Apr. 21,1923 Occurrence and Significance of Phloem Necrosis 
243 
TRUE PHLOEM NECROSIS AND THE LEAFROLL PROBLEM 
In the microscopic examination of potato vines one observes, occasion¬ 
ally, pathological changes associated with certain types of diseased plants 
which by virtue of their peculiar external appearance are grouped under 
the collective name of le^roll. Quanjer ( 6 ), while investigating the leaf- 
roll disease, noticed this correlation, and as the result of subsequent studies 
arrived at the following conclusion: 
The lignification of the phloem is a dependable diagnostic symptom for the identi¬ 
fication of leafroll, and the physiological disturbance, occasioned by the destruction 
of the conducting system for plastic materials, accounts for the change in the mor¬ 
phological structure and for the reduction in yield. 
Stem sections of a typical leafroll plant exhibit, as a diagnostic internal 
symptom, a necrosis and lignification of the phloem groups. In case of 
severe external symptoms the diseased groups pervade the entire plant 
with the occasional exception of the underground organs. The distal 
stem region is commonly affected, and in nearly every instance the 
necrotic changes are of an extreme type. The basal stem region always 
shows necrotic changes when external symptoms become evident while 
the plant is still young. As a rule necrosis of the phloem in the lower 
stem means general necrosis of the plant throughout its extent, but the 
symptoms may decrease toward the distal end or disappear altogether. 
At any given height of the stem the node is typically more severely affected 
than is the intemode. This condition is especially observed in the initial 
stages of the disease, but during subsequent development either region 
may be equally affected. 
In petiole and midrib necrotic phloem groups may appear at a much 
later period, but the extent of necrosis is as a rule correlated with the 
severity of the rolling and the discoloration of the foliage. In the young 
leaf most advanced necrosis is observed in the middle part of the petiole 
probably because this part of the leaf is ontogenetically the oldest. In 
mature leaves, however, the midrib is often the organ which is most 
severely affected. 
In the underground organs of diseased plants the phloem strands are 
usually normal, but in severe cases both stolons and tubers will show 
evidence of necrosis. 
The lateral distribution of phloem necrosis is also subject to a great 
deal of variation. This is true for the phloem groups of a certain region, 
but the difference becomes even more apparent when outer and inner 
phloem are compared. In a given cross sectional area one may observe 
phloem groups in various stages of degeneration. Entire groups may be 
affected in whole or in part. Often one notices perfectly healthy groups 
side by side with diseased ones, which seems the more remarkable if one 
recalls how closely the groups are connected with each other through 
branching and anastomoses. In the apical stem region the first stages 
of necrosis are found in the external phloem and only later in both 
regions. In the basal stem either region is found to be diseased, but often 
the inner phloem alone is completely destroyed while the outer phloem 
is altogether normal or shows, at the most, only initial stages of dis¬ 
organization. 
In petiole and midrib, necrosis is primarily restricted to the outer 
phloem. In advanced stages the bundles flanking the comers of the large 
lateral bundles are necrotic, and occasionally all the inner phloem is also 
diseased. 
