294 
Joumal of Agricultural Research 
Vol. XXX, No. 4 
with a few of the larger lateral branches. 
No other type of root rot has been found 
which permits the pulling out of these 
vascular strings. This test does not 
apply in the earlier stages of disease and 
sometimes fails to give results when the 
soil is exceedingly hard and dry. Ex¬ 
amination of the roots themselves will 
reveal the extent of injury by the soft 
decay of the cortex. In early stages 
there is a pale yellow discoloration of 
the decayed tissue not readily distin¬ 
guishable from the discoloration asso¬ 
ciated with the invasion of the mycor- 
rhizal fungus described in a previous 
paper (9). 
The soft decay and shrinking of the 
cortex of the roots and of the portion 
of the stem below ground usually dis¬ 
tinguishes this disease from the turgid 
roots with the mycorrhizal fungus. 
After a time the dead cortical tissue 
usually becomes blackened (pi. 1) so 
that it can not readily be distinguished 
by this character from other forms of 
root decay. If the visible symptoms 
already described fail to distinguish 
the disease with certainty, it is almost 
always possible at any stage in the de¬ 
velopment of the disease to distinguish 
it by an examination of the roots with 
a microscope. The characteristic spores 
of the fungus, described later, are al¬ 
ways formed more or less abundantly 
in some of the dead cortex of root (pi. 2) 
or more rarely in that of the stem, and 
serve to determine the presence of this 
parasite beyond possible doubt. 
HISTORY OP THE DISEASE 
Since this disease has not been dis¬ 
tinguished previously under any name, 
it has no unmistakable written history. 
There are a few references to pea-root 
troubles, ascribed to other fungi, but 
on such insufficient evidence that it is 
not impossible that they may be per¬ 
tinent here. The first of these is by 
Wittmack 4 (19), who found in decay¬ 
ing roots of peas sent to him by 
Sadebeck from Hamburg, Germany, 
oospores apparently belonging to a new 
Pythium, which he called P. sade- 
beckianum. 
In the United States the disease has 
undoubtedly been present for a long 
time and has compelled the abandon¬ 
ment of intensive culture of peas for 
canning purposes in certain restricted 
areas. Recently Clinton (%) has de¬ 
scribed a root rot of peas in Connecti¬ 
cut evidenced by the presence of 
oospores which he believed to be those 
of Phytophthora cactorum. The senior 
writer has been unable to produce any 
extensive decay in pea roots growing 
in soil with a culture of this species, 
and in no case were oospores formed in 
the few small lesions produced. Al¬ 
though infection of root ends with a 
pure culture of a species of Pythium 
has been obtained by the writer with 
the formation of oospores in the decay¬ 
ing tissue, and though spores appar¬ 
ently of this species have been found 
occasionally in field material, neverthe¬ 
less the measurements and drawings 
given by Clinton appear for the most 
part to agree more closely with the 
spores of the Aphanomyces species, to 
which reference was made in a brief 
abstract (7) and which is fully de¬ 
scribed in this paper, than with those 
of any other fungus that has been en¬ 
countered. There are other references 
to rootrot of peas in the United States 
which undoubtedly indicate this dis¬ 
ease, but which do not contain a suffi¬ 
ciently adequate description to make 
exact determination possible. 
THE FUNGUS 
HOST PLANTS 
Although it seems unlikely that this 
fungus is parasitic only on peas, a 
search for other host plants has thus 
far been in vain. The roots of many 
species of plants growing among dis¬ 
eased peas have been examined for the 
presence of the characteristic oospores 
and decayed cortex which might have 
been produced by this fungus. In 
order to test thoroughly the possible 
relation of this fungus to the species 
of Aphanomyces on sugar beets, de¬ 
scribed by Peters (IS, 14), seedlings 
of this plant and of cress (Lepidium 
sativum) have been repeatedly inocu¬ 
lated without any infection resulting. 
The conclusion that these fungi are 
distinct, at least in their physiological 
capacities, is further borne out by the 
fact that examination made of fields 
of beets grown on pea-sick soil, made 
both by the senior writer and R. E. 
Vaughan, has discovered no trace of 
the European disease. On the other 
hand, although it appears generally 
true that beets grow well on old pea 
fields and vice versa, experience is 
reported otherwise in the Salt Lake 
Valley, Utah. It may be of interest 
to note in this connection that in the 
course of making isolation from dis¬ 
eased peas from this valley with the 
4 Wittmack, L. ueber eine durch pythium veranlasste krankheit der erbsenwurzeln. 
Paper presented at 64. Versamml. zu Halle, Gesell. Deut. Naturf. u. Aerzte, Sep. 1891. Not published. 
Title in Verhandlungen 2:108. 1892.] 
