POTATO WILT, LEAF-BOLL, AND BELATED 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) The leaf -roll occurred most severely where no fertilizer was used. 
(2) The 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 this view: " Through such means as the selection of the smallest 
potatoes for seed stock, poor preparation of the soil, excessive appli- 
cations of commercial fertilizers, 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 in Germany and Austria. It has everywhere 
proved the most susceptible to leaf-roll, 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 susceptibility or resistance of American varieties 
there are almost no data. The Pearl, in the West, seems more liable 
to the trouble and may have to give way, like the Magnum Bonum. 
In Germany, however, extensive records are already kept by the 
German Potato-Culture Station (Von Eckenbrecher, 1912) and others. 
