PYTHIUM ROOTLET ROT OF SWEET POTATOES 1 
By L. L. Harter 
Pathologist, Office of Cotton, Truck, and Forage Crop Disease Investigations , 
Bureau of Plant Industry, United States Department of Agriculture 
The genus Pythium, certain species 
of which are known to cause “damping- 
off ” of a number of plants in the seed¬ 
ling stage, has never been reported on 
the roots of sweet potatoes so far as the 
writer is aware. Sweet potato plants, 
the rootlets of which were partially 
decayed by Pythium, were first col¬ 
lected in New Jersey in 1914. Since 
then it has frequently been observed 
and collected from various types of 
hotbeds in New Jersey, Delaware, 
Maryland, Virginia, and in many of the 
States in the South and West. It is not 
unlikely that under suitable environ¬ 
mental conditions the disease may occur 
wherever sweet potatoes are grown. 
Although the Pythium rootlet rot 
of sweet potatoes occurs principally 
in the hotbed or seedbed, it has been 
found on the roots of plants from the 
time they were set in the field until 
they were dug. The amount of injury 
actually caused by the disease is diffi¬ 
cult to estimate. Undoubtedly plants 
having the ends of the smaller roots 
dead are at a disadvantage and would 
be slow in starting to grow after being 
set in the field. Furthermore, obser¬ 
vations have shown that infected plants, 
especially if the soil conditions are un¬ 
favorable, remain stunted the entire 
summer. It is probable that a consider¬ 
able amount of the loss hitherto attrib¬ 
uted to the socalled “sick soils” is 
actually due to the injury to the root 
system by Pythium throughout the 
summer. It is interesting to note in 
this connection that Rhizoctonia is 
sometimes associated with the Pythium 
in the decayed ends of the rootlets. 
Pythium rootlet rot has been found 
in hotbeds or seedbeds prepared by the 
use of soil which was almost pure sand. 
It is more prevalent in old beds or beds 
in which the soil or sand has been used 
for several years. The amount of 
infection is apparently increased by an 
abundance of moisture in the soil. If, 
on the other hand, the bed later 
becomes dry, the injury to the plants is 
increased as a result of the reduction of 
the root system. Pythium rootlet rot 
is primarily a disease of the small 
rootlets, as shown by the accompany¬ 
ing illustration (PI. 1). The infections 
take place at the tip ends of the rootlets 
and from there the fungus grows 
progressively up the root, killing it for 
a distance of from to 3 cm. or more 
from the tip. 
In the spring of 1924 twenty-one 
varieties of sweet potatoes were bedded 
in soil that had been used for a sweet- 
potato bed the two preceding years. 
The soil contained a considerable 
amount of organic matter in the form 
of stable manure and decayed roots and 
vines. When the plants were pulled 
they were carefully examined and 
Pythium rootlet rot was found on all 
the varieties. There was, however, 
considerable variation in the amount of 
infection among the different varieties, 
as shown by the following data: The 
amount of infection on different varie¬ 
ties of sweet potatoes by the Pythium 
rootlet rot showed severe infection on six 
varieties, Big Stem Jersey, Key West, 
Creola, Little Stem Jersey, Nancy Hall, 
and Porto Rico; moderate infection on 
12 varieties, Red Jersey, Georgia, 
Triumph, Yellow Belmont, Gold Skin, 
Dooley, Haiti, Dahomey, Red Brazil, 
Gen. Grant Vineless, Pierson, and 
Southern Queen; and slight infection on 
three varieties, Pumpkin, Yellow Stras- 
burg, and White Yam. This classifica¬ 
tion is merely an estimate and was 
determined by a careful examination of 
the roots of a number of plants. In 
many cases it was difficult to deter¬ 
mine to which group a certain variety 
belonged, and as a matter of fact, there 
are several varieties in the “moderately 
infected” class which are on the border 
line of either the “severely” or 
“slightly” infected group. 
1 Received for publication Aug. 30, 1924—issued January, 1925. 
Journal’of Agricultural Research, 
Washington, D. C. 
( 53 ) 
Vol. XXIX, No. 1 
July 1, 1924 
Key No. G-424 
