Nov. 1, 1925 
Infection of Wheat Plants by Ophiobolus Graminis 839 
In these experiments of the writers it was found that all infected 
{ ►lants did not show signs of the disease on the aboveground parts, 
n many cases plants which appeared healthy before removal from 
the soil were in reality almost devoid of a root system. This condi¬ 
tion has been noted also by one of the writers (McKinney) in several 
commercial wheat fields which were affected by take-all. Although 
data presented here show that relatively high soil moistures favor 
infection and injury, it is evident that these soil moistures also were 
favorable for the development of the host. At these soil moistures 
many plants sent out new roots in an attempt to replace those which 
had become rotted. In the present studies the writers did not 
subject any particular lot of plants to various temperature and 
moisture changes in order to determine the exact influence of hot, 
dry conditions following a period of cold, moist conditions or vice 
versa. However, on a basis of our knowledge of plant responses it 
is only reasonable to assume that even though cool, moist conditions 
do favor infection and injury, a sudden reduction in water supply 
and increased transpiration would produce marked leaf yellowing 
and killing among those apparently nealthy plants which in reality 
have badly rotted culpas and roots. One of the writers (McKinney) 
has noted an increase in the amount of leaf yellowing in infested 
fields after the beginning of a hot, dry period. Under such condi¬ 
tions, it might be assumed that dry, hot weather actually favors 
infection, but it appears that such weather only brings out the 
expression on the aboveground parts of injuries below ground which 
are favored by the opposite set of conditions. It would seem that 
the nonappearance, in the fall and extremely early spring, of take-all 
on winter wheat on infested land is probably accounted for on a 
basis of cool, moist conditions which favor the growth of the wheat 
plants although already infected. However, when the warm spring 
days arrive the infected plants turn yellow and finally die. 
Although the optimum temperature for the development of the 
parasite is more limited than that for the host, it is of interest to 
note that the two optima appear to be very close together. On the 
other hand, the optimum temperature range for the disease is con¬ 
siderably lower than that for either the host or parasite. This 
relationship is exactly the reverse of that existing with the Helmin- 
thosporium disease of wheat, the latter disease being favored by 
soil temperatures which are above those favorable for the host and 
parasite. In a paper dealing with this latter disease, McKinney 
{12) suggested that the high temperature optimum for disease occur¬ 
rence might be accounted for on the basis of the weakening of the 
host at the high, unfavorable temperatures. However, it is not 
possible to explain the results with Ophiobolus graminis on such a 
basis. At 12° and 16° C. the wheat plant is thrifty and robust, yet 
it is attacked severely by this parasite. Ophiobolus graminis pro¬ 
duces severe injury at unusually low temperatures in comparison 
with the other wheat parasites such as Gibberella saubinetii , studied 
by Dickson {2 ), and Helminthosporium sativum , studied by the senior 
writer {12). It seems that the explanation of this will require the 
study of factors relating to both the host and the fungus and which 
are far more basic than the phenomena of growth rate and vigor 
as expressed by the ordinary methods of weight and measurement. 
It is believed that this also applies to the explanation of the behavior 
