Apr. 3, 1916 Climatic Conditions and Cercospora heticola 53 



the humidity remained above 60 for not less than 15 to 18 hours, but 

 very few were produced when the humidity remained above 60 for less 

 than 10 to 12 hours daily. 



With a view to determining the approximate number of conidia pro- 

 duced on a sugar-beet plant under a favorable temperature and humid- 

 ity, one representing a heavy infection in August was selected. After 

 the infected leaves were measured a representative portion of conidia 

 were carefully washed off into sterilized water and counted. The count, 

 which was made by means of a dilution method, showed 250,000,000 

 conidia on the plant at that time. 



CoNiDiAL DISSEMINATION. — That a period of low humidity, with its 

 accompanying factors, is favorable to the dissemination of conidia was 

 frequently observed (fig. 6, curve R). For instance, it was found that 

 the amount of conidia diminished on September i, 6, 14, and 15, when 

 the humidity remained above 60 for 5, 6, 10, and 4 hours, respectively; 

 while, on the other hand, there was no diminution in the amount present 

 on September 3 to 5 and 8 to 10, during which periods the humidity 

 remained above 60 for 12 to 16 hours. 



Rainfall is also an important factor in the dissemination of conidia, as 

 was noted in several instances. On July 19 (fig. 6, curve F) rain fell, 

 and as a result many conidia were washed off, and the same was true in 

 the case of rains on July 23 (curves C, D, E, G, H), August 9 (curve K), 

 September 4 (curves N, O, R), and September 16 (curves Q, S, T, U, V, 

 W). After rains on July 19, August 9, and September 4, however, there 

 were more conidia present than before, but this was probably due to the 

 fact that more we re produced under the favorable humid conditions attend- 

 ing these rains than were washed oft'. It was also found that the conidia 

 were disseminated more rapidly from the upper than from the lower sur- 

 face of the spots (fig. 8). This was due probably to the greater exposure 

 of the former to wind and rainfall. 



RELATION OF INFECTION CYCLES TO CLIMATIC CONDITIONS 



For the purpose of determining the relation of infection cycles to 

 climatic conditions, a study was made of the increase and spread of dis- 

 ease in a field of sugar beets planted about May i ^ at Rocky Ford and 

 one planted about two weeks later. Both fields had been in beets for two 

 or three years, and as very few, if any, of the tops were removed after 

 the harvest of 191 2, infection appeared early in 191 3 and was generally 

 distributed. 



Three plants in the early field (Table VII) and ten in the medium- 

 early (Table VI) were selected, the leaves tagged and numbered con- 

 secutively, beginning with the outermost or oldest and continuing with 

 the new ones as they appeared. The spots on each leaf were counted at 



' ConidJal production and dissemination were also studied in this field. 



