A SERIOUS LETTUCE DISEASE. 
125 
on the seventh day and it was not until the twelfth day that any signs 
of infection were seen. The leaves had by that time wilted badly and 
these three cases can not be regarded as cases of parasitism. 
To still further study infection by ascospores leaves were inoculated 
as is indicated in Table XIII. In the case of the inoculations with 
mycelium the vigorously growing mycelium was placed upon cover 
glasses in a drop of lettuce broth so that only the tips of the mycelium 
could touch the leaf by growing over the edge of the cover glass. The 
leaves were kept in culture dishes so as to preserve a humid atmosphere. 
TABLE XIII.—REGARDING INFECTION BY ASCOSPORES. 
Number of 
Leaves 
Surface of 
Leaf 
Ascospores + 
Sterile Water 
Ascospores + 
Lettuce Broth 
Mycelium 
10 
Upper_ 
No leaves infected.. 
No leaves infected.. 
10 leaves infected 
10 
Lower_ ... 
2 leaves infected_ 
10 leaves infected_ 
10 leaves infected 
10 
Bruise_ .. 
2 leaves infected_ 
10 leaves infected_ 
10 leaves infected 
10 
Control. _ 
No leaves infected.. 
No leaves infected .. 
No leaves infected 
Summarizing all of the experiments upon this point, it appears 
that direct infection by ascospores seldom, if ever, occurs but that in¬ 
fection from the mycelium follows in 100 per cent, of the cases. 
Spores placed upon a lettuce leaf in lettuce broth with a small bit of 
torn lettuce leaf also gave a reasonably high per cent, of infection. The 
use of a drop of lettuce broth upon the leaf in which to place the 
spores also usually gave infection. In only two cases, however, of all 
the trials made did any infection result from placing spores upon 
the leaves in pure water or upon the bare surface of the leaf. In 
view of this very small percentage of positive results we are not will¬ 
ing to accept this as evidence that the fungus can ever enter upon its 
parasitic existence without at first having attained vigorous headway 
saprophytically. 
Parasitism and Saprophytism. 
The fungus is clearly a saprophyte under many conditions as is at¬ 
tested by its luxuriant growth on various nutrient media and upon 
dead organic matter. That it may be parasitic as well is obvious from 
its inroads upon lettuce in our experiment beds and in many com¬ 
mercial beds of this State. The exact degree of its parasiticism and 
the extent to which it can exist as a saprophyte can, however, only 
be told by careful experimental work. S;nce these relations are of 
great economic significance, with strong bearing upon methods of proph¬ 
ylaxis, close attention was given to ascertain under what conditions 
and ut what stages of its development this fungus is capable of para- 
citism and how long and upon what nutrients it can exist as a sapro¬ 
phyte, and in particular to what extent it can exist in and migrate 
through soil. 
The following additional test of ascosporic infection was made. 
Thirty lettuce leaves were placed in culture dishes and upon the first 
