820 
Journal of Agricultural Research 
Vol. XXXI, No. 9 
That all of the inoculated plants of the four experiments became 
diseased, regardless of their age at the time of inoculation, is very 
significant. In two of the experiments part of the plants were in 
the flowering stage at the time of their inoculation, but these plants 
showed distinct lesions produced by the fungus, and by October 
Ophiobolus perithecia had formed on them. The same was true with 
the plants inoculated earlier, except that these were attacked more 
severely. 
From these results it is evident that the varieties of wheat studied 
are susceptible to attack by the fungus during their entire growing 
E eriod. The disease is most severe during the seedling stage of the 
ost, and only a small portion of the plants attacked at this stage 
ever recovered. Whether, under field conditions, they would recover 
sufficiently to produce a crop is not known, but reports from field 
observations make this seem improbable (18/9). Plants infected 
during the late stages of development, or plants recovering from 
earlier infections, produce symptoms known as “ white heads. 77 (I) 
Although the inoculation method used in these experiments caused 
considerable root mutilation, it is believed that this does not seriously 
affect the results obtained. This is supported by the fact that the 
most severe cases of disease always occurred in the plants gown in 
the soil which was inoculated at the time of sowing the seed. Further¬ 
more, results discussed later show that the fungus hyphae readily 
penetrate the unbroken epidermis of the roots. 
PARTS OF WHEAT PLANT SUSCEPTIBLE 
Preliminary studies to determine what parts of the wheat plant are 
susceptible to attacks of Ophiobolus graminis were carried on in large 
glass tubes containing cultures of the fungus. When the surface of 
the medium was thoroughly covered with mycelium, a 4-inch layer of 
cooled Crone 7 s plant agar was poured over the colony. Wheat seed¬ 
lings, previously germinated under sterile conditions, were then placed 
in the tubes, and the cultures were placed in a light place in the labora¬ 
tory. Plants for experimental controls were grown in the same 
manner, except that no fungus was in the tubes. 
The roots of the seedlings were the first parts to be attacked. 
In from two to three days after planting, dark lesions began to show 
on the roots, and soon the entire roots were blackened. The culms 
did not show any signs of attack until four to five days after the roots 
had shown symptoms of invasion. The control plants remained 
entirely healthy. 
In studying the histology of diseased tissues, as reported later, the 
fungus was found to enter the roots at any point when the plants 
were grown in the soil. Mycelial threads could be seen entering 
through the epidermis at various points along young roots, except 
near the distal ends where they were elongating rapidly. Single 
lesions also were noted on roots that had healthy tissues for several 
centimeters above and below the diseased portion. These ob¬ 
servations, together with the fact that the roots could be infected 
while the rest of the plant was free from the fungus, definitely prove 
that the roots are susceptible to attack by Ophiobolus graminis. 
