838 
Journal of Agricultural Research 
Vol. XXXI, NO. 9 
From the data presented, it is evident that Ophiobolus graminis 
is a vigorous root and tiller-base parasite, and also that infection 
is greatly influenced by soil temperature and soil moisture. Al¬ 
though the temperature and moisture optima shifted slightly in the 
several experiments, it is evident that infection and injury are 
favored by moderately low temperatures (12° to 16° C.) and by 
fairly high soil moistures (70 to 80 per cent). In these studies it 
was found that 0 . graminis causes practically no injury to the very 
young seedlings just before emergence. The germination of seeds 
planted in the inoculated soils was not consistently lower than that 
of seeds sown in the uninoculated soil; in fact it was not uncommon 
to find that the germination of seed in the control soil was actually 
lower than that of seeds sown in the inoculated soil. It was found in 
these studies that infection seldom took place until the seedlings were 
well advanced. This was found to be the case even in the experiments 
in which the greatest quantities of inoculum were used. When small 
quantities of inoculum were used the plants showed little infection 
until they were well advanced beyond the seedling stage. 
An examination of the summary curves in Figures 2, 3, and 4 shows 
that the optimum temperature for tiller-base and root injury was 12° 
C. in soil containing the medium amount of water, whereas 16° C. 
was the optimum temperature for these injuries in soils containing 
low and high soil moistures. Whether this shift represents an actual 
relationship between the joint influences of temperature and moisture 
or merely experimental variation is a question. The data given 
for the individual experiments show that the temperature optima 
shifted from time to time, regardless of the moisture content of the 
soil. This indicates that some other factors which were not so well 
controlled as soil temperature and soil moisture may have a decided 
influence on the occurrence of the disease and that the temperature 
optimum probably extends over a range of at least 4° to 6°. The 
quantity of Ophiobolus inoculum placed in the soil has a marked 
influence on the behavior of the take-all disease, as was also found in 
the studies on Actinomyces scabies (8) and Helminthosporium sativum 
(12 ). However, it is much more difficult to standardize the amount of 
inoculum of Ophiobolus graminis than is the case with the inoculum of 
the other parasites mentioned, and as the strains of 0 . graminis em¬ 
ployed in these studies differed in their virulence it was frequently found 
that the quantity of inoculum used was in excess of that whicn would 
give the sharpest indications of the temperature and moisture optima. 
DISCUSSION 
Although these studies have only opened up the general subject of 
environmental influence on the development of the take-all disease, 
it is believed that the data presented represent the general influences 
of temperature and moisture on the infection of wheat seedlings and 
young wheat plants by Ophiobolus graminis . While it is difficult or 
impossible to correlate many of the field observations which have been 
recorded on this subject, it now appears from the experimental data 
herein presented that some of the seemingly contradictory reports 
may be in accord with the facts. As pointed out earlier, the quantity 
of infectious material in the soil influences the amount and severity of 
take-all, and this relationship doubtless is the basis for many dis¬ 
crepancies in field observations. 
