OZONIUM ROOT EOT 6 



Ozonium strands on roots of native plants growing about the station 

 at San Antonio. Therefore, there can be no question that Ozonium 

 is present on the roots of native plants of this southwest region. 

 Many more observations must be made to determine whether or not 

 the fact that the mesquites which cover the entire range where root 

 rot has been reported can be correlated with the presence of 

 Ozonium strands found on their roots. 



SYMPTOMS OF THE DISEASE 



The first symptom of root rot on cotton is a bronzing or yellowing 

 of the leaves about 24 hours before wilting occurs. This symptom 

 was observed occasionally in Texas. The wilting of the diseased 

 plant is very rapid and can be observed at any hour of the day. It 

 is more pronounced, however, during the hot midday. Sometimes a 

 wilted plant may temporarily recover during a cool night, in wet 

 weather, or during irrigation, but as a rule when once a plant wilts 

 there is no recovery. The wilting of the top occurs first, followed 

 quickly by the wilting of all the leaves from the top to the bottom of 

 the plant. Within 24 hours after wilting the leaves dry up and turn 

 brown but remain attached to the branches. A plant attacked by 

 root rot can be diagnosed from the aboveground symptoms with 

 some assurance by the peculiar manner in which the leaves hang 

 down over the branches and stem and the way they persist on the 

 plant for some time (pi. 1, fig. 1). When a plant, showing the first 

 signs of wilting, is examined, the root system is found to be fairly 

 well rotted at the point of attack. The amount of mycelium found 

 on the wilted plants depends more or less on the percentage of soil 

 moisture and the extent of breaking down of the root tissues. Sev- 

 eral days after the plants wilt the bark tissues disintegrate, and the 

 mycelium disappears from the surface of the roots. 



The wilting of alfalfa plants attacked by root rot is so sudden that 

 no aboveground symptoms have been observed prior to wilting. 

 The wilting of alfalfa is similar to that described for cotton, except 

 that as the leaves dry on the branches they drop off more readily. 

 Likewise the root system is very badly rotted before wilting occurs. 

 The taproot of alfalfa, being softer and more succulent than the 

 cotton root, favors the more rapid development of the mycelium and 

 consequently the more rapid destruction of the roots. 



Large fruit trees may wilt completely in a day, and there appears 

 to be no way of determining by any aboveground symptoms whether 

 or not a tree is attacked by root rot until it is wilted. It happens 

 occasionally, however, that one or more branches on one side of a 

 tree will die, and several months may elapse before the other 

 branches die. In such instances it is usual to find only part of the 

 root system diseased. By the time complete wilting occurs the 

 roots of the trees are badly diseased. 



TIME OF FIRST APPEARANCE 



In cotton fields the date of the first appearance of root rot depends 

 on (1) the age of the plants, (2) the prevailing weather conditions, 

 and (3) the number of centers of active mycelium in the soil. An 

 average over a period of years shows that it begins its activity dur- 

 ing the first week of June, becoming more serious as the season 



