= 
FOOT-ROT DISEASES OF WHEAT IN AMERICA 13 
take-all. In nearly all cases conclusions have been based on rather 
casual observations, which in many instances have been made with 
regard to foot-rots in general rather than a specific foot-rot. Obvi- 
ously, many uncontrollable factors enter into field observations 
which make it very difficult or impossible to interpret properly the 
exact influence of seasonal and climatic factors, and it becomes nec- 
essary to carry out experiments in which the conditions can be con- 
trolled to a greater degree than is possible in the field. Such experi- 
ments dealing with the influence of temperature and soil moisture 
have been carried out by R. J. Davis and the writer, and the results 
will be published in another paper.® In general, these results show 
that fairly low soil temperatures (12° to 16° C.) are most favorable 
for the infection and injury of seedlings and young wheat plants. 
They also indicate that take-all is more severe when the soil is too 
wet or too dry for the best development of the wheat plant. How- 
ok : as 
8 fea 16 2o ede 2& 32 36 
SOL TLMFERAIT CCE DLCRELE CLIV7/ GRADE 
Fic. 2.—Curves representing the influence of soil temperature on the infection 
and injury of seedlings and young plants of winter wheat when grown in ex- 
perimental soils infested with pure cultures of Ophiobolus graminis, Helimnin- 
thosporium sativum (45), and Gibberella saubinetti (18). As these curves 
are based on experiments conducted under somewhat different conditions and 
as the data were not all obtained in exactly the same manner the curves do 
not express with complete exactness the relative infective powers of the three 
parasites 
ever, wet soil seems to be more favorable for the disease than dry 
soil. The general summary of the results obtained in temperature 
studies is represented by the curve shown in Figure 2. Other phases 
of the take-all disease must be studied before it will be possible to 
analyze thoroughly the temperature and soil-moisture data now in 
hand, and these latter factors also need further study. As pointed 
out earlier in this bulletin, Ophiobolus graminis behaves in a very 
perplexing way at times in regard to both fruiting and infection; 
and, aside from apparent differences in strains of the fungus, it 
seems probable that environmental factors may be playing an im- 
portant role in bringing about some of these irregularities. 
The exact influences of soil type and fertility on the development 
of take-all are not definitely known, but it seems evident from the 
» 
® McKinney, H. H., and Davis, R. J. The Influence of Soil Temperature and Moisture 
on Infection of Wheat Seedlings by Ophiobolus graminis. (Unpublished manuscript.) 
