26 BULLETIN 64, U. S. DEPARTMENT OF AGRICULTURE. 



oxysporum of Smith and Swdnglo is different from any European 

 species yet laiown. 



Leaf-roll diseased plants in America have been free from fungous 

 infection in so far as the winter's observation goes, except for certain 

 cases in Colorado, which were plainly mixed infections with Fusa- 

 rium oxysporum, and here many other plants in the same field were 

 fungus free. The external appearance of leaf-roll and ^vilt present 

 many differences already given in detail. 



Other fungi than Fusarium have also been reported in connection 

 with leaf-roll, but for the most part mthout verification, e. g., 

 Solanella rosea {Y&vihai, 1910), Phoma, Bacteria, etc. (Stoermer, 1910). 

 The burden of proof is now on those who attribute leaf-roll to fungi 

 to identify 'their organism through pure culture and reproduce the 

 disease by inoculation. For a more extended summary of this 

 phase of the subject, see Krause (1912). 



LEAF SPOTTING IN RELATION TO LEAF-ROLL. 



The occurrence of spots or flecks on potato leaves is not an inva- 

 riable symptom of leaf-roll, but is often observed in connection with it, 

 particularly in the severer types of leaf-roll. The spots observed by 

 the writer were small, dark-brown flecks in the tissues of the terminal 

 leaves, generally between the-veins. They have also been found on 

 plants not attacked by leaf-roll. These spots are apparently free 

 from fungi and are beheved to be due to physiological causes. 

 Frank (1897) connected these spots with several types of what he 

 termed "Krauselkrankheit," but Appel is undoubtedly correct in 

 pointing out that there is no connection between the spots and the 

 curly-dwarf or the leaf-roll. 



OTHER LEAF-ROLLS. 



Typical leaf-roll must be differentiated from several similar appear- 

 ances, due to other causes, as follows : 



(a) Temporary leaf -roll due to water-logged soil. There are not infrequent cases 

 in poorly drained land or in seasons of excessive precipitation when the potato plants 

 suffering from lack of soil aeration show this by a rolling of the leaves. This can, 

 however, be distinguished from the true leaf -roll, as the symptom disappears when 

 the cause is removed, while true leaf -roll is inherited. The plants, moreover, do not 

 undergo the same color changes. (Appel and Schlumberger, 1911.) 



(6) A leaf-roll condition, usually of temporary duration, may be induced by heat 

 or drought, or by the use of excessive quantities of fertilizer, especially potash (Quan- 

 jer, 1913). In such cases the rolling may be more marked on the lower leaves. 



(c) Blackleg {Bacillus phytophlhorus) produces an upward rolling of the leaves, 

 with a yellow color. The later stages of this disease may at first glance exactly simu- 

 late leaf-roll, but as blackleg is invariably associated with a blackening and shriveling 

 of the base of the stem the two can not be confused after the plants in question have 

 been pulled up. 



(d) Curly-dwarf is perhaps an allied malady, but differs in that there is a pro- 

 nounced shortening of the stem and branches, a crinkling or downward curling of 



