Oct. 29. 1921 Relation of Soil Temperature to Onion Smut 



255 



data collected from this field plot are tabulated in Table XII. Obser- 

 vations were made by pulling plants at several points in each plot, and 

 examining for smut lesions after the chlorophyll had been removed by 

 means of alcohol and acetic acid. The first observation was made on 

 the twenty-first to the twenty-third day after planting. Subsequent 

 observations were made as indicated in the table. 



TabIvE XII. — Development of onion smut in successive plantings in the field at Madison, 



Wis., ig20 



" Covered for 15 days only. 

 •6 Observation 39 days after planting. 



«^ Extent of infection: Systemic, 26 per cent; confined to dead cotyledon, 14 per cent. 

 <* Extent of infection: Systemic, 10 per cent; confined to dead cotyledon, 66 per cent. 

 « Extent of infection: Systemic, 5 per cent; confined to dead cotyledon, 81 per cent. 

 / Extent of infection: All cotyledon infections. 

 Extent of infection: All systemic infections. 



An analysis of the data secured can be made by referring to Table XII 

 and figure 2. It will be seen that the soil temperature mean gradually 

 rose until July 23 to 29, after which there was a gradual drop. At no 

 time did the mean in the uncov red plot reach the inhibiting temperature 

 (29° C), but it closely approached this point during the warmest portion 

 of the season. In the covered plot, however, the mean remained above 

 29° continuously until the cover was removed on August 3. The two 

 important points to be considered in the respective plantings were (i) 

 the amount of original infection which was determined three to four 

 weeks after planting (see first and second observations in Table XII) 

 and (2) the extent to which the disease either became systemic or was 

 entirely outgrown by the plants during the following four or five weeks 

 (see third observation in Table XII). 



