20 BULLETIN 1417, U. S. DEPARTMENT OF AGRICULTURE 



Part II— THE PATHOLOGICAL ANATOMY OF OZONIUM 



ROOT ROT 



By George L. Peltier and Rayburx TV. Samson 



INTRODUCTION 



In order to determine the type of hypha? involved in the pene- 

 tration of the roots of susceptible plants, the modes of entrance, and 

 the subsequent development of the disease, material representing 

 various stages of root rot on alfalfa and cotton plants was obtained 

 at the L T nited States Field Station at Sacaton, Ariz. A few roots 

 of the common mesquite (Prosopis glandulosa Torr.) and the gran- 

 jeno (Momisia, pallida (Torr.) Planch.) with Ozonium strands 

 present were also collected. 



This study does not presume to be complete in many respects, 

 for the quantity of material studied was not sufficient nor was it 

 representative of all conditions found in the field. Further, time 

 did not permit an elaborate examination of all the material at hand, 

 but enough was studied to cover the whole field in a preliminary 

 way. 



To avoid repetition, the writers assume that the readers are famil- 

 iar with the macroscopic symptoms and development of the root rot 

 and the various mycelial forms of this fungus, as heretofore de- 

 scribed. 



MATERIAL AND METHODS OF STUDY 



The material representing the cotton and alfalfa roots was obtained 

 at the time root excavations were made at Sacaton, Ariz., during 

 the summer of 1924 (p. 5). 



One criticism of the material used in this study can be offered, 

 especially where the later stages of the disease are involved — the 

 presence of secondary invaders in the plant tissues. It is realized 

 that material from pure-culture inoculations would avoid any mis- 

 interpretation. As has been pointed out. very few successful in- 

 oculations have been made with Ozonium on susceptible plants, so 

 that material gathered in the field had to be employed. Pure cul- 

 tures of Ozonium, however, were obtained from the material which 

 was collected for this study. 



The roots of the plants representing the various stages of root rot 

 were placed in chromo-acetic acid, Fleming's solution (medium and 

 weak), and Gilson's fixatives and finally embedded in paraffin in 

 the usual manner. 



Two differential stains were employed, those of Durancl {2) and 

 Vaughan (9). The latter, a modification of Pianese's stain, pro- 

 duced the best results and was used for the most part in coloring the 

 sections. 



THE STRAND HYPHJE 



Contrary to general opinion, the strand hyphse are composed of a 

 large central hypha (pi. T, A), which is generally filled with a mass 

 of granular material which colors a deep red with Pianese's stain. 

 Around this central cylinder are small, rather irregular-shaped, 

 thick-walled hypha?. From three to six bands of these hypha? are 



i 



