Peb. is, 1925 Root Rot of Peas Caused by Aphanomyces Euteiches 
321 
tricts of Montana, Idaho, and other 
Western States, this crop is sometimes 
more profitable than others, and since 
the suitable land area is limited the 
temptation to grow peas repeatedly is 
strong. Under the pressure of these 
economic reasons which favor the in¬ 
tensive culture of peas, many questions 
regarding rotation have become of vital 
interest to growers—how long peas 
may be grown before rotation is neces¬ 
sary, how short the rotation can be 
made with safety, and whether per¬ 
chance new pea-growing districts may 
be found from which dis.ease can be 
excluded or in which it will not develop. 
Rotation as a control measure in 
combating pea diseases is generally 
discussed, not as a remedy for one of 
the many diseases of foliage and of 
roots, but as a control measure suited 
to secure relief from all of them in so 
far as they occur in the locality. 
Therefore, even though it is properly 
outside the limits of this paper to 
discuss more than the rotation that is 
needed to secure protection from the 
root rot caused by Aphanomyces 
euteiches , it will be futile to recommend 
a rotation which is suitable to pro¬ 
tect from this disease alone. In the 
following pages this matter has been 
kept in mind, and, although it will be 
shown that a suitable rotation for peas 
is a local matter which must be deter¬ 
mined with reference to the soil and 
climatic conditions existing there, it 
may be said that it appears generally 
true that any rotation which will con¬ 
trol this form of rootrot will be ade¬ 
quate to hold other diseases sufficiently 
in check, at least in so far as rotation 
can control them. 
First of all, it may be said that 
even when peas are grown intensively, 
an adequate inspection of fields can 
always or nearly always detect the 
presence of disease before it has begun 
to reduce yields, so that the crop can 
be discontinued on infested soil before 
losses have occurred. New soils have 
never been found heavily infested with 
the. fungus unless perchance there is a 
badly infested field near by. The 
disease first appears under usual con¬ 
ditions in small areas, whence it 
spreads in successive years to the rest 
of the field. Usually the disease does 
not reach serious extent until the 
second year after diseased plants can 
be readily found. Careful inspection 
of fields can be made an adequate safe¬ 
guard against loss wherever there is 
economic advantage in growing peas 
intensively, provided peas are not re¬ 
turned to land on which disease has 
appeared for many years. 
It is very difficult to predict how soon 
disease will begin to appear upon any 
given tract of land. Experience shows 
enormous variation in this regard. 
Sometimes the third successive crop is 
rendered unprofitable, while neighbor¬ 
ing fields may grow peas for 5 or 6 years 
successfully, and there are instances 
known both in Wisconsin and in irri¬ 
gated districts in which peas have been 
grown in fields nearly every year for 
10 or 12 years without a single failure. 
There are two possible explanations for 
such differences in time before the ap¬ 
pearance of disease; either the parasite 
may have been absent originally from 
the fields which grew peas without 
disease for the longer period, or some 
soil condition may have prevented its 
rapid distribution and development. 
There is some evidence which indicates 
that the first condition furnishes the 
explanation for the late appearance of 
disease in some districts where peas 
have not been grown previously. There 
is a widespread belief among growers 
that fields in a new district will grow 
peas successfully much longer than 
fields in a district where root rot is 
known. That this is not always true is 
shown by the following instance. In 
the summer of 1922 a canning factory 
in Wisconsin far north of any locality 
where peas have been grown inten¬ 
sively was growing its third crop of 
peas. One field had been planted the 
three years in succession, four had been 
planted two years, and the remaining 
fields were on new ground. When an 
examination of these fields was made, 
about 25 per cent of the plants in the 
field producing its third crop were 
found dying with root rot, a few small 
groups of infected plants were found in 
each of the fields producing the second 
successive crop; while no diseased plants 
were discovered in any of the several 
other fields that were carefully searched. 
Although it may be true that the ab¬ 
sence or scarcity of the fungus from 
some localities may explain in part the 
tardy appearance of root rot, it is cer¬ 
tainly difficult to obtain adequate evi¬ 
dence in support of such an opinion. 
On the other hand, there is. abundant 
evidence that the disease develops 
much more rapidly and severely in 
fields upon some soil types than in 
neighboring fields upon other types. 
For instance, fields on Superior red 
clay in Wisconsin appear to develop 
root rot earlier and more severely than 
on any other type of soil. A minor 
factor that may contribute to the sever¬ 
ity of the disease upon this type of soil 
may be due to the fact that, because it 
can not usually be worked in the spring 
