$6 BULLETIN 64, U. S. DEPARTMENT OF AGRICULTURE. 
oxysporum of Smith and Swingle is different from any European 
species .yet known. 
Leaf-roll diseased plants in America have been free from fungous 
infection in so far as the writer'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 wilt 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 without verification, e. g., 
Solanella rosea (Vanha, 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 believed 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.) 
(b) 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 phytophthorus) 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 
