potato so characteristic of scab, 

 and that in a form to prove it the result of the inoculation, thus 

 proving satisfactorily that scab is the result of the growth of germs 

 in the skin of the potato. 



In this connection, perhaps, it should be stated that the scab 

 fungus will grow on the surface of a potato, if given warmth and 

 moisture, whether the tuber is growing or not. In consequence of 



