Feb., 1923] CLAYTON — TEMPERATURE AND FUSARIUM WILT 
75 
were equally increased or decreased by a change in temperature, the degree 
of disease-expression might remain the same even though fundamental 
changes had taken place. It is more reasonable to expect that changes in 
temperature would not equally increase or decrease both the attacking 
power of the fungus and the resistance of the host, or, in other words, that 
pathogenicity might be increased out of proportion to resistance, or vice 
versa. It is also quite conceivable that a temperature change ( e.g ., from 
medium to low) might increase the resistance of the host and at the same 
time reduce the pathogenicity of the parasite. 
Since the amount of disease produced in the case of the Fusarium 
disease of tomatoes did vary at different temperatures, it seemed possible 
that a study of the temperature relations of the host plant and of the 
fungus separately might aid in interpreting the variations in the amount of 
disease produced under the different conditions. 
In so far as the host plant is concerned, differences in temperature 
would affect the processes of food manufacture and also the utilization of 
the elaborated substances in respiration and other activities. Some micro¬ 
chemical work was undertaken in an attempt to discover the gross differences 
in composition which might be correlated with differences in the amount 
of disease. Reducing sugars, starch, and nitrates were measured quantita¬ 
tively. It was not presumed that these materials were necessarily directly 
associated with qualities of resistance, but they are intimately concerned in 
the processes of growth, and the quantities present at any particular time 
vary widely with conditions of nutrition and environment. They might 
serve also as indicators of still other materials, possibly derived from them, 
which are more closely associated with resistance. 
No definite interpretation can be made of the results of these chemical 
tests. When the plants were grown at approximately 17 0 C. there was no 
disease, while at 27 0 C. there was a maximum of disease, yet the quantita¬ 
tive differences in the compounds tested for in the two sets of plants were 
not marked. Again, with conditions of warm soil and cool air the lower 
portions of the plants held at the warm temperature were readily invaded 
by the fungus, which did not penetrate the upper parts surrounded by cool 
air. The chemical analyses, however, did not indicate marked differences 
in the composition of the tissues of these two regions. 
There is a closer correlation, perhaps, between the temperature ranges 
of the host, the parasite, and the disease, considered separately, in the case 
of the Fusarium wilt of tomatoes than is ordinarily found. This correlation 
is especially marked at the optimum points, e.g., it has been noted that the 
temperatures at which the host makes its most rapid growth range from 
24 0 to 31 0 C. The fungus also grows most rapidly at these temperatures, 
having its optimum at approximately 28° C., and the disease is most destruc¬ 
tive at soil and air temperatures of 25 0 to 31 0 C. There are, however, 
differences. Thus, the disease develops more rapidly at 31 0 than at 25 0 , 
