The Take-all Disease of Cereals and Grasses 29 



a per ot ration tube entered and passed down the root hair into the root; 

 l.iii in the other four cases of root-hair penetration observed, appressoria 

 were not formed. 



Infection. — In wheal plants growing on agar plates the first evidence 

 of infection appeared seven days after the fungus had come in contact with 

 the roots, and consisted of small, brown, necrotic spots. These spots 

 enlarged so rapidly that within a period of from three to five days the 

 entire root for a distance of one-quarter to one-half inch was dead and 

 brown, all of the tissues being permeated with mycelium. Robinson 

 (1907) states that in Australia young seedlings planted in diseased soil 

 may be affected almost as soon as they appear above ground. McKinney 

 (1922) reports that plants in the greenhouse show severe seedling blight 

 and often die within fourteen days after sowing. 



Under New York field conditions, symptoms of the disease have not 

 been observed on the culms before May 20, but lesions have been found 

 on the roots as early as March 19. The fungus has been isolated from 

 such lesions. 



The death of the roots reduces the supply of moisture and mineral 

 nutrients to the plants. To overcome this, many fine rootlets are 

 developed above the lesion. The plant may also send out new roots at 

 its crown. These efforts of the plant to recover seem to be of little value 

 when it is attacked while young, since the mycelium grows upward inside 

 and outside of the roots to the crown, attacking and killing all roots 

 with which it comes in contact. The mycelium enters the leaf sheaths 

 ;tt the crown, permeating the tissues. 



The septate mycelium is of two general types, colorless and brown. 

 The very young mycelium, and nearly all the mycelium within the culm 

 and the roots, is colorless. This consists of rhizomorphic strands, the 

 individual cells of which are cylindric, thin-walled, hyaline, and 10-30- 

 jiuttulate. These cells are from 15 to 50 /z long and from 2 to 6 jj. wide. 

 The cells of the branches are very variable, ranging from knob-like cells 

 with a diameter of 5 /j,, to cells resembling the main rhizomorphs. The 

 mycelium is abundant in the xylem tubes and grows intracellularly in 

 all the other tissues. 



By the time the mycelium has entered the leaf sheaths, the roots are 

 usually so severely affected that the plant becomes yellow. Soon after 

 this symptom appears, a mass of mycelium is formed between the inner 

 leaf sheath and the culm. This mass is composed of hyphae which 

 intermingle to form a plate, which usually adheres to the culm when the 

 leaf sheaths are stripped away. The plate mycelium thus formed con- 

 sists of flat, ribbon-like strands resembling somewhat compressed rhizo- 

 morphs, which run definitely parallel to one another. The individual 

 cells of these strands are cylindric, and are from 20 to 260 yu long and 

 from 3 to <; 5 /jl wide. Their contents are sparingly guttulate and hyaline. 



