OZONIUM ROOT EOT 9 



occur. At the crown of most of the trees examined the strands were 

 profuse on the bark and on the surface roots arising from this point. 

 As no fruit .trees were examined prior to wilting, the point and 

 manner of entrance of the fungus could not be determined. How- 

 ever, it was noted that the outer ends of all the surface roots were 

 healthy and that the rot on these roots progresses out from the collar. 



SPREAD OF ROOT ROT 



Taubenhaus and Killough (8) frequently make the assertion that 

 " root rot spreads underground from contact of infected roots of one 

 plant with adjoining healthy ones of another." All the observa- 

 tions made by the writers show that the spread of the fungus from 

 plant to plant in the field occurs through the spil, irrespective of the 

 root contacts of the susceptible plants, although it may spread more 

 rapidly in one crop than another. Our observations show that the 

 point of attack is nearly always on the taproot within the first foot 

 of soil, whereas the lateral or side roots are usually diseased only 

 at the point of attachment with the taproot. The rotting of the root 

 system is much more rapid down into the roots than upward. As 

 a rule most root parasites tend to advance from the ends of the roots, 

 so that root rot differs in this respect. 



If the spread of Ozonium occurred through the contact of a 

 diseased root with a healthy one, then we should expect to find 

 that the rot began at the ends of the laterals and worked up to 

 the main taproot. If such were the case, wilting would be less 

 rapid and more nearly like the wilts caused by Fusarium spp. 



Taubenhaus and Killough (8) also state that if Ozonium does not 

 spread " by underground contact of the roots ... we should 

 find hundreds of plants dying all at once in a given spot." They 

 fail to consider that the active mycelial zone is always present in 

 advance of the Avilting plants, while the mycelium in the zone of 

 wilted plants serves as a food reserve for the advancing strand 

 hyphaB. Thus, through the agency of the advance mycelial strands, 

 the spread of Ozonium takes place directly through the first foot 

 of soil from plant to plant and not by contact of diseased and 

 healthy roots, as maintained by Taubenhaus and Killough (8). 



Although the tendency of the fungus is to spread in a circular 

 fashion, the shape of root -rot spots depends to a large degree on the 

 quantity and extent of the underground parts of the susceptible 

 crops in a field. 



Generally, the root-rot spots in an alfalfa field assume an almost 

 circular shape, especially under irrigation. In cotton fields the 

 spots are of an irregular shape, although occasionally they may 

 be circular. Two such spots were observed near Sacaton, Ariz., 

 during the summer of 1924. One of them is shown in Plates 3 

 and 4. 



Although a number of factors are probably involved in the forma- 

 tion of these circles, the most important factor is the quantity of 

 underground parts of the host plant present. In a cotton field, 

 Ozonium may spread faster down a row than across it, because of 

 the fact that this is a line of less resistance and the soil is not dis- 

 turbed by frequent cultivations. 

 92792—26 2 



