OZONIUM BOOT ROT 19 



SUMMARY OF PART I 



Typical strands of Ozonium omnivoruru have been found on the 

 roots of several plants native to the Southwest, both on roots of 

 sprouts in recently cleared land planted to cotton under root-rot 

 ^pots and on roots of these plants growing in their native habitat. 



Ozonium omnivorum appears to be indigenous in the soils of cer- 

 tain regions of the southwest section of the United States and yearly 

 causes large economic losses, principally to cotton, alfalfa, and fruit 

 trees in Texas and in certain irrigated valleys of Xew Mexico, 

 Arizona, and southeastern California. 



A study of exhumed root systems of susceptible plants at various 

 stages of infection gave no evidence that the mycelium of Ozonium 

 spread from plant to plant by underground contact of diseased 

 with healthy roots, but indicated that the fungus usually progresses 

 radially from a center through the soil, irrespective of the root 

 systems, and attacks the plants in the line of the advancing 

 mycelium. The plants are generally attacked on the taproot within 

 the first foot of soil, and the laterals usually rot from the point of 

 attachment with the taproot. 



The actual agent in the penetration of the roots is the strand 

 hyphse, which are found approximately 38 to 75 centimeters in 

 advance of the wilting zone of plants. The hyphse composing the 

 mycelial weft found on the wilting plants appear to complete the 

 breaking down of the roots and act as a reserve food supply for the 

 advancing strand hyphse. This mycelial weft disintegrates and 

 disappears as soon as the roots are rotted. 



At the close of the growing season the only active mycelium is 

 slightly in advance of the periphery of the root-rot spots, and it is 

 at this point that new centers of infection usually occur the follow- 

 ing year. From the evidence at hand it does not appear necessary 

 that the roots on which these strand hyphse are present remain alive 

 during the winter. From these new centers of infection the 

 organism can again spread out radially. 



To judge from the scattered distribution of root rot in fields in 

 the irrigated districts of Arizona and southeastern California and 

 from the difficulty of infecting plants with pure cultures, the 

 organism is not commonly spread by any aboveground agency, in- 

 cluding the spores of the conidial stage. 



The difficulty encountered in attempting to infect susceptible 

 plants with pure cultures of Ozonium may be due to the polymor- 

 phic nature of the fungus, which tends to produce in culture the 

 large-celled type of mycelium rather than the strand hyphse found 

 in the advance zone under field conditions. Of the inoculations 

 made at Sacaton, Ariz., during the summer of 1924, the only suc- 

 cessful one was by direct contact. 



Because of the extremely erratic behavior of the fungus in the 

 field, no correlation could be made with certainty with any environ- 

 mental factor or factors. The only factor which seemed to have any 

 influence on the spread and severity of root rot was the amount of 

 precipitation occurring during July, August, and September, and 

 this appeared to be of only slight influence in the irrigated region. 

 Until the relation of the environmental factors can be determined 

 under controlled conditions, the data already collected under field 

 conditions can not be interpreted. 



