Apr. 3. 1916 Climatic Conditions and Cercospora beticola 33 



ture usually lower among the beet plants than the temperature shown by 

 the Weather Bureau records, which were taken on top of a four-story 

 building about a mile from the sugar-beet field. These wide variations 

 between the air temperature taken near the ground among the plants 

 and that taken 5 feet above the field and between the former and the 

 temperature taken at the Weather Bureau stations show that for correla- 

 tion with fungous activities only the records taken among the plants 

 should be used. 



Relative; humidity. — There was also a wide variation in the humidity 

 near the ground among the plants and 5 feet above the field. For instance, 

 the daily minimum humidity at Rocky Ford from June 13 to 29, with two 

 exceptions, was higher and remained above 60 generally for a longer 

 period in the air above the plants than among the leaves near the ground 

 (fig. 4), owing to the higher temperature at the surface of the ground as 

 a result of the small amount of covering afforded by the young plants. 

 During this period the daily variation of humidity among the leaves was 

 extreme, ranging from 99 to 10 on June 13, from 99 to 16 on June 25, and 

 from 100 to 8 on July 2. After June 29, on the other hand, the minimum 

 humidity was generally higher, the humidity remained above 60 for a 

 longer time among the leaves than in the air above, and the daily varia- 

 tion among the leaves was less extreme than earlier in the season. These 

 conditions were due mainly to the greater amount of covering afforded by 

 the larger plants and consequent longer retention of moisture among the 

 leaves. The humidity both among the plants and in the air 5 feet above 

 the field remained, on an average, above 60 for a longer time each day 

 during midsummer than during June, owing in part to the increased use 

 of irrigation water as the season advanced and the increased amount of 

 moisture in the surrounding air resulting from the increased transpiration 

 of the larger plants. 



Comparison of the Madison and the Rocky Ford records (fig. 5) of the 

 number of hours that the relative humidity remained above 60 each 

 day among the sugar-beet plants shows that throughout the season it was 

 higher, on an average, at Madison. Here it remained above 60 for a 

 longer time each day during the latter half of June, when the records 

 were started, and for a shorter time each day during August than during 

 any other summer month. This was due to difference in the amount of 

 rainfall, there being frequent rains during the former period and com- 

 paratively dry weather during the latter. At Rocky Ford the facts were 

 reversed, the humidity remaining above 60 for a longer time each day 

 during midseason than during the latter half of June or the first part of 

 September. This was probably due to more frequent irrigation and the 

 increased covering afforded by the larger plants of midseason. 



