Apr. 3 1916 Climatic Conditions and Cercospora heticola 35 



RAINFALIv AND IRRIGATION 



The rainfall records made during the summer season of 191 3 in the 

 beet field at Rocky Ford in which infection was studied in detail (fig. 

 4, 7) where obtained by means of a rain gauge placed at the edge of the 

 sugar-beet field (PI. IV, fig. i). Most of the rain was in the form of local 

 showers, the amount varying greatly within a radius of less than 2 miles; 

 but occasionally general rains fell. The efifect of the increased relative 

 humidity resulting from rainfall usually lasted longer among the leaves 

 than in the air 5 feet above (fig. 7). 



The effect of irrigation on humidity was found to be similar to the 

 effect of rain. On July 2, before the field was irrigated, its humidity 

 was as low as 8 and on July 3 and 4 remained above 60 for 7 and 6 hours, 

 respectively. On July 4 and 5 the field was irrigated and the humidity 

 remained above 72 on the 4th and above 60 during 23 hours of the 5th. 

 On July 27 the field was again irrigated and the humidity remained 

 above 60 for 15 hours that day and 21 hours the following day. On 

 August 19 and 20 the field was irrigated the third time and the humidity 

 remained above 60 for 12 and 13 hours, respectively, and the next day 21 

 hours. The general humid conditions necessary for leaf spot infection, 

 however, are developed much better by rain than by irrigation, because 

 of the latter being comparatively local and unaccompanied by the 

 atmospheric conditions attending rainfall. 



WIND 



Records of wind velocity at Rocky Ford were taken by means of an 

 anemometer placed 6 feet in the air at the edge of the beet field (PI. IV, 

 fig. i), the readings being made at irregular intervals and the velocities 

 computed being the hourly averages from one reading to the next. As 

 the records were not made daily, accurate hourly velocities for different 

 intervals during the day can not be obtained from the records. They 

 show, however, that the average seasonal velocity from June 12 to Sep- 

 tember 22 was 5.3 miles per hour. Occasionally two daily readings 

 were made, one in the morning and the second late in the afternoon. 

 These show that the average velocity of the wind was always higher 

 during the day than at night, the greatest velocity usually prevailing 

 in the afternoon during the period of lowest humidity. While no gen- 

 eral dissemination of conidia was correlated with high wind velocity, 

 the afternoon combination of highest wind with lowest humidity ap- 

 parently favored the dissemination of conidia. In fact, in the case of 

 air cultures made at different times during several days, it was found 

 that the fungus grew usually only on those exposed during the afternoon. 



