May IS. 1919 Influence of Soil Environment on Rooirot of Tobacco 69 



entail too many readings. Temperature readings taken at 7 a. m, and 

 at I and 5 p. m. for one week (July 6 to 13, 191 6) showed that the tem- 

 perature was anywhere from i to 

 5 degrees lower at 7 a. m. than at 

 I p. m. and to average slightly 

 higher at 5 p. m. than at i p. m., 

 indicating that the maximum 

 perhaps was reached at some 

 time between i and 5 p. m., and 

 on clear days at about 3 p. m. 

 A recording soil thermograph 

 was also used at a depth of 4 

 inches in 191 7. From these rec- 

 ords it may be noted that the 

 highest temperature usually oc- 

 curred about 4 p. m. (fig. 2). 



A more important consider- 

 ation, however, is the general 

 rise or fall of temperature during 

 extended periods of a week or 

 more, or the general trend of the 

 temperature for one season as 

 compared with another. 



In the northerly latitudes the 

 growing period of tobacco, 

 practically speaking, lies within 

 the months June, July, and Au- 

 gust. Although much tobacco 

 remains in the field during the 

 month of September, practically 

 all the growth must be made be- 

 fore that time. July undoubt- 

 edly is the critical month in 

 which most of the growth should 

 be manifested, although if 

 growth is retarded until August, 

 and a warm fall follows, with ab- 

 sence of frost until late into 

 September, a marketable crop 

 may often be produced. In the 

 northern districts nearly all to- 

 bacco is planted in June. From 

 the standpoint of temperature this is the most favorable month for the 

 rootrot. Heavy infestation in June followed by a warm July, however, 



