450 Wisconsin Academy of Sciences, Arts, and Letters. 
peratures the disease occurs and destroys the plants at any and 
all temperatures down to the lowest limit at which the tobacco 
can grow, viz., about 10° C., showing that at the lower tempera¬ 
ture the range of parasitic activity for the fungus goes quite as 
low as that for the growth of tobacco. The tobacco is favored 
by warm soils, and as the soil temperature rises, if in soil free from 
Thielavia, the plants show an increasingly vigorous stem and leaf 
development up to about 26 °—30 °C. The optimum for root devel¬ 
opment occurs a little lower, at 20°-25°C. (PI. XXXVI, fig. 2). 
On the other hand, the disease is most serious at about 17°—18°C., 
and as the temperature rises the evidence of its injuries is gradu¬ 
ally lessened until at 26°C. it ceases to be a noticeable factor in 
checking the host plant (PI. XXXVII), while at 30° or above no 
further lesions appear on the roots. The tobacco, on the other 
hand, with its optimum development at 26 °—30° and capable of 
enduring soil temperatures at least ten degrees higher, makes a 
vigorous development at soil temperatures of 28 °C. or above even 
in the “sickest” soil. 
These experiments furnish an adequate explanation of the 
farmer’s experience in the variations in this disease with soil and 
season. In the first place, they show that the direct and sole 
cause is the parasitic soil fungus Thielavia. If this fungus is 
present in the seed beds, it attacks the roots of the seedlings in the 
earliest stages of their growth because of its ability to develop at 
the lowest temperatures at which tobacco will start. In old tobacco 
fields it is also, as a rule, present in the soils ready to attack the 
roots immediately following transplantation, but even with the 
newer soils it is very likely to be carried to the fields from the seed 
bed on the roots of some of the transplants. It thus gets a foot¬ 
hold and tends to increase year by year. Soil temperature is low 
enough every season for it to make at least some development, 
once started in the field, and with a dominance of weather suffi¬ 
ciently cool to hold the soil temperature below 25° C. during July 
and August, it continues to rot off the new roots about as fast as 
they originate, so that through midsummer the plants remain sta¬ 
tionary even if they are not killed outright. When, on the other 
hand, the summer continues hot during July and August, the roots 
escape disease even in the “sickest” old tobacco fields. The more 
usual thing under Wisconsin climatic conditions is to have relatively 
cool weather during the first part of the summer with a dry, hot 
period during late July or August. When this occurs the tobacco 
