1034 Journal of Agricultural Research voi. v, no. 32 
leaves were killed by the fungus, the plants were forced to produce more 
new leaves in an effort to keep up their normal activities. Under such 
conditions the new leaves formed, appeared to mature earlier than usual, 
and never became as large as normal. Thus, they became susceptible 
to infection by Cercospora beticola quite early in their development, 
and often became infected while comparatively small. 
This difference in the susceptibility of the different leaves is shown 
in a general way in Table XI by the diagonal grouping of the three types 
of leaves—namely, the very young, the mature, and the old. The upper 
diagonal indicates either no increase in spots on the old leaves, or a slight 
increase on those which were still somewhat active. The lower diagonal 
indicates the very young leaves on which there occurred few or no spots, 
while the middle section represents the mature, active leaves of the plant 
on which the greatest increase in infections took place. A great increase 
in the number of infections developed on either the same leaf (reading 
to the right) or on the entire plant (reading diagonally) as the season 
advanced. 
The mature leaves therefore show the greatest susceptibility to leafspot 
infection and possess the characters which allow the freest penetration 
of the host tissue by the fungus. Such leaves, as previously shown, have 
on the average a stomatal count on the upper surface of approximately 
ioo per square millimeter with a stomatal pore length of 28 fi and exhibit 
the greatest stomatal movement. Thus, the greatest susceptibility 
to infection becomes concomitant with the greatest stomatal movement, 
for they both occur on the leaves of the same degree of maturity. 
STOMATAL MOVEMENT AND GERM-TUBE PENETRATION 
It may then be concluded that a favorable daily temperature (70° to 
90° F.), combined with a relative humidity which does not fall below 60 
at any time, together with daylight, will offer conditions under which 
the stomata on the mature leaves should remain open throughout the 
day. This condition of the host associated with favorable growth 
factors for the parasite would usually allow germ-tube penetration and 
leafspot development. 
With these factors active in producing stomatal opening, detailed 
studies were made of germ-tube penetration from material that had 
been collected in the field during controlled tests. For these experiments 
newly formed conidia from recently developed leaf spots were sprayed 
on mature sugar-beet leaves about 7 p. m. After an incubation period 
of 11 days numerous typical leaf spots appeared. Portions of these 
leaves were taken 24, 36, 48, 60, and 72 hours after inoculation, killed 
and stained according to modifications 1 of the method given by Vaughan 
1 These modifications were suggested by Miss Pearl M. Smith, of the Botany Department of the Univer¬ 
sity of Wisconsin. After the acetic alcohol had acted for 12 to 24 hours, the material was washed for 6 to 8 
hours in 95 per cent alcohol, stained in Pianeze’s stain overnight, and destained with acid alcohol until the 
leaf tissue became a clear red, or even pink in places. The material was washed in 95 per cent alcohol until 
the acid was removed and mounts made in Euparal. Balsam, as a mounting medium in these studies, 
was not found to give a good differentiation between the stomata and the penetrating fungous mycelium. 
