4 8 
Vol. XXI, No. i 
Journal of Agricultural Research 
becoming infected from the tuber or when only a few centimeters tall 
show it in the lower leaves first, usually followed by rolling of progress¬ 
ively higher leaves later in the same season until most or all are rolled. 
Some other diseases have characteristics in common with leaf roll, 
but they can be distinguished from it. Leafroll is marked by rigidity 
or stiffness of the affected foliage (PI. i, A, B) in great contrast with the 
flaccid, wilted condition of the rolled leaves found on vines affected 
with wilt diseases. It causes the leaves to be rolled symmetrically 
instead of asymmetrically as on plants whose leaves, usually a few top 
ones, are injured by certain insects. It closely resembles when in the 
top alone the type of rolling found on plants affected with Rhizoctonia, 
but the effect of Rhizoctonia will be disclosed by examining more closely 
especially for the characteristic underground stem lesions. The type 
of leafroll caused by blackleg usually appears on the entire plant, accom¬ 
panied by more pronounced yellowish discoloration, and disclosure of 
the blackened underground stem will distinguish this type further. 
A nonparasitic type of leafrolling resulting from excessive soil moisture 
resembles very closely the kind considered in this paper. However, 
in the former the leaves on the entire plant may become rolled at any 
time in the development of the vine, the foliage does not become as 
stiff and rigid, and the leafrolling is limited to segregated areas where 
soil moisture is excessive. Another nonparasitic type is caused by 
drouth conditions, but usually'is limited to the lowest leaves on the 
side of the plant most exposed to sunlight and is accompanied by limp¬ 
ness rather than by rigidity of the leaves. ^ 
NET-NECROSIS 
Orton first used the name “net-necrosis” in connection with an illus¬ 
tration { 15 , PI. II, fig. 2 ). As described previously by other writers 
(6, 7, 9), the most apparent effect of this disease in the tuber is a dis¬ 
coloration of the fibrovascular bundles, originating at the rhizome scar. 
This is dark, varying from brown to black, and is in the form of a net¬ 
work (PI. 2, B) which may extend throughout the whole tuber. The ap¬ 
pearance of affected tubers in section is shown in Plate 3. The writers 
have seen net-necrosis occasionally absent in the stem end while pro¬ 
nounced in the bud-end half. While it is not difficult in severe cases to 
distinguish, with the unaided eye, the type of net-necrosis under consid¬ 
eration from wilt types of vascular discoloration (75, PI. II), the writers 
often have found it very difficult and even impossible to do so in slight 
and, sometimes, in moderate cases. Recourse to microscopic examina¬ 
tion then has been necessary, the phloem being the tissue showing necrosis 
instead of the xylem. As will be described more in detail later, other 
leafroll characteristics besides necrosis of the phloem appear to be'insep¬ 
arable from net-necrosis. Net-necrosis tubers are often hard and firm 
(PI. 4, A), as are leafroll tubers, and bluish on the stem end. Frost- 
