Apr. 3, 1916 Climatic Cotiditions and Cercospora beticola 59 



SUMMARY 



(i) The life of the fungus Cercospora beticola overwintering in sugar- 

 beet-top material varies with different environment. When exposed to 

 outdoor conditions, the conidia die in from one to four months; but when 

 kept dry live as long as eight months. The sclerotia-like bodies, which are 

 more or less embedded in the tissues of the host, are more resistant than 

 the conidia, living through the winter when slightly protected, as, for 

 instance, in the interior of a pile of hayed sugar-beet tops or buried in 

 the ground from i to 5 inches, and become a source of infection for the 

 succeeding crop. Notwithstanding the difference in temperature and 

 soil-moisture conditions, similar results from the overwintering experi- 

 ments were obtained at Rocky Ford, Colo., and Madison, Wis. 



(2) Climatic conditions and the development of the leafspot can be 

 correlated only when all records are taken at the same relative positions, 

 as shown by comparisons of the Weather Bureau records and the records 

 taken among the plants and 5 feet above the field. 



(3) The maximum temperature is much higher near the ground than 

 5 feet above early in the season, but the difference diminishes as the 

 season advances. 



(4) Throughout the season the maximum relative humidity was higher 

 among the leaves than 5 feet above the field. Early in the season, while 

 the plants were small, the humidity remained above 60 longer each day 

 5 feet above the field than among the plants near the ground ; but after 

 the plants attained a good size this condition was reversed. Because of 

 this difference, only records collected among the leaves should be con- 

 sidered in correlating climatic conditions and conidial production and 

 infection. 



(5) The effect of rainfall and irrigation on the increase of relative 

 humidity and its duration is apparently much the same. 



(6) Thermal tests with artificial cultures showed (a) that exposure to 

 constant temperatures of 35° and 36° C. is fatal to the growth of the 

 fungus ; (b) that growth occurred when cultures after exposure for 3 days 

 to either of these temperatures were changed to 30.8°, and also when 

 they were held at either for 8 hours and then at 20° for 16 hours; and (c) 

 that a temperature of 40.5° was fatal in all combinations tested. 



(7) Temperature and relative humidity influence the production of 

 conidia and infection in much the same way. A temperature of 80° or 90° 

 F., with a night minimum preferably not below 60°, is most favorable to 

 conidial production, while it is checked by a temperature of 100° or 

 higher and greatly checked by a range from below 50° to 80°. A maxi- 

 mum humidity ranging above 60 for not less than 15 to 18 hours each 

 day induces a good growth of the fungus. 



(8) Because of the higher humidity on the lower than on the upper 

 surface of the leaf, the conidia are generally more abundant on the lower 

 surface of the spots, but because of the action of rain and \\ind they 

 disappear more rapidly from the upper surface. 



