38 Journal of Agricultural Research voi. vi. No. i 



on cultures exposed to temperatures of 14.5° and 19.2° C. averaged 2.7 

 mm. in diameter; after exposure for the same length of time to 14.5° and 

 21.6°, 14.5° and 28°, and 14.5° and 30.8° the growth gradually increased 

 until it reached a maximum diameter of 8.8 mm; but when exposed to 

 higher temperatures (20° and 34.7° or 20° and 35.8°) the growth grad- 

 ually diminished until finally it equaled approximately that attained 

 under 14.5° and 28°. There was no growth on cultures exposed for nine 

 days or longer to 20° and 40.6° (Table III, series C). 



SERIES D (high varied changed to low varied temperatures) . — A 

 plate culture exposed for three days to temperatures of 20° and 40.5° C, 

 being held 16 hours at the lower and 8 hours at the higher, and then for 

 six days at 20° and 30.6°, developed 23 colonies, averaging 10.3 mm. by 

 the end of the latter period, while a check plate exposed constantly to a 

 temperature of 30.6° developed 100 colonies by the end of the latter 

 period. A plate exposed to the higher temperatures — 20° and 40.5° — 

 for five days and then held at 20° and 30.6° for six days developed six 

 colonies at the end of the latter period, while a plate exposed to 20° and 

 40.5° and then held at 20° and 30.6° for seven days developed no growth 

 of the fungus. 



Later on in this paper the fact that high minimum and maximum tem- 

 peratures inhibit the growth of the fungus, as brought out by these tests, 

 is correlated with the effect of existing high field temperatures, with 

 their consequent accompanying factors, on the leaf spot. Although 

 the optimum temperature variations — 20° and 30.8° C. — were found to 

 be very favorable to the development of leafspot in the field, little or 

 no increase in the disease was observed to follow high night and day 

 field temperatures — 20° and 40.5°, respectively. 



It was also observed that different temperatures affect conidial septa- 

 tion. The normal average septation varies from 6 to 11, but during 

 warm, humid periods the conidia were usually found to be many septate, 

 sometimes as high as 20-septate, while after a cooler period, such as 

 usually occurs in September, they were only from 2- to 4-septate. 



relation of conidial production and dissemination to climatic 



conditions 



For the purpose of studying the relation of temperature and relative 

 humidity to the production and dissemination of conidia, detailed life 

 histories of a large number of individual spots on 10 plants in the medium- 

 early field at Rocky Ford were kept during the season of 191 3. The tem- 

 perature and humidity records used in these correlations were those 

 taken among the beet leaves near the ground and, together with rainfall 

 and dates of irrigation, are shown in figure 7. 



Beginning with the outermost or oldest, the leaves were tagged and 

 numbered consecutively, and the location of the spots on each was indi- 



