July i, 1921 
Pathogenicity of Corticium vagum on the Potato 
461 
exposure, soil moisture, and the growth of the new tissues, together with 
other possible factors, might easily have accounted for this observed dif¬ 
ference. 
Balls (j), on the other hand, presents data from critical field observa¬ 
tions which indicate definitely that cool temperatures favor the patho¬ 
genic action of the fungus Rhizoctonia solani on cotton. The data pre¬ 
sented also suggest that the cool weather increases the severity of attack 
both by retarding the growth of the young cotton seedlings and by 
favoring definitely the action of the fungus upon the plant tissues. At 
the higher soil temperatures it appears the plant may escape unharmed 
even in a heavily infested field. This writer states his conclusions as 
follows: 
There is 110 doubt, in view of this year’s experience, that given a supply of healthy 
mycelium in the soil of average texture, the amount of damage done to the cotton 
crops depends upon the temperature of the soil. 
By pure culture work Balls (2) was further able to show that “Rhizoc- 
tonia” could attack the cotton plant vigorously at 22 0 C., but he ob¬ 
tained no infection at 33 0 C. 
GENERAL CONSIDERATION OF THE FACTORS INVOLVED 
Progress in the analysis of so complex and fluctuating a condition as is 
found in parasitism can be made best by the common procedure of vary¬ 
ing one factor in the environment while holding all others under control. 
Because of the complex nature and the interrelation of the factors in¬ 
volved, an absolute control of the possible variable is rendered difficult if 
not impossible. Proper evaluation must be given those variations out¬ 
side the limits of control. The problem, however, becomes further com¬ 
plicated in that the variables concerned must be controlled as nearly as 
possible, at a range approximately optimum for the parasitic relation 
under consideration. Wholly safe conclusions as to the importance of 
any one factor are possible, therefore, only when the value of several of 
the most important relations contributing to a parasitic state have 
become known. 
Such factors as are concerned in influencing the parasitic action of 
Corticium vagum on the potato might be considered as both hereditary 
and environmental in nature. Of the former group, strain differences of 
the fungus and host susceptibility are of first importance. The control¬ 
ling features in the environment, however, are more numerous and may be 
listed for convenience as follows: (1) the oxygen supply of the soil, 
(2) the soil temperature, (3) the soil moisture, (4) the acid or alkaline 
reaction of the soil, (5) the organic matter in the soil, (6) the soil texture, 
and (7) the soil flora and fauna, including the degree of soil infestation of 
the strains of the parasite under investigation. At present, the true rela¬ 
tion and importance of any one of these various factors to the parasitic 
