74 
Journal of Agricultural Research 
Vol. XXI, No. i 
coloration in litmus solution when from net-necrosis tubers but not 
when from healthy tubers. Possibly the discoloration of net-necrosis is 
due to an accentuation of a normal tendency of the potato tuber tissues to 
darken upon exposure to air. Darkening has been attributed by Barthol¬ 
omew (4) in the case of blackheart to melanin or humin formed by the 
interaction of tyrosinase and tyrosin in the presence of oxygen. It 
may possibly be due, in net-necrosis, to identical or similar physiological 
changes resulting from, or productive of, leafroll infection. This is in¬ 
dicated by the finding of tyrosinase in greater quantities in leafroll 
tubers ( 8 ). 
RELATION OF NET-NECROSIS TO SPINDUNG-SPROUT 
In the preceding sections of this paper evidence has been submitted 
indicating that net-necrosis of one kind is a symptom of leafroll infection. 
It may be a cause of spindling-sprout, and the latter may thus become 
a symptom of leafroll. Spindling-sprout was distinctively pronounced 
in the net-necrosis half of the tuber shown in Plate 5, A. It was 
consistently characteristic of the net-necrosis tubers selected from lot 
V-148, sublot 3, of Table VIII, half of which sublot is shown in Plate 
12, A, and was accompanied by a reduction in the number of lengthened 
sprouts. Such reduction in length is also apparent to some extent in 
sprouts of the companion sublot 2 from leafroll hills, but they show no 
decided spindliness (PI. 12, B). Plate 3, B, depicts the appearance of 
representative tubers from the two groups shown in Plate 12, A. Plate 
4, A, illustrates the accompaniment of net-necrosis and another leaf- 
roll symptom—namely, maintained rigidity of the tuber—by spindling- 
sprout in diseased tubers kept with healthy ones in an uncooled room 
from June 1 to August 11. The type of spindling-sprout caused pre¬ 
sumably by net-necrosis has been described also by Stewart and Sirrine 
(25, p. 140-141 ) and other types with apparently different causes by 
the same writers (25), by Coons (7, p. 29), and by Haskell (10, PL 13 , 
Fig. B). 
RELATION OF LEAFROLL TO MOSAIC 
As indicated previously, Quanjer considers that both mosaic and 
leafroll are virus diseases. The writers are of the same opinion. That 
is, they consider both to be transmissible or infectious diseases for which 
no pathogene or other cause has been isolated or synthesized. 1 
Regardless of such similarity between the two diseases, it appears to 
the writers unwise yet to regard the behavior of one as indicating exactly 
what that of the other will be, at least in commercial fields. Each va¬ 
riety of potato commonly grown in Maine apparently differs in suscepti¬ 
bility both in regard to the two diseases and in comparison with the other 
1 Such isolation or synthesis apparently will be necessary before it will be decided whether the conven¬ 
ient term “virus’’ represents an ultramicroscopic organism or an enzym. The two sides of the controversy 
are represented by Allard (/, 2) and Chapman (5). 
