Apr. i, 1921 Leafroll, Net-Necrosis , and Spindling-Sprout 6 1 
1915-E) or doubtful cases of net-necrosis and no leafroll plants (lots 
1919-54 and 1919-96). It will be noted that whenever all the tubers 
in a lot were unquestionably or severely affected the plants produced 
by them were all leafroll or spindling or both. 
The preliminary studies having indicated that severe net-necrosis 
was fundamentally a tuber phloem-necrosis and was always associated 
with leafroll, attempts were made in 1920 to differentiate more exactly 
microscopically between net-necrosis and various types of vascular 
discoloration not associated with leafroll. A tuber was considered as 
showing at least a medium stage of net-necrosis if a longitudinal section 
disclosed vascular discoloration exceeding 1 cm. in extent from the 
rhizome scar, running through several layers, lacking uniformity in degree 
of discoloration, and with soggy-looking parenchyma near by. It was 
considered as being in the severe stage if in addition to the foregoing the 
discoloration extended into the bud-end half or extended beyond the 
stem-end third with many of the strands black. 
During the planting season of 1920, about 8,000 tubers were exam¬ 
ined for the presence of net-necrosis and were planted. Of these, 75 
were recorded as being in the severe stage, 89 in the medium, and 87 in 
either the severe or medium stage with no distinction made. Of the 75, 
all produced leafroll plants except 3 which did not grow above ground. 
Of the 89, all but 2 produced leafroll plants. One of the 2 exceptions 
was among the unusual cases showing the disease only in the eye-end 
half of the tuber. The second was dug July 26 with another net-necrosis 
hill of the same lot. It was flabby, with the discolored strands hard 
and brittle and containing fungous hyphae, discolored xylein, and very 
little discolored phloem, while the tuber that produced a leafroll plant 
was rigid, with the discolored strands smaller, more numerous, more 
extensive, lighter in color, not hard, and containing neither apparent 
fungous hyphae nor discoloration outside the phloem. It evidently 
had not been possible to distinguish readily at the time of planting be¬ 
tween a wilt tuber and a phloem-necrosis tuber. Of the 87, all but 3 
produced leafroll plants, 2 of the exceptions producing wilt plants and 
the third probably being a wilt tuber. 
The practical restriction of net-necrosis diagnosis to leafroll potatoes 
as just described is made more striking by the following facts. Of tiie 
8,000 tubers examined and planted, 1,000 came from about 10 lots which 
in 1919 were grown so as to be readily exposed to leafroll infection, or 
were controls to such lots, and furnished 135 of the diseased tubers, 
while the remaining 7,000 came mostly from lots concerned with the 
study of mosaic and furnished only 116 diseased tubers. Furthermore, 
each of the latter group of 116 tubers came from a lot that either was 
partly leafroll in 1919 or was artificially inoculated in some way with 
leafroll, or was near—not exceeding 12 meters at the most—to leafroll 
lots in 1919. 
