Feb. 15,1925 Root Rot of Peas Caused by Aphanomyces Euteiches 313 
while a growth of 1.5 cm. occurred at 
9° to 11° in the same time. The opti¬ 
mum temperature for mycelial growth 
appeared to be between 15° and 34°. 
Formation of zoospores. —The 
temperatures at which zoospores were 
discharged from sporangia and be¬ 
came motile was determined as follows: 
A culture upon oatmeal mush six days 
old was washed as free as possible 
from its substratum, cut into frag¬ 
ments, and placed in sterile water in 
watch glasses which were distributed 
in incubators at a series of tempera¬ 
tures. The results are presented in 
Table I. 
drops of sterile water to which spore¬ 
bearing mycelium with as little sub- 
stratum as possible hasbeen 
transferred. Germination has been 
secured at temperatures from 6.5° to 
31° C. Germination with the pro¬ 
duction of zoospores occurs in 24 to 48 
hours at 14° to 28°, a range that may 
be regarded as an optimum. Germina¬ 
tion with the production of a few 
zoospores has been secured at a 
temperature as low as 9° to 10° in 
four days, while germ tubes which did 
not discharge zoospores developed in 
the same time at 6° to 7°. Thus the 
oospores germinate in practically the 
Table I.— The effect of temperature upon the formation and motility of zoospores of 
Aphanomyces euteiches and the germination of zoospores 
Time in hours 
Temperature °C. 
: 
18 
25 
42 
49 
73 
97 
33-35__ 
Groups. 
Groups. 
Disintegrating. 
! 
i 
21-22_ 
Groups. 
Groups. 
Groups. 
Inactive. 
: 
Zoospores. 
19.5-21_ 
Groups. 
Groups. 
Groups. 
Inactive. 
; 
Zoospores. 
Zoospores. 
15-16__ i 
Groups. 
Groups. 
Groups. 
Groups. 
Groups. 
Zoospores. 
Zoospores. 
Zoospores. 
Zoospores. 
Inactive. 
13-14_! 
Groups. 
Groups. 
Groups. 
Groups. 
Zoospores. 
Zoospores. 
Zoospores. 
Inactive. 
9-11_ 
Groups. 
Groups. 
Groups. 
Zoospores. 
Zoospores. 
Zoospores. 
8-9.5_ 
Groups. 
Groups. 
5-6.5_ 
Note.—T he word “Groups" indicates the presence of groups of discharged encysted zoospores at the 
time of observation, and “zoospores” the presence of motile conditions. 
Germination of oospores. —The 
determination of the effect of tempera¬ 
ture upon the germination of oospores 
has been rendered difficult by reason 
of the fact that no dependable method 
has been developed whereby a quan¬ 
tity of mature oospores can be pro¬ 
duced at a given time. Oospores from 
the roots of peas have not been seen 
to germinate. Frequently cultures on 
corn-meal agar three weeks old will 
germinate readily when placed in 
water, but sometimes only one culture 
among many made at the same time 
will respond in this way. Spores on 
the scant aerial mycelium which can 
be separated from the substratum 
usually germinate sparsely, while spores 
on or in the substratum often begin 
germination as soon as mature. In 
spite of the difficulties which have 
arisen from these irregularities in 
behavior, fragmentary records have 
been made which appear to delimit 
closely the range within which germi¬ 
nation occurs and the optimum. Ger¬ 
mination has been studied in hanging 
entire range of temperature at which 
vegetative activity occurs, and the 
discharge of zoospores from the spo- 
ranges produced by the germination is 
affected by temperature in exactly the 
same manner as in the previous 
experiments where sporanges derived 
more remotely from oospores were 
used. 
PATHOGENICITY 
PENETRATION 
A study of lesions in early stages 
occurring in the field or in greenhouse 
inoculations rarely reveals the first 
penetration of the fungus into tissue. 
In roots entry is accomplished without 
discoloration or visible indication of 
the fact, and later the entering my¬ 
celium, devoid of contents, is practically 
invisible. In the base of the stem a 
yellow discoloration in a few epidermal 
cells often indicates a point of entry. 
The invasion of the base of the stem 
is usually accomplished by the advance 
of the fungus upward from the root 
