324 
Joumal of Agricultural Research 
Vol. XXX, No. 4 
permits the infestation of the soil 
with the fungus to proceed as rapidly 
as when the most susceptible peas are 
grown. With intensive culture these 
resistant peas will be subjected to 
increasingly severe infection from year 
to year. Resistant varieties, however, 
may afford a very satisfactory escape 
from root rot where it is not extremely 
severe. But it remains to be deter¬ 
mined experimentally whether any 
resistance has yet been found in varie¬ 
ties of commercial utility which will 
continue to produce profitable crops 
with intensive culture on soil in which 
this disease reaches its greatest severity. 
SUMMARY 
1. Of the several root-rot diseases of 
peas occurring in the United States 
which have been distinguished and 
studied during the past five years, 
the disease caused by the fungus 
described in this paper appears to be 
the most important. It occurs in 
nearly all of the important pea-growing 
districts with a varying severity which 
depends largely upon the degree to 
which intensive culture of peas permits 
the accumulation of the fungus in the 
soil and upon the conditions of soil 
temperature and moisture favoring 
early infection and rapid decay of the 
invaded roots. No other crop than 
peas has been found subject to disease 
from this fungus. 
2. The effect of this disease upon the 
appearance of the plant in the field and 
upon the yield of the crop varies with 
the stage of development of the plant 
at which infection takes place and upon 
the number of infections. If the root 
system is invaded extensively when only 
three or four nodes have been formed, 
the plant may wilt and die suddenly. 
Later invasions may result in dwarfing 
of growth with drying out of foliage 
from the ground upward and in unpro¬ 
ductivity. The disease can hardly be 
distinguished by the appearance of the 
top of the plant, but it can usually be 
identified readily when plants are pulled 
from the ground by the behavior of the 
root, which instead of breaking near the 
planted seed, pulls out as a fibrous 
string consisting of the vascular cylin¬ 
der of the root freed from the decayed 
cortex. 
3. The fungus enters only the cortex 
of the roots and base of the stem, where 
it produces a softening and rapid decay 
of the tissue, leaving the vascular 
cylinder exposed to decay by other 
organisms. In most varieties of garden 
peas the smaller roots thus denuded of 
cortex die immediately. A large num¬ 
ber of oospores are formed by the fungus 
in the invaded cortex, and it appears to 
be from these spores, which increase in 
the soil from year to year with inten¬ 
sive culture of the crop, that infection 
originates each season. 
4. The fungus can be isolated in pure 
culture only with considerable diffi¬ 
culty, both because the period during 
which the mycelium is growing actively 
in the tissue is brief and because it is so 
closely associated with bacteria and 
other fungi that the separation of the 
parasite from its associates is not 
readily accomplished. However, 12 
cultures from different localities have 
been obtained for comparison and study. 
5. Within the host tissues or on a 
suitable solid substratum the mycelium 
soon gives rise to resistant thick-walled 
bodies, the oospores which result from 
the development of oogonia following 
their fertilization by antheridia, of 
which from one to five are associated 
with each female cell. Depending on 
the presence or absence of food mate¬ 
rials, the oospores germinate either 
directly by the production of one to 
several vegetative hyphae, or indirectly 
by the proliferation of a single germ 
hypha of limited growth, within which, 
as well as within the oospore wall, the 
protoplasm is divided into portions 13 
to 18 in number, which are promptly 
discharged in the manner characteristic 
of the genus. 
6. Asexual reproduction resulting in 
the formation of great numbers of 
motile zoospores ensues whenever ac¬ 
tively growing mycelium is provided 
with suitable conditions. Young thalli 
may become almost entirely involved 
in sporogenesis, the individual spo¬ 
rangia being represented by portions of 
mycelium delimited by septa and often 
including a moderate number of well- 
developed branches. These sporangial 
units discharge their zoospores by one 
or several elements, the distal portions 
of which are considerably constricted. 
7. The fungus shows much similarity 
to two aquatic congeners possessing 
smooth oogonia, Aphanomyces laevis 
and A. helicoideSj differing especially, 
however, from the former in having a 
greatly thickened oogonial envelope 
with characteristically sinuous internal 
contour, and from the latter in its 
antheridial branches not exhibiting any 
well-defined spiral habit. It is de¬ 
scribed as a new species, Aphanomyces 
euteiches Drechsler. 
8. Inoculation of pea plants with 
pure cultures under conditions of 
controlled soil temperature and mois¬ 
ture show that infection of peas may 
take place at temperatures between 
10° and 30° C., but that optimum 
