30 TECHNICAL BULLETIN 17 



ranged from moderate to heavy minus. The root systems of the dis- 

 eased plants were considerably less extensive, brown lesions were 

 numerous, and the roots were very easily broken. The contrast between 

 diseased and check plants was greatest in this type of soil. 



In the peat soil the average number of culms on inoculated plants 

 was 2.7, on uninoculated 2.6. Basal infection was light 'to moderate. 

 There was very little difference in general appearance of the plants 

 grown in inoculated soil and in ^uninoculated soil. The best plants in 

 both series were obtained in the peat soil. 



Under the conditions studied, the root-rot inhibited the growth of 

 Lion barley most, during the first six weeks, in the peat soil. The 

 effect of the disease was less evident in the heavy loam and the sand, 

 and least evident in the sandy loam. During the same period, the 

 growth of Marquis wheat was least inhibited in the heavy loam. The 

 effect of the disease on the growth of the plants was markedly in- 

 creased, and to practically the same extent, in the other three types of 

 soil. By the time of maturity, however, the disease had developed 

 much more severely in the sand, as evidenced by the smaller size of the 

 plants, their decreased vigor, the amount of basal browning, and the 

 breaking down of the root system. The effect of the disease was almost 

 as severe in the heavy loam. In both the sandy loam and the peat 

 there was only a very slight difference between the plants grown in 

 inoculated and uninoculated soil. 



In analyzing the factors involved in these various soils, it may be 

 pointed out that in the loam soils, in addition to the change in physical 

 texture brought about by adding increasing quantities of sand to the 

 original heavy loam, there has been a dilution of the mineral nutrients 

 of the host, a decrease in the water-holding capacity, a decrease in the 

 amount of organic matter in the soil, and an increase in the amount of 

 soil aeration. All these factors may be assumed to have an influence on 

 both the host and the pathogene. On the other hand, in the peat soil, 

 we have a high organic content, a high water-holding capacity, and an 

 optimum of mineral nutrients for the host. The abundant moisture 

 and high organic content of the peat soil should seemingly be conducive 

 to extensive saprophytic growth of Helminthosporium, thus greatly in- 

 creasing the amount of inoculum and the chance for infection of the 

 growing host. This tendency, however, seems to be counterbalanced by 

 the optimum conditions offered for the growth of the host. On the 

 other hand, the greater severity of the disease in sand and heavy loam 

 suggests a possible influence of the soil water. These results led to a 

 further study of the influence of the soil moisture and of soil fertility 

 on the development of the disease. 



