30 Robert S. Kirby 



Saprogenesis 



With the formation of the plate mycelium the plant usually succumbs, 

 and thereafter the fungus leads a saprophytic existence. Perithecia are 

 now formed on the inner side of the outer leaf sheath and in mycelial 

 mats covering the roots. These leaf sheaths and roots are nearly always 

 dead before perithecia are developed, and often the entire plant has died 

 before the perithecia reach their full size. Fully developed perithecia have 

 been formed on wheat in the greenhouse within eight weeks after planting 

 of the seed, but in the field they have not been found before June 4. The 

 ascopores which are discharged early in the summer are probably Avashed 

 to the ground, where they germinate and produce mycelium which may 

 live saprophytically in the soil on organic matter until the fall, when 

 wheat is planted. The mycelium in the soil about the cereal or grass 

 plants may also live saprophytically until fall. 



The ascospores in the perithecia are only partly discharged at any one 

 time, as already pointed out. Hori (1901) states that the ascospores 

 may germinate and live saprophytically on the decayed culm until autumn, 

 when the mycelium attacks the young cereal plants. 



While the life cycle of this pathogene is usually initiated by ascospores, 

 mycelium spreading through the soil also may infect healthy plants. 

 Hori (1901), McAlpine (1904), Waters (1920 a), and others have noted 

 that the infection occurs and seems to spread in circular areas. Spafford 

 (1917) states that mycelium spreads from root to root by dark brown 

 mycelial strands. During the present investigations it was noted that 

 in infested wheat fields the margin of the spots killed by take-all was 

 composed of a border, ranging in width from 6 inches to several feet, 

 wherein the infected plants were not stunted and seemingly had' been 

 attacked late in their development. The 1921-22 plot gave ample oppor- 

 tunity for observing the spread of the pathogene through the soil, since 

 inoculum was placed only at known points in the field. In these plots 

 it was clearly shown that during the period of growth of winter wheat the 

 mycelium could not have spread through the soil for a distance greater 

 than one foot. Only a fraction of one per cent of the plants were killed 

 in their later stages of growth at this distance from inoculum, consisting 

 of pure cultures or diseased wheat straw, placed in the ground at planting 

 time. Thus it seems that the fungus is able to spread through the soil 

 for very short distances only and initiate new life cycles. 



ecology 



Moisture and temperature 



There seems to be a direct correlation between such climatic factors as 

 moisture and temperature, and the amount and severity of take-all. It 

 is said (Anonymous, 1922 b) that climatic factors have a tremendous 



