16 



DEPARTMENT OF AGRICULTURE 



another, as it did in one plat studied. In other words, the behavior 

 of Ozonium from one season to another is so erratic that no series 

 of rotation or tillage experiments over a period of years can be 

 undertaken with the idea of eliminating root rot from the soil. 

 When a susceptible crop is grown during a season favorable for 

 root rot following a nonsusceptible crop, the loss may be as high as 

 on those plats grown continuously to a susceptible crop. 



Further, the loss from root rot in duplicate plats may vary so 

 widely that averages can not be justified. For example, two series 

 of plats with a four-year rotation including cotton showed a differ- 

 ence over an eight-year period of 1.5 and 14.3 per cent, respectively, 

 in loss from root rot. In other words, the percentage of root rot 

 present from year to year may vary very greatly from plat to plat, 

 and therefore comparative experiments can not be run satisfactorily. 



There seems to be no correlation between the time of first appear- 

 ance of root rot early in the season and the number of plants killed 

 in the plats. Likewise, temperature does not seem to have any influ- 

 ence on the time of appearance or severity of root rot during the 

 growing season. 



In the vicinity of San Antonio, Tex., the most important factor 

 involved in the seasonal loss from root rot appears to be precipita- 

 tion. (Table 1.) A season ideal for root-rot activity would be one 

 in which generous showers occurred frequently during the months 

 from June to September. 



Table 1. — Precipitation at the field station near San Antonio. Tex., during a 

 -five-year period for the three critical months of root-rot activity 



Month 



Rainfall (inches) 



1919 



1920 



1921 



1922 



1923 



July 



7.61 

 2.22 



7.06 



1.66 



2.39 



.03 



1.40 

 .02 



6.77 



0.12 

 .02 

 1.07 



2.16 





2.50 





3.02 









Total rainfall.. . ..... ...... 



16.8 

 26.05 



4. 08 8. 19 



1.21 

 8.1 



7.68 



Average percentage of root rot for all plats 



13.2 



35.2 



41.9 



A digest of the data collected in past years at the United States 

 Field Station at Sacaton, Ariz., was also made. King (3 and £) has 

 issued two reports dealing with certain phases of this work. The 

 most striking point observed at this station was the scattered dis- 

 tribution of root rot in the experimental plats. In many of these 

 plats no root rot has ever been observed since the establishment of 

 the station in 1907, whereas at the station at San Antonio, Tex* 

 some loss from root rot has been recorded for every plat. It has 

 been noted at the Sacaton station that the disease spreads from the 

 point where it was first observed to adjacent plats, but the advance 

 has been relatively slow. The spotted distribution of root rot 

 throughout the irrigated valleys in Arizona is very characteristic. 

 The same condition exists in southeastern California, as has already 

 been pointed out. Owing to the fact that all crops are grown under 

 irrigation, the percentage of loss from root rot from year to year is 

 much more uniform than in Texas. 



