OZONIUM ROOT EOT H 



mycelium is beyond the wilted circle of plants. Likewise, when the 

 crowing season is over the only points where the active mycelium 

 of Ozonium is likely to be present are on the periphery ot the dead, 

 areas. Ozonium mycelium in the form of strand hyphse may. per- 

 haps, live through "the winter on live cotton and alfalfa roots, as 

 on native plants^ already described, and when conditions are again 

 favorable new centers of infection begin at these points where this 

 type of mycelium has withstood the winter. It is not absolutely 

 essential that the roots remain alive, for many observations have 

 been made of dead cotton roots covered with Ozonium mycelium 

 which were not only the source of conidial mats but also producing 

 centers of infection. 



Scofielcl (5) and Taubenhaus and Killough (8) show by diagrams 

 that root-rot spots in cotton fields frequently do not occur in the same 

 place from season to season, but reappear close by in other spots. 

 Perhaps this can be explained by the following analogy : The spread 

 of Ozonium can be compared to the advance of a grass fire. To 

 begin with, a lighted match thrown into the grass may or may not 

 start a fire. If conditions are favorable it may spread rapidly. 

 When conditions »are unfavorable it may do little damage and die 

 down. So with root rot: a number of centers of infection in June 

 may spread very rapidly if all conditions are right. On the other 

 hand, they may make moderate progress or very little, depending on 

 the conditions^ favoring or inhibiting their spread. During some 

 years Ozonium does much more damage than in others, only a small 

 percentage of plants dying one year and nearly 100 per cent the next. 

 It is especially noticeable in plat experiments that a high death rate 

 over the plat one year is often followed by a much lower death rate 

 the next season. Further, when practically all the plants are killed 

 in a plat one season, the plants begin dying the following year at 

 the outer borders of the plat. A difference between the spread of a 

 grass fire and root rot is that the fire goes out eventually, whereas 

 root rot is able to begin its activities each season and start its de- 

 struction from the points where it left off the previous season. Thus, 

 in cotton fields new centers of infection at the periphery of the old 

 spots formed the preceding year tend to advance in all directions, 

 killing all the plants in the line of advance but less completely invad- 

 ing the spot formed the previous year. 



These observations may explain why cotton root rot may not 

 recur in the same spots. King (4) has shown that in alfalfa fields 

 the new centers start about 70 centimeters from the periphery of the 

 old spot, and subsequently these new centers merge and advance 

 outward, the extent of regression depending on the number of 

 reestablished plants that are present in the old spot for the fungus 

 to attack. 



PURE-CULTURE ISOLATION OF OZONIUM OMNIVORUM 



A summary of the pure-culture isolations of Ozonium omnivonim 

 showed that it could be cultured only from the small fresh lesions 

 (depressions) on the roots. Only rarely were cultures obtained 

 from the taproots, where the mycelial weft and strands were pres- 

 ent in any quantity. No cultures were obtained from plants which 



