Oct. ii, 1915 
Phytophthora infestans in Irish Potato 
73 
RELATION OR MOISTURE AND TEMPERATURE TO THE SPREAD OP THE 
MYCELIUM OP PHYTOPHTHORA INPESTANS IN THE TUBERS 
In order to learn something as to the influence of environmental factors 
upon the spread of the mycelium, tubers naturally infected with P. infes¬ 
tans were taken and the boundary line of the infected area marked with 
india ink. Thirty-three tubers were thus treated and buried in steam- 
sterilized sand in boxes 6 inches deep. The sand was kept continuously 
well moistened. The infected areas included from 20 to 90 per cent of 
the total surface area of the tubers, but there were at least two living 
eyes. The boxes with these tubers were kept in a greenhouse where the 
temperature was held at 16 0 C. at night and 22 0 in the daytime. After 
12 days the tubers were all taken up and the boundary line between the 
sound and infected areas again traced. In every case the fungus was 
found to have made progress at some point on the tuber, but the progress 
was not uniform, the lines coinciding at some points and diverging as 
much as 1 inch at others. The spread seemed to have been more rapid 
in the vicinity of the eyes, although this was not always the case. On 
7 of the 33 tubers sprouts were found that were infected with P. infestans , 
which at the time of planting, 12 days earlier, were sound. 
Again the tubers were planted in the moist sand and allowed to de¬ 
velop for 9 days more. When dug up this time, the fungus was found to 
have spread over all the remaining sound surface area, except in the case 
of two tubers, and even in these it had made material growth. This time 
four more sprouts were found to be infected with P. infestans, its presence 
being proved by cutting the sprouts off and holding them for 24 hours 
in a moist atmosphere, the fungus in the meantime fruiting on them. 
Under the conditions of this experiment, therefore, it was found that in 
three weeks P. infestans had spread over 10 to 80 per cent of the surface 
area of the tuber, that in n cases it had spread into the eyes and traveled 
out into the sprouts, and that in the majority of the cases it spread most 
rapidly in the vicinity of the eyes. 
This experiment was repeated under the same conditions, except that 
the sand was kept very dry and the tubers were held in it for six weeks. 
Without stating any of the details, which were much like those already 
given, it may be said that the fungus spread very slowly; and, while there 
was growth in some cases, in many the infected area remained as it was 
in the beginning. The tubers remained free from soft rots and germinated 
freely. From the results of this experiment it is very strikingly evident 
that to produce rapid spread of the mycelium in the tubers the sand must 
be kept well moistened. 
In still another experiment the temperature, was reduced instead of 
the moisture, the former being held at 4 0 to 6° C. In this case the fungus 
made little or no growth or spread in the tubers, and the potato gave as 
little evidence of activity, showing that both moisture and temperature 
exert a marked influence on the growth of the mycelium. 
