Feb. 15,1925 Root Rot of Peas Caused by Aphanomyces Euteiches 297 
writer, John W. Carlson, of the Utah 
Experiment Station, obtained among 
other fungi a culture of Pythium 
( Rheosporangium ) aphanadermatum 
(Edson) Fitzpatrick, one of the well- 
known sugar-beet parasites. This is 
one of the species of Pythium which the 
writer has found capable of producing 
root rot of peas, and if it is widely 
distributed in this valley it may be the 
common parasite of these plants which 
renders pea growing on old beet fields 
unprofitable. 
GEOGRAPHIC DISTRIBUTION AND ECO¬ 
NOMIC IMPORTANCE 
In the United States the disease has 
been found in practically every pea¬ 
growing district that has been searched 
with care. In the Eastern and Central 
States it occurs frequently, and often 
very destructively. It has been found 
in Utah, Idaho, and Montana, where 
it appears to be unimportant at 
present except under special conditions 
described later. In the Pacific Coast 
States it has been found but once, in 
diseased pea plants sent from Santa 
Clara, Calif. It appears that, although 
the disease is very widely distributed, 
it requires special soil and climatic 
conditions generally present only in 
the Eastern and Central States in 
order to become important when peas 
are grown intensively. It is probably 
this disease more than any other factor 
which has compelled the growing of 
peas in a comparatively long rotation, 
and has thus limited the culture of 
this food crop. Certain it is that were 
it not for the accumulation of this 
disease and others with intensive pea 
culture, the cost of producing canned 
green peas and probably of dried peas 
would be greatly reduced. In the 
absence of any accurate survey of 
fields in the region where this disease 
occurs, it is impossible to state approxi¬ 
mately the number of acres that are 
damaged or destroyed each year; in 
some of the older districts in un¬ 
favorable years as much as 25 per cent 
of the acreage seems to be infested. 
From the reports of growers and county 
agents, combined with a limited per¬ 
sonal survey, it appears that several 
thousand acres of peas are rendered 
unprofitable or destroyed in the United 
States each year. 
ISOLATION OF THE FUNGUS 
The fact that this important parasite 
of peas has remained undescribed so 
long is undoubtedly due to the difficul¬ 
ties encountered in isolating it in pure 
culture. One cause of failure is. due to 
the brief period of time during which 
the fungus flourishes in an active vege¬ 
tative stage at any point in the host 
tissue. When the fungus first begins 
to invade tissue, it can hardly be in¬ 
duced to grow out on the culture 
medium in preference to the living 
cells. Only a few days are required at 
ordinary temperatures to exhaust and 
destroy the host cells, whereupon the 
mycelium begins to transfer its con¬ 
tents into the large oospores, which are 
formed abundantly. After this process 
is well under Way it is again almost 
impossible to secure growth on culture 
media. Thus it is necessary to obtain 
diseased plants in which the fungus is 
growing vigorously just prior to ex¬ 
tensive oospore formation if success in 
isolation is to be attained. 
Another cause of failure is due to the 
large number of vigorous saprophytes 
which follow closely the invading 
Aphanomyces. Besides abundant bac¬ 
teria, in some seasons there is almost 
always present a species of Pythium so 
much more vigorous in growth upon 
culture media that it usually submerges 
the Aphatfomyces. As yet no method 
of surface disinfection tried has given 
material aid in destroying the sapro¬ 
phytes present. The parasite is exr 
tremely sensitive to bichloride of mer¬ 
cury, and its use almost invariably 
brings failure. 
In order to secure cultures of the 
fungus from localities remote from 
facilities for making isolations or from 
plants which are so far decayed that 
direct isolation is impossible, the senior 
writer has been accustomed to pack 
diseased roots with soil from the field 
in tin cans kept tightly sealed awaiting 
a convenient time in which to combine 
this material with steam-sterilized soil. 
In this soil mixture peas are then 
grown properly protected from outside 
infection until plants are obtained 
which have developed the stage of the 
disease suitable for the isolation of the 
parasite. In this way cultures have 
been obtained for comparison from 
nearly all of the regions where the 
disease has been found. Fragments of 
tissue are selected in which the my¬ 
celium of the fungus is seen under the 
microscope to be filled with granular 
contents, are thoroughly washed in 
sterile water, and are placed on plates 
of 2 or 3 per cent clear agar, or prefer¬ 
ably prune agar, as recommended by 
Hartley (4) for the isolation of Pyth¬ 
ium. The parasite always grows 
sparsely, sending out long, straight 
filaments with comparatively short 
lateral branches through or over the 
agar. It will almost always outgrow 
the bacteria which soon develop abund- 
