The Take-all Disease of Cereals and Grasses 



35 



In order to determine the effect of acidity on the amount of infection, 

 the following test was made in the greenhouse. Sodium hydroxide and 

 sulfuric acid were added in varying amounts to lots of soil so that eighteen 

 degrees of acidity, ranging from pH 2.3 to 8.8, were obtained. The soil 

 comprising each degree of acidity was distributed in five 5-inch pots on 

 February 17, 1922, and five seeds of No. 6 Junior wheat were planted in 



mm. diam. of 

 colony 



3.5 



4.5 



7.5 



8.5 



9.5 



5.5 6 6.5 7 



pH of media 



Figure 4. rate of growth of ophiobolus cariceti and fusarium moniliforme on agars 

 having varying degrees of acidity 



each pot. In three of the five pots a pure culture of Ophiobolus cariceti 

 was added at the time of planting. Within three months after the date 

 of planting, the plants growing in inoculated soils having a pH value from 

 5.2 to 8.6 were all dead. The plants growing in soils having pH values 

 of 3, 4, and 2.3 were still alive and showed few symptoms of take-all. 

 These plants remained alive until the test was discontinued at the end 

 of nine months. They never grew higher than a few inches, because of 

 the unfavorable soil condition. The plants in the soil having a pH of 

 8.8 remained alive for about six months, when they all succumbed to the 

 attack of the disease. This experiment seems to show that when suf- 

 ficient inoculum is present and the plants are grown under greenhouse 

 conditions, no degree of acidity at which plants will normally grow to 

 maturity will allow them to escape being killed by the disease. The 

 plants in the uninoculated pots produced the most heads and grew 7 the 

 tallest at pH values between 7.8 and 8.4, and no heads were produced by 

 plants growing in soil having a pH value as low as 3.4. 



