Feb., 1923] 
CLAYTON — TEMPERATURE AND FUSARIUM WILT 
85 
and the rapid acceleration during bright weather have occurred consistently 
throughout the work. Thus, while during the dark month of January 
plants which were inoculated and held at a temperature of 27 0 to 30° C. 
developed the wilt and died, the incubation period was several days longer, 
and the progress of the disease was not so rapid as with plants held at the 
same temperature range in April. 
From these observations it would seem that with a constant temperature 
conditions most favorable for the development of the wilt can not be at¬ 
tained. While soil and air temperatures of 27 0 to 30° C. seem optimum 
for the wilt disease, it is actually true that with a constant temperature of 
27 0 to 30° C. and a sudden rise in temperature of 4 0 to 5 0 associated with 
bright sunlight, the wilt develops the most rapidly. It is, of course, entirely 
possible that the strong sunlight might have a direct effect quite apart from 
the temperature influence. However, for the* present the problem will be 
considered only from the standpoint of temperature. 
Experiment VI. In order to observe further the influence of alternation 
of temperature on the development of the disease, at the conclusion of the 
air-soil temperature experiments a lot of twenty plants which had been 
growing in inoculated soil at a low temperature for two months was brought 
into the medium temperature house and allowed to remain there for ten 
days. The plants all appeared healthy at the end of this time. They 
were then divided into two lots, of which Lot 1 was allowed to remain 
always at the medium temperature (about 27 0 C.) while Lot 2 was carried 
into the high-temperature (33 0 C.) house during bright, sunny days, in 
order that it might be exposed to excessive air temperatures for short 
periods toward the close of the normal incubation period. The normal 
incubation period under these conditions would be about 16 days. The 
results were as follows: 
The incubation period of Lot 2, occasionally exposed to high air tempera¬ 
ture, was two days shorter than the incubation period of Lot 1, which was 
maintained at a constant medium temperature (27 0 C.). The plants of 
Lot 2 wilted quickly, the progress of the disease being much more rapid 
than that in Lot 1. Thus, one week from the date of the first appearance 
of the disease several of the plants in Lot 2 were completely dead and most 
of the others were badly diseased, while in Lot 1 a number of the plants 
showed signs of disease, but in none was it well advanced. 
Summary 
This paper deals with (1) the relation of soil temperature to the occur¬ 
rence of the wilt of temate caused by Fusarium lycopersici , and (2) the 
effect of combinations of different soil and air temperatures on the disease. 
When the fungus was exposed in Petri-dish cultures to temperatures 
ranging from 4 0 to 38° C., the optimum for its growth was found to be about 
28° C., although abundant growth was secured all the way from 18 0 to 31 0 
C. (See Plate VIII.) 
