PATHOGENICITY OF H. SATIVUM 31 



INFLUENCE OF SOIL MOISTURE 



Preliminary series of experiments were carried out in the green- 

 house in the following manner in order to determine the effect of soil 

 moisture on the development of H . sativum on Lion barley. Light loam 

 soil was sifted through a 5 millimeter screen, packed into jars, and 

 sterilized. The sterilized soil was mixed with a culture of H. sativum 

 grown on sterilized oats seed. Five degrees of soil moisture were main- 

 tained more or less uniformly by adding definite amounts of water each 

 day. In the fifth series the soil was kept saturated by standing the porous 

 pots in jars of water. In the other four series the soil was in glazed jars 

 and the soil moisture was regulated by adding different amounts of 

 water. Each moisture series was carried out in triplicate, in both inocu- 

 lated and uninoculated soil. The seed was sterilized with silver nitrate 

 before planting. 



Comparative results on the infection above ground at the end of 

 three weeks and below ground at the end of four weeks are summarized 

 in Table X. In this table the infection is designated by fractions; the 

 denominator represents the number of plants in one pot, the numerator 

 the number that were infected. On examining the data, it is seen that, 

 as far as the above ground parts of the plants are concerned, the per- 

 centage of infection, as well as the severity, is increased as the amount 

 of soil moisture is increased. Comparatively few infections occurred 

 on the check plants. 



The relation of soil moisture to root infection is a little more difficult 

 to see, as here the development of the roots in inoculated and uninocu- 

 lated soils with the same moisture content must .be compared, and then 

 these differences compared for the various series. The roots were most 

 severely rotted in the saturated inoculated soil, and the difference in 

 the extent of the root systems of diseased and check plants was greatest 

 here. The next greatest difference was in the first series, with a soil 

 moisture content averaging 9 per cent, while the least difference was 

 found in the third and fourth series. In these two series the plants 

 grew best of all, in both the inoculated and uninoculated soils. Injury 

 to the roots is brought about by rather limited local lesions which kill 

 the root tips or cut off portions of the roots when the lesions occur back 

 from the tips. Very often the roots are rotted off near the seed. 



These results would indicate that plants suffer most from root infec- 

 tion by H. sativum in soils containing both maximum and minimum 

 extremes of moisture. When conditions are more nearly favorable for 

 the optimum growth of the plants, the effect of the disease can be over- 

 come, and root systems are developed in inoculated soil almost equal 

 in extent to those in clean soil. 



