446 
Journal of A gricultural Research 
Vol. XXV, No. n 
GENERAL CONSIDERATIONS 
The uniform parasitic behavior of Corticium vagum on its several hosts 
in relation to the temperature of the soil is of special interest. Figure 6 
shows the temperature range for the pathogenicity of Corticium vagum 
on the three hosts—pea, bean, and potato—in comparison with the tem¬ 
perature requirements for the growth of these plants. 12 It is evident that 
the optimum temperature for the growth of the bean, as undoubtedly as for 
the cotton (see Table I and fig. i), is entirely outside the temperature range 
for dangerous pathogenic action of the fungus, and at such temperatures 
as would permit these plants to grow at their best, practically uninjured. 
On the other hand, the optimum for the continued growth of the pea and 
for the potato (18° C.) lies well within the range and approximates 
closely the temperature found most favorable for the pathogenic activity 
of the fungus. It would appear evident from the data accumulated, 
that this close approximation in the case of the pea and the potato is 
merely coincident, both the hosts in question being cool-temperature 
plants with similar temperature requirements. The fact that its optimum 
* temperature for pathogenicity remains the same for the cotton and bean 
as for the pea and the potato would indicate that the temperature require¬ 
ments for the parasitic relations of Corticium vagum and its various 
hosts is not influenced seriously by the species of hosts attacked, nor by 
the temperature requirements of any host, but is without doubt a con¬ 
dition determined primarily by a fixed physiological characteristic of the 
pathogen. It is further suggested that in general the resistance of these 
hosts as affected by the temperature of the soil plays a minor part in 
influencing the pathogenic state. 
It is significant that the temperature range so far as determined * for 
the parasitic activities of Corticium vagum agrees essentially with that 
found for its growth as a saprophyte in pure culture. While slight growth 
in pure culture occurs as low as 4.6° C., the severe damage which was 
found on the potato stems at 9 0 (ij), the lowest temperature main¬ 
tained, indicates that the fungus may tecome pathogenic under specially 
favorable conditions at temperatures considerably lower than 9 0 and 
probably as low as the minimum found for hyphal growth.; Further, it 
should be recalled, in view of the relative low maximum (32.6°) for 
the growth of the fungus in pure culture; that under severe conditions, 
such as obtained for the pea and the bean in steaniT&terilized soib the 
highest temperature (29. 5 0 ) tried in the experiment did not entirely 
inhibit tissue destruction. It is important to note in this relation that a 
considerable difference appears between the optimum temperatures for 
pathogenic action and for hyphal growth (fig. 6). As previously indicated, 
however, we have no accurate data at present as to the possible optimum 
for growth of the fungus under the different conditions in the soil. It 
appears definite that the exact optimum for the saprophytic growth will 
vary within a considerable range, dependent upon a number of factors. 
Results obtained by Balls, together with the decreasing rate of growth 
noted at temperatures above 22.4 0 , suggest a possible optimum for 
saprophytic activity in the soil not far different from the low optimum 
found for its parasitic activity. 
There appears no explanation of the fact, however, that the optimum 
temperature for tissue destruction on the various hosts falls considerably 
below that found most favorable for mycelial growth in pure culture. 
12 Temperature data for the potato in figure 6 are obtained fron the earlier publication of this series (rj). 
The reader is referred to this article for a full discussion of this relation. 
