Dec. 15. I9I9 Cotton Rootrot Spots 307 



The distribution of the diseased areas on this same plot in 191 8 is 

 shown in figure 3. The diagram for that year was made on October 28, 

 1918, when a count of the living and dead plants showed that 42.0 per 

 cent of the total number were dead, apparently from the effect of rootrot. 



The west half of this plot again showed some indication of a progres- 

 sive spread of the disease, in that there was a V-shaped area of living 

 plants in approximately the same place where nearly complete infection 

 had been noted the previous season. The 191 6 area of infection was 



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S6.8 



Fig. 2. — Diagram of plot B5-4, showing by the heavy lines the portions of the rows in which the cotton 

 plants were killed by rootrot in 191 7. 



again infected, as were also the areas in the southwest comer of the plot 

 and in the southeast corner of the west half, areas that had been free from 

 dead plants in 191 6 and 191 7. However, this tendency toward alternate 

 occurrence or progressive spread was not shown in the east end of the 

 plot or in other plots with sufficient regularity to be considered significant. 



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 C2x? <2<ft? ^.S /OS eO.S- ^J^<3 4^4510 



Fig. 3. — Diagram of plot B5-4, showing by the diagonal hatching the portions of the rows in which the 

 cotton plants were killed by rootrot in 1918. 



SUMMARY OF THREE YEARS' RECORDS 



If the records of the occurrence of the disease in this plot for the three 

 years be brought together as in figure 4, it will be seen that almost the 

 entire plot has been affected within that time. Yet during the last two 

 years less than half the plants have been taken by the disease. 



It seems clear from the evidence here presented that these rootrot 

 spots do not carry over from year to year. This may explain the diffi- 

 culty that has been experienced by investigators in attempting to 

 determine, by a comparative study of the soil conditions inside and out- 

 side the rootrot spots, what conditions permit tlie disease to become 

 destructive. 



