144 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
Comparison of the Effects of Low Soil Moisture and Low Soil 
Temperature on the Host, in Relation to Development 
of the Disease 
The resistance induced by low soil moisture seems different from that 
induced by low soil temperatures. Thus, plants could be produced which 
had tops that were susceptible and roots which were resistant to the disease, 
or roots that were readily invaded and tops that were not attacked. A 
soil temperature of 15 0 to 20° C. checked the progress of the fungus while 
it was in the roots, but exerted no influence if the parts above ground became 
infected, the temperature of the air being the deciding factor in that case. 
On the other hand, soil-moisture conditions low enough (14-18 percent) to 
check the disease exert an influence through the plant as a whole, roots and 
tops both being made resistant. 
Temperature effects on disease were not correlated with apparent 
differences in the host plant; thus, the decrease from a minimum of disease 
at a temperature of 27 0 to 30° C. to no disease at 19 0 to 20° C. was not 
associated with a marked change in the appearance or in the composition 
of the host plants. 
The case of soil moisture was different, for the decrease from a maximum 
of disease with a soil moisture of 30 to 33 percent to a minimum of disease 
with a soil moisture of 13 to 14 percent was correlated with a marked re¬ 
duction in the vegetative vigor of the host plant. This decrease in the 
amount of disease did not begin to manifest itself until the soil moisture 
was reduced to a point where the plants actually began to show symptoms 
of moisture shortage, and the more acute this shortage became, the more 
pronounced was the decrease in disease. The plants grew well with soil 
moistures from 22 to 33 percent, and there was a virulent development 
of the disease. The plants growing with a soil moisture of 18 to 19 percent 
were the first to show perceptibly increased resistance to the disease, and 
likewise the first to show a marked reduction in vegetative vigor of growth. 
During periods of excessive transpiration they suffered severely. A soil 
moisture of 13 to 14 percent brought the plants close to the point of per¬ 
manent wilting, and these plants showed a maximum resistance to disease. 
Thus the evidence now at hand indicates that soil moisture and soil tempera¬ 
ture act in different ways to increase or diminish the amount of disease. 
Interpretation of the Temperature and Moisture Results in 
Terms of Field Conditions 
In the early work with temperature no attempt was made to control 
soil moisture accurately, water simply being added every day in amounts 
sufficient to keep the soil moist. In the later experiments the moisture was 
held constant by weighing the pots and adding the amount of water needed 
to restore them to the constant weight. However, since the range of mois¬ 
ture conditions almost equally favorable to the development of the disease 
