FOOT-ROT DISEASES OF WHEAT IN AMERICA 35 
extent in connection with infection studies, but experience indicates 
that it is of questionable value in establishing the pathogenicity of 
organisms as they occur in the field. Under test-tube conditions and 
the conditions obtained in a roll of cloth, such as a rag-doll seed 
tester, the wheat seedling is at a decided disadvantage, whereas the 
fungus has an ideal opportunity to attack the plant. Even in the 
case of carefully planned soil-inoculation experiments it is extremely 
difficult at times to obtain results which can be interpreted and com- 
pared satisfactorily with conditions occurring in nature. The writer 
has demonstrated repeatedly that the amount of inoculum introduced 
into soil greatly influences the number of plants which become in- 
fected and the severity of the infection on the plants attacked. Up 
to a certain point large amounts of inoculum have always caused an 
increased amount of infection, which might easily be confused with 
the factor of virulence. This relationship was observed repeatedly 
in the work with Actinomyces scabies (36), Helminthosporium sati- 
vum (45), and Ophiobolus graminis. When it is possible the writer 
introduces only the spores and mycelium of the fungus to the ex- 
perimental soil, and when nonsporulating forms are used they are, 
when possible, increased on inert materials, such as straw and leaf 
mold. In some cases, it has been necessary to use cooked oat and 
barley grains for culturing sterile forms, but in these cases it is 
always the aim to avoid the addition of too much of such culture 
material to the soil. 
In the study of the factors which influence parasitism among soil 
fungi, we are continually reminded that the distinction between the 
pathogenic and saprophytic types is very largely relative. Certain 
parasites are more aggressive than others, some are capable of 
attacking the host throughout a wider range of environmental con- 
ditions than others, and if the right conditions are obtained some 
of the so-called saprophytes become pathogenic. In view of these 
relationships it becomes strikingly evident that the mere statement 
that an organism is pathogenic or nonpathogenic means very little 
to the student of plant diseases. Information regarding the environ- 
mental conditions, the methods used, and the general conditions 
under which experimental or field observations are made is highly 
iunportant and should be recorded. 
The control of foot-rot diseases is the ultimate thing sought in the 
studies being carried on by the Government and State agencies. 
Although certain information is available on this phase, it is strik- 
ingly evident from the material presented in this bulletin that much 
more exact knowledge should be obtained. As these diseases are 
caused by soil-inhabiting parasites, it is evident that seed treatments 
alone can not solve the control problems. The soil is the important 
source of infection, and it is evident that this aspect of the problem 
must be given considerable attention. In order to combat this source 
of infection several methods are open for investigation, some of 
which are more practical than others. 
The host may be aided in escaping infection by several methods, 
but the practicability of these depends very largely on the circum- 
stances surrounding each farmer’s problem. ‘The soil-temperature 
studies which have been carried on with certain soil-inhabiting 
parasites have shown that the date of seeding may be regulated so 
as to assist the wheat plant in escaping infection_er serious injury 
