121 Robee r S. Kihhy 



culture and diseased straw in the full of 1922. In the same tests, 209 

 rod-rows of winter wheat growing more than one foot distant from inocu- 

 lated rows, which received no inoculum, produced no signs or symptoms 

 of take-all. Typical symptoms of the disease were produced when the 

 roots of fifty sterile wheat seedlings were inoculated with a pure culture 

 of the organism growing in large individual test tubes. At the same 

 time, twenty uninoculated seedlings growing under the same conditions 

 showed no sjnnptoms of the disease. 



The fungus was reisolated at least twelve times from the lesion areas 

 on plants that had been inoculated with pure cultures, but it was never 

 isolated from bits of the host tissue taken adjacent to the discolored area. 

 The reisolated fungus agreed in every essential with that in the original 

 cultures. Furthermore, when used as inoculum at planting time, these 

 cultures gave typical symptoms of take-all on the growing wheat plants 

 in the field. There can be, therefore, no doubt that the fungus Ophiobolus 

 cariceti is the cause of the take-all disease. 



Life history 



The life history of this fungus exhibits no definite separation into 

 primary and secondary cycles. Infection appears to take place at any 

 time during the year when moisture and temperature conditions are 

 favorable. .It appears to be initiated usually by ascospores, but there is 

 evidence that the fungus may spread from plant to plant by mycelium 

 growing through the soil. 



Pathogenesis 



Inoculation. — The chief sources of inoculum are the lower internodes 

 and the roots of diseased plants. Upon these perithecia are formed 

 during a period of four weeks preceding harvest. The rootstocks of 

 Agropyron repens have been observed to harbor mycelium of the take-all 

 pathogene for twelve months. Wheat planted among these rootstocks 

 soon showed signs and symptoms of the disease. Thus mycelium-harbor- 

 ing grasses are another important source of inoculum. Winter wheat 

 planted in September in soil removed from infested wheat fields the 

 previous spring has, in both 1920 and 1921, become infected. In this 

 case mycelium harbored in the soil during the summer probably acted 

 as the inoculum in the fall. Concerning the source of this inoculum, 

 Spafford (1917) states that it appears to come wholly from the soil, 

 particularly from straw of affected plants left in the field. 



Field observations have indicated that the take-all fungus may be 

 harbored with the seed to a very small extent. However, repeated plant- 

 ings of seed from diseased plants in the greenhouse have given negative 

 results. In an effort to ascertain whether the pathogene is transmitted 

 with the seed, the following test was made in the fall of 1921 on land 



