8 4 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
symptoms of disease. The symptoms, however, were not marked, con¬ 
sisting simply in a yellowing of the lower leaves, from which isolations of 
the fungus were made. 
The Influence of Alternation of Temperature on the 
Development of the Disease 
All through these experiments it has been observed that, while the wilt 
developed with a fair degree of rapidity when the temperatures were con¬ 
trolled, it was during bright, sunny days when the temperature of the 
house rose considerably that the disease was most virulent. Since this was 
noted with plants grown under conditions of constant soil moisture, the 
wilt cannot in this case be associated with water shortage due to drying 
out of the soil. Likewise, from the experiments with high air temperature 
it can be definitely stated that continuous exposure of the plants to air 
temperatures as high as those attained during the bright weather is not 
effective in increasing the amount of wilt. In the high-temperature (33 0 
C.) plants, just as in the warm-temperature (27 0 C.) plants, the disease 
developed slowly during cloudy weather and with great rapidity during 
bright days. 
Table 3 illustrates the degree to which the virulence of the disease is 
accentuated by clear, bright weather. 
Table 3. 
Date 
4/21 
4/22 
4/23 
4/24 
4/25 
4/26 
4/27 
4/28 
4/29 
4/30 
5/i 
5/2 
5/3 
5/4 
5/5 
5/6 
The Relation of Sunlight to the Amount of Wilting of Tomato Plants 
Produced by Fusarium lycopersici 
Weather Conditions No. Plants which 
Wilted during Day 
. . Clear. o 
. . Cloudy. 5 
. . Cloudy. 4 
. . Clear. 7 
. . Partly cloudy. 5 
. . Cloudy. 1 
. . Cloudy. o 
.. Clear. 6 
.. Partly cloudy. 14 
.. Partly cloudy. 8 
. . Clear. 9 
. .Clear. 16 
. . Clear. 17 
. . Cloudy. o 
. . Clear. 22 
. . Clear. 43 
From this record, kept during one of the soil-reaction experiments, it 
will be noted that on clear or partly clear days the disease was much more 
active than on cloudy days. 
The retardation of the development of the disease during dark weather 
