.1 Dry Rot of the Irish Potato Tuber 
17 
A few cultures were grown on filter paper soaked with a 1 per 
cent glucose solution. A potato agar of the following composi- 
tion wa c ; used : 
500 c.c. water. 
100 grams glucose. 
500 grams potato extract. 
15 grams agar. 
Potato plugs, raw potato plates, and stems were used for the 
determination of the morphological characters; while the agar 
was used for the isolation, for color study, and for the study of 
the rapidity of growth of the organism. The liquid media were 
used for color study and for spore study. 
The growth of the fungus w r as studied at the following tem- 
peratures and humidities (the humidities given being those of the 
air which surrounded the tubes or plates in question) : 
8° to 10° C, (humidity 60 per cent) ; 25° to 27° C, (humidity 
98 per cent) ; 20° to 25° C., (humidity 40 to 55 per cent) ; 22° to 
35° C, (humidity 40 to 55 per cent) ; 25° to 45° C, (humidity 10 
to 40 j>er cent) ; 1.1° C, —3.9° C. ; and —22° O. 
Cultures were grown in direct sunlight, in diffuse light, in 
darkness, and under double walled bell jars filled with solutions 
of copper sulfate and ammonium hydrate, and potassium bichro- 
mate prepared according to Bulletin 55 of the Bureau of Plant 
Industry, page 49. Cultures in the dark were made by wrapping 
the Roux tubes with the black paper which is used in wrapping 
photographic plates. 
The original isolation was begun by opening an infected tuber 
and cutting out pieces of the tissue near the affected area with 
sterilized needles. These pieces were then transferred to agar 
plates and no contaminations of any sort appeared. When the 
spore material was examined it was apparent either that there 
were six to eight types of Fusarium in the culture or that it was 
a pure culture showing a great diversity as to spore form. Con 
sequently it was absolutely necessary to isolate single spores and 
then grow these to determine whether these spore forms were all 
from one progenitor or from several. 
Various methods were employed by the writers for single 
spore isolation; but none, save one by which they could actually 
examine the inoculum before inoculation and determine whether 
a single spore was present or not, is satisfactory and reliable 
The use of the ordinary dilution and pouring method is admis 
sible after one has pure cultures and w r hen the spores do not stick 
together. The same thing applies to the streak plate method. 
The examination of poured plates is a cumbersome and in- 
