POTATO WILT, LEAF-EOLL, AND RELATED DISEASES. 35 



entrance of fungi into the vessels. In this connection it may be 

 noted that no fertilizers are used in Colorado, but that, according to 

 Headden (1910), an abnormally large amount of nitrogen is present 

 in these soils. 



Experimental evidence on the effect of fertilizers is brought forward 

 by Osterspek (Appel and Schlumberger, 1911), who comes to the fol- 

 lowing conclusions: 



(1) Tlie leaf-roll occurred most severely where no fertilizer was used. 



(2) Tlie second degree of severity was where the potash salts were left out. 



(3) The absence of phosphoric acid favored the leaf-roll to a lesser degree, though 

 still perceptibly. 



(4) The use of a complete fertilizer, with nitrate of soda, superphosphate, and 

 potash salts, tends to reduce the prevalence of leaf -roll. 



(5) A second application of nitrate of soda after stable manure or after a complete 

 commercial fertilizer reduced the leaf-roll. 



Many practical growers have attributed leaf-roll to defective 

 cultural conditions, poor soil, etc. Stormer (1911) also subscribes 

 to tliis view: "Through such means as the selection of the smallest 

 potatoes for seed stock, poor preparation of the soil, excessive appli- 

 cations of commercial fertihzers, heating of the potatoes in the silo, 

 etc., a degeneration of the stock may be brought about and with this 

 the leaf -roll." However, he has not yet exact proof of this. He 

 believes that a hereditary leaf-roll may be caused by soil influence, 

 "that one and the same potato may degenerate or remain healthy, 

 according to the place where grown." He reports having succeeded 

 in bringing up the vigor of a weak stock by growing it in one year on 

 a poor, sandy soil. This leads us to the consideration of the problem 

 from the varietal viewpoint. 



VARIETAL SUSCEPTIBILITY AND RESISTANCE TO LEAF-ROLL. 



The first appearance of leaf-roll in Germany was on the variety 

 Magnum Bonum and was considered as an evidence of varietal 

 deterioration (Schultz [Soest], 1905). Magnum Bonum is one of the 

 older varieties. It has also been one of the most popular and, since 

 its introduction from England, has become one of the most widely 

 cultivated potatoes m Germany and Austria. It has everywhere 

 proved the most susceptible to leaf-roU, but those who take this to 

 be proof of the general ''running out" of the variety have to meet 

 several counter arguments. Healthy stocks of Magnum Bonum are 

 still to be found. The leaf-roll attacks many other varieties, and it 

 occurs even on plants grown from seed. 



As to the relative susceptibihty or resistance of American varieties 

 there are ahnost no data. The Pearl, in the West, seems more liable 

 to the trouble and may have to give way, Hke the Magnum Bonum. 

 In Germany, however, extensive records are already kept by the 

 German Potato-Culture Station (Von Eckenbrecher, 1912) and others. 



