10 BULLETIN 1417, U. S. DEPARTMENT OF AGRICULTURE 



The rapidity with which Ozoniuni can spread from a given center 

 of infection depends on a number of known and unknown factors. 

 Under irrigated conditions in Arizona, root rot advances in an 

 alfalfa field at the average rate of 1 to 2 meters a month over a 

 period of about seven months. In cotton fields the rate of spread 

 under both dry-land and irrigated conditions is very erratic, and 

 no exact measurements can be given. Under optimum conditions 

 root rot may spread down a row or rows of cotton plants much 

 faster than in an alfalfa field. 



The fact that Ozonium is indigenous and appears in some fields 

 -and not in others when they are first cleared of native vegetation 

 lias already been pointed out and can be further illustrated by the 

 following example : The United States Yuma Field Station at 

 Bard, Calif., is and has been free from root rot since its establish- 

 ment, in spite of the fact that root rot is present and causes severe 

 losses on other farms in the vicinity; in fact, root rot is prevalent 

 on a farm just across the road from the station. If Ozonium can be 

 spread readily by any aboveground agencies, including the spores 

 of the conidial stage, why has root rot not appeared at the Bard 

 station during the past 15 years? 



No instances are known where root rot has appeared in a field after 

 it has produced healthy crops of a susceptible plant for a number 

 of years, even though root rot may be present in neighboring fields, 

 likewise, few instances are known where root rot once well estab- 

 lished in a field has ever disappeared, even after a long rotation with 

 nonsusceptible crops. In other words, if root rot does not appear in 

 a field planted to a susceptible crop within one or two years after 

 it is cleared of native vegetation, it is indicated that Ozonium is 

 not present in the soil and that no losses from the disease will 

 result, whereas if root rot does appear, no practicable methods of 

 tillage or rotation are known which can be relied upon to control it. 

 It has been shown, however, that small isolated areas may be freed 

 from root rot by thorough disinfection with a suitable fungicide. 



In many ways the spread of Ozonium through the soil is similar 

 to that of many Basidiomycetes which produce fairy rings. It is a 

 well-known fact that the production of the conidial stage takes 

 place just ahead of the wilting zone of plants in cotton and alfalfa 

 spots. While the wilting of the plants occurs in this zone, the 

 advance mycelium is well out in front of the zone (38 to 75 centi- 

 meters), attacking the root systems of the plants which still appear 

 healthy above ground. It is suggested that the front line of advanc- 

 ing mycelium uses the reserve food material of the mycelium found 

 on the dying plants, and after the reserve food is used up this 

 mycelium disintegrates. King (4) showed that Ozonium is not 

 active in the dead zone of an alfalfa spot by planting such an area 

 to cowpeas, a very susceptible crop, which remained healthy. 



By examining Plates 3 and 4 it will be noted that most of the cot- 

 ton plants in the interior of the wilting zone which surrounds the 

 root-rot circle are healthy. This root-rot spot is clearly indicative 

 of the way the fungus mycelium moves outward from a given center. 

 The aboveground appearance here shown is rather unusual in cot- 

 ton, but common in alfalfa fields. 



Thus, during the growing season, as root rot spreads, the old 

 mycelium in the dead zone of plants disintegrates, while the active 



