448 Wisconsin Academy of Sciences, Arts , and Letters. 
that with the first planted series, in which the tips of the primary 
shoots in the cold soil were cut off or killed by the canker, the 
secondary sprouts arising from the bases of these same stems later, 
when the soil was warm, developed almost uninjured. (See 
Plate XXXY.) 
The influence of soil temperature on this potato canker is, 
therefore, as clearly evident as with the tomato wilt but in an 
entirely different way. With the tomato disease a soil temper¬ 
ature below 19 °C. inhibited the attack, and 27°C. was most fa¬ 
vorable. With the potato disease at 27 °C. the host is practically 
uninjured, whereas the worst attacks occur below 19°C. If one 
seeks the explanation it is natural to recall that the tomato 
fungus (Fusarium) developed most aggressively in culture at 
about 27° C. and to suspect that the potato fungus (Rhizoctonia) is 
favored by the lower temperature. As a matter of fact, however, 
although the potato fungus is not as sensitive as the tomato para¬ 
site to temperature fluctuations, it too grows fastest at the higher 
temperatures (24°-26°C.). For adequate explanation of the 
escape of the potato shoots at the higher temperature we are, 
therefore, forced to turn to one or both of two other theories. The 
first was suggested some years ago by the British botanist, Balls 4 , 
for a similar if not identical disease of cotton; viz., that the fungus 
loses its pathogenicity at the higher temperatures as a result of 
some form of self-poisoning if the food supply is limited or of 
improper character, a condition which may occur in the cells un¬ 
der these conditions. Richards prefers to consider that it may be 
due to inability, at the higher temperatures, to develop certain 
enzyms or toxins essential to its parasitism. He has, however, 
shown that the more rapid growth of the potato shoots at the 
higher temperature offers the important reason and perhaps the 
chief one for their escape. Thus in his trials he found that with 
rather dormant potato tubers planted 5 inches deep the shoots broke 
ground in 18 to 25 days at 24°C., whereas they were delayed 
nearly two weeks longer at 9° to 12°C. In view of the fact that 
the chief injury by the fungus results from a rather slow attack 
upon the growing tips of the shoots while yet below ground, he 
finds evidence for the conclusion that at a temperature of about 
21 °C. or above the shoot is able to grow away from the pest in 
4 Balls, W. L. The physiology of a simple parasite. Yearbook Khedivial 
Agr. Soc. Cairo 1905:184. 
