A SERIOUS LETTUCE DISEASE. 
127 
vigorously growing mycelium, some placed as near the center of the head 
as was possible without tearing the plant seriously, others by laying the 
mycelium upon the lower leaves on the shaded side of the plant. All 
of these plants showed clear cases of infection at the end of fifteen 
days or earlier, and all the plants were dead and dried up on the 
first of April, 1908. 
To determine the rapidity of growth of the mycelium of the fungus 
in a plant and its migration through the tissues of the plant a leaf, 
after it had become infected on one edge, was placed in a large petri 
dish and daily record was made of the growth of the mycelium by 
marking the outside of the plate. The daily growth as is shown by 
the accompanying diagram, Tig. 27, was on an average, about 11 mm., 
while the maximum for a single day was 15 mm. and the minimum for 
the same period 8 mm. The lines of the diagram indicating the 
growth each day, represent as nearly as possible, the place where the 
green and uninfected portion of the leaf joined the translucent or 
infected portion. The average temperature for the five days was 54.5 
degrees F. with a maximum of 75 degrees and a minimum of 39 
degrees. 
After the mycelium exhausts the food supply from the portions of the 
plant which are attacked, it reaches into the air or over the earth; no 
aerial mycelium is noted upon newly infected tissue. 
The Maximum Infection Distance for Mycelium. 
A few lettuce leaves were placed on moist sand in a glass dish, in¬ 
oculated and kept covered. After a vigorous growth had developed 
and the originally infected leaves were nearly consumed fresh leaves 
were placed at various distances from the infected mass. The maxi¬ 
mum distance at which leaves were infected in this way, i. e., the 
maximum distance that the fungus could travel under such condi¬ 
tions, was 2.5 cm. White mycelium might be seen with the naked 
eye to a distance of about 8 mm. from its source of nourishment, no 
farther. A very few small sclerotia formed on the naked sand near 
the lettuce leaf. 
In order to determine to how great a distance the mycelium would 
spread over strictly non-nutritious surfaces small pieces of lettuce leaf 
were laid in a large sterile petri dish and inoculated wfith mycelium. 
The dish was then placed in a moist chamber. The following is the 
record showing the distance to which the mycelium spread, using vari¬ 
ous amounts of lettuce leaf for inoculation. 
It is evident that with a very small amount of lettuce from which 
to draw nourishment the mycelium spreads only a short distance. By 
increasing the amount of nutrient material the distances to which the 
mycelium can spread is increased up to a maximum of about 22 or 
23 mm. It is thus apparent that the spread of this fungus over a 
soil to any great distance, unless the soil provide nutrient material, 
is not to he feared. It is noted also in this experiment with the small 
amount of nutrient material such as 50 sq. mm. of lettuce leaf no 
sclerotia are formed. With as much as 100 sq. mm., however, a sclero- 
tium, though small, was made. 
