46 TECHNICAL BULLETIN 17 



In phosphoric acid-potassium hydroxide solutions, the spores 

 germinated through a wide range of hydrogen-ion concentrations. A 

 double optimum occurred, both maxima falling on the alkaline side of 

 neutrality at pH 8.2 and pH 9.2. In Czapek's solution minus the sugar, 

 the maximum germination occurred at pH 6 and pH 8. In general, 

 the spores germinate better in alkaline solutions than in acid solutions. 

 The spores will tolerate high degrees of alkalinity. 



Leaf infection increases directly with the amount of moisture 

 present. Greenhouse experiments indicate that the effects of root and 

 foot infections are more severe in extremely dry and extremely wet 

 soils than in soils containing an optimum amount of moisture for the 

 growth of the host plant. 



During one year's field experimentation, no correlation was found 

 between the fertility of the soil and the development of foot- and 

 root-rot. 



The pathogenic effect of H. sativum isolated from barley plants in 

 Minnesota was compared with that of a Helminthosporluni isolated 

 from stunted wheat in Illinois and with Fusarium culmorum isolated 

 from scabby wheat. Experiments were made to determine the ability 

 of these organisms to cause root- and foot-rot of Marquis wheat and 

 Lion barley. Under the conditions of the experiment, the Helmin- 

 thosporiums caused more injury than the Fusarium. The Minnesota 

 strain of Helminthosporium caused the greater amount of seedling 

 injury on the Lion barley, while the Illinois strain caused the greater 

 dwarfing of the mature plants on both wheat and barley. 



As a result of these studies, the wide-spread occurrence of H. 

 satwum may be explained by the fact that the fungus responds sapro- 

 phytically to such a wide range of environmental conditions. Neither 

 the effect of temperature nor acidity seems to be a limiting factor in the 

 development of the disease so far as spore germination is concerned. 

 As a parasite, the fungus causes rather limited local infections. The 

 amount of injury is determined largely by the number and size of the 

 lesions. A direct correlation exists between the amount of moisture 

 present and the number of lesions. The severity of the infection is 

 greater at rather high temperatures than at low temperatures. The 

 disease may be expected to develop most severely, therefore, at high 

 temperatures in the presence of sufficient moisture. 



Root and foot infections are more severe in certain soils than in 

 others. This is probably largely due to differences in soil moisture 

 and temperature. In general, the disease causes the greatest injury 

 under conditions unfavorable to the growth of the host. Factors, such 

 as soil fertility, which might then be expected to influence the disease, 

 apparently have little effect. 



