18 



BULLETIN 1417, U. S. DEPARTMENT OF AGRICULTURE 



When the number of widely spaced plants which died from root 

 rot was compared with those closely planted it was found that for 

 the area of the ground which was occupied by each the percentage 

 of plants killed was the same. In other words, although many more 

 plants were killed by root rot in the closely planted rows, the advance- 

 of Ozonium in the soil was just as rapid in the widely spaced row& 

 of cotton. 



The decided fluctuations in the death rate of the plants can be • 

 explained by the fact that after every irrigation a smaller addi- 

 tional number of plants showed symptoms of the disease. However, 

 the effect was only of a temporary nature, as can be seen by the 

 larger mortality a few days later, so that, although the application 

 of water retarded the diseased plants from wilting for a short period, 

 the application of irrigation water really increased the death rate of 

 the plants. No doubt a period of wet, cloudy weather would have 

 the same effect. 



The data obtained on alternate days on the number of dead plants 

 and their location were plotted to determine whether the spread of 

 Ozonium in the soil could be determined from day to day, but the 

 behavior of the disease was so erratic that no definite correlation 

 could be made. Although definite centers of infection were present 

 when the count was begun, these centers did not spread in a regular 

 fashion. Some centers, after a number of plants were killed, ceased 

 to grow larger; others ceased for a time and later enlarged again. 

 Occasionally one plant after another down the rows for a consider- 

 able distance was progressively killed off. At other times the fungus 

 would skip for several feet, kill a few plants, and then spread to an 

 adjacent row. Then, as time went on, the plants not killed at first 

 would begin to die off rapidly. At the end of the season, however,, 

 the irregular spots united so that the last plants killed by root rot 

 were more or less on the periphery of the circular areas. 



A number of other borders were charted in the same manner as 

 the one just mentioned. Table 2 is representative of what was 

 found. In these borders root rot appeared somewhat later than in 

 border C 2-24, but an idea of the progress of root rot is given. This 

 series of borders includes an experiment that is being conducted on 

 the influence of manures in the inhibition of root rot. 



Table 2. — Bimonthly count of the number of cotton plants killed by Ozonium 

 root rot in a series of borders manured and untreated at Sacaton, Ariz., 

 during the summer of 1924 





[Each border had approximately 2,600 plants] 









Borders 



Treatment 



Number of plants dead 



July 31 



Aug. 15 



Sept. 2 



Sept. 17 



Oct. 2 



Oct. 31 



Total 



C 2-16.... 



Untreated 



21 



4 

 22 



4 

 18 



3 

 38 



5 



42 

 22 

 102 

 4 

 13 

 17 

 76 

 17 



116 

 31 



129 

 28 



106 

 28 



113 

 4 



67 

 97 

 96 

 51 



33 



20 



27 



1 



17 

 25 

 47 

 13 



447 



C 2-17.... 



Manure 2 years 



133 



C 2-18.... 



Untreated 



136 141 



541 



C 2-19.... 



Manure, 4 vears 



14 

 58 

 101 

 186 

 54 



15 

 93 

 114 

 163 



78 



42 



C 2-20... . 



Untreated ... 



266 



C 2-21.... 



Manure, 2 vears . 



357 



C 2-22.... 



Untreated 



606 



C 2-23— 





218 



















