139 



of the affected leaf, and also rotting without spotting, resulted. On the 

 leaves in the humid air of the rag doll occasional spontaneous infections 

 were noticed. In such cases the infection rapidly spread, involving nearly 

 all of the leaf, which first turned pale, then very slightly brown. Aerial 

 mycelium and conidia were profuse over the diseased portion. 



SUMMARY CONCERNING ETIOLOGY OF FOOT-ROT 



The evidence is conclusive that Helminthosporium is the cause of 

 the basal rot of the wheat-stems. It is the only parasite constantly 

 present, and has been repeatedly, and by many methods, proved capable of 

 causing such rot. This conclusion is in accord with the findings of Beck- 

 with (14), who as early as 1911 showed that Helminthosporium is a very 

 common parasite within the tissue of wheat-plants. Bakke (6) in 1912 

 reported that when conidia of H. teres were placed on barley seeds, 

 "At the end of two weeks' time there were not over seven seedlings to 

 the row [originally there were twenty-five]. The roots were not in any 

 sense indicative of a healthy state of growth." Oats and fescue-grass 

 were not susceptible. A seedling blight of wheat observed since 1910 has 

 been described by Stakman (113) in Minnesota, where in 1918-19 it became 

 seriously injurious. The symptoms include dwarfing, foot-rot, and root- 

 rot. The disease appears to be closely like, if not quite identical w r ith, 

 the one which is the subject of this paper. She proves conclusively that 

 the cause is a Helminthosporium. A foot-rot of wheat due to a Hel- 

 minthosporium having quite different morphological characters is also 

 known in Sudan (see No. 46, page 184). 



Certain of Ravn's experiments (91) conducted by inoculating seeds 

 on wet filter-paper in a Petri dish, gave conditions much like those in 

 the rag dolls. He makes no mention, however, of infection of the sheath 

 nor of the occurrence of "rotting of the basal region. 



II. Evidence and Discussion of the Occurrence of 

 Saltation within the Genus Helminthosporium 



INTRODUCTORY 



Early in my study of this Helminthosporium of foot-rot of wheat 

 (herein designated as H. No. 1) it was noted that occasionally certain sectors 

 of a colony growing on an agar plate differed more or less from the rest of 

 the colony (PI. XXII, 5; PI. XXIII,!). This phenomenon is of rather 

 common occurrence in work involving Petri-dish cultures of either fungi or 

 bacteria, and little significance was at first attached to it; but later, when 



