Aug. i, 1921 
Two Sclerotium Diseases of Rice 
CAUSAI* ORGANISM 
Small, brown, spherical sclerotia were found constantly associated 
with the decayed roots of blighted seedlings. The coarse, white mycelium 
typical of Sclerotium rolfsii was present at the base of the stems, on the 
diseased roots, and in the soil immediately surrounding the roots. 
Fragments from freshly diseased tissues, plated on sterile agar, gave pure 
cultures of 5 ". rolfsii, as did the sclerotia which were surface-sterilized 
and sown on sterile media. The organism grows vigorously and produces 
sclerotia abundantly on boiled rice. In culture it appears slightly differ¬ 
ent morphologically from the forms occurring on soybean, wheat, and 
tall oat grass (Arrhenatheruvi elatius (L.) Beauv.), but more nearly like 
the form from wheat. The difference, however, in any case is very 
slight. The sclerotia of the form on rice are more uniformly spherical, 
the mycelium is not so coarse in appearance, and there is also a physio¬ 
logical difference brought out later in comparing the ability of these 
forms to parasitize rice. There is not enough difference, however, to 
warrant their separation as distinct species. 
The rice fungus is not a vigorous parasite. It attacks germinating seed 
and young seedlings before the root system is sufficiently established to 
combat the invasion. It is very destructive to germinating seed but 
acts as a slow parasite on seedlings that have emerged, gradually destroy¬ 
ing the tissues of the roots from the surface inward (PI. 124) until the 
seedlings are dead. It kills seedlings readily when grown in tubes on 
nutrient agar (PI. 123). If there is an abundance of the organism in the 
soil the stand of plants may be greatly reduced (PI. 122, B). On soil 
inoculated by sowing sclerotia with the seed the stand was reduced as 
much as 50 per cent in some cases. Comparisons were made with the 
various strains of Sclerotium rolfsii mentioned above—namely, from rice, 
soybean, wheat, and tall oat grass. The results of these experiments, 
given in Tables I and II, show that the forms from rice and wheat are 
equally parasitic to rice, while the forms from soybean and tall oat grass 
did but little damage to rice. This does not mean necessarily that all 
cultures from the soybean and tall oat grass would be such weak para¬ 
sites to rice. They also might become more parasitic after a period of 
adaptation accomplished through contact with the rice plant. 
For experimental purposes, large galvanized cans 16 inches square 
and 15 inches deep were used. These were filled with about 10 inches 
of soil from a field where rice was never grown, obtained in such a way 
as to avoid contamination as much as possible. Each can had a drain 
pipe at the bottom for the purpose of regulating the water supply. The 
cans were arranged in series of 10, and soil was banked against them on 
the sunny side to prevent overheating and too rapid evaporation (PI. 
122, B). The soil was inoculated by sowing the fungus sclerotia with 
the rice seed. Honduras rice was used for these experiments, and 100 
seeds were sown in each can. 
