BLIGHT AND LEAF-Sl^OT OF CARROT 275 



Laboratory Experiments with Fungicides 



Toxicity tests were made by the method of Reddick and Walhice (6). 



Bordeaux mixture, 2-2-50, was found to jirevent all germination of the 

 spores of the liliglit fungus, Mnrro.'iporiuni rmotae; and Bordeaux mixture, 

 1-1-50, permitted slight germination, only 4 jier cent as compared with 100 

 per cent in tlie check. 



Micro-slides were lightij' dusted witli a cojiijer dust containing 9.5 per cent 

 monohydrated copper svdfate. These slides were then sprayed with water 

 and dried for twenty-four hours, when drops of water containing tlie spores 

 of this fungus were placed on them. There was no germination of tlie spores 

 in the presence of the copper, and the germination of ciieck sjiores was 100 

 per cent. 



The spores of tlie leaf-s})ot fungus, Cercospora apii carotae, were found to 

 be even more susceptible to the toxic action of a copper fungicide. Bordeaux 

 mixture 1-1-50, tested by the metiiod of Reddick and Wallace, ])revented all 

 germination of these spores. 



A copper fungicide which controls blight may be expected to control leaf- 

 spot of carrot. 



The effect of sulfur on tlie germination of the spores of the blight fungus, 

 Macros por'nim cnrofae, was determined. Blighted leaves of carrot were dust- 

 ed with sulfur and placed at controlled constant temperatures, as were also 

 blighted leaves not sulfured. At hourly intervals spores were removed and 

 ])laced under optimum conditions for germination. It was found that sulfur 

 is not toxic to th.e spores of this fungus. Even after as much as twenty hours' 

 exposure to sulfur at a temperature as high as 87° F., these spores germinated 

 well, and as well as spores not sulfured. Previous wetting of the spores did 

 not affect their great resistance to the toxicity of sulfur. It is evident from 

 this that sulfur cannot lie expected to protect carrot foliage against infection 

 by blight. 



In this connection it is of interest to note that the spores of the closely 

 related fungus, Alternaria solani, which causes early blight of potato, were 

 found by Doran (7) to be very resistant to the fungicidal action of sulfur. 



Spraying and Dusting Experiments in the Field 



Field exjieriments with co]ij)er fungicides for the control of blight and leaf- 

 spot of carrot were conducted in five years In two of them, 1921 and 1922, 

 blight was more than usually severe. In three of them, 1923, 1925, and 192(). 

 there was but little blight. 



For lack of more suitable equipment at the time, the spraying was done by 

 carrying a long hose through the carrots from a power sprayer at the end 

 or side of the field. Aside from the inconvenience of this method, the carrots 

 were well sprayed. 



In 1921, carrots were sjirayed with Bordeaux mixture 4-6-50 at intervals of 

 about eleven days beginning August 22 and ending October 14. The yield 

 per acre was 15 per cent greater on plots sprayed than on plots not sprayed. 



In 1922, carrots were sprayed with Bordeaux mixture of the same formula 

 at intervals of about ten days, beginning August 21 and ending October 15. 

 In that year, the yield of carrots per acre was 22 per cent greater on sprayed 

 plots than it was on jilots not sprayed. 



In 1923, 1925, and 1926, years of little blight, spraying carrots with Bordeaux 



