DOWNY MILDEW OF CUCUMBERS 15 



Table 4. — Effect of Sulfur on Germination of Conidia 



Conditions of exposure to sulfur (dry) 

 Temperature 



(°C.) 



26 

 26 

 26 

 26 

 24 

 22 



For purposes of comparison, other downy mildew fungi were similarly used. 

 It was found that the conidia oi Bremia lactucae were killed by contact with dry 

 sulfur for 6 hours at 23" C. and the conidia of Peronospora effusa (Cirev.) Ces. 

 were killed by contact with dry sulfur for 4J4 hours at 26° C. Both Erwin (12) 

 and Guba have found that the conidia of Bremia lactucae may also be killed by 

 exposure to vaporized sulfur. 



Sulfur was toxic to the conidia of P. citbensis when applied dry as above 

 described, but it was found to be non-toxic to them when they were in water. 

 The conidia germinated freely when placed in water on micro-slides or leaves 

 which had been dusted with sulfur. Kendrick (24) noted the same lack of effect 

 of sulfur on the germination of the conidia of Bremia lactucae when they were 

 placed in drops of water on a surface which had been dusted with sulfur. Copper- 

 lime dust on the other hand was found by the writer to prevent the germination 

 of the conidia of P. cubensts when used in water this way. 



The water necessary for the germination of these conidia does not interfere 

 with the fungicidal action of copper, but it prevents the toxic action of sulfur, 

 at least to P. cubensts and to B. lactucae. This is naturally a very serious limita- 

 tion to the value of sulfur for use against downy mildew of cucumber (and of 

 lettuce), for it is least effective under conditions when it is most needed, that is, 

 when there is water on the leaves. Sulfur does, however, have some slight fungi- 

 cidal value against downy mildew of cucumber, as Weber (loc. cit.) concluded 

 from his field experiments. This is attributable to its effects on the conidia in 

 the absence of water, and on sporulation. 



Protection of Cucumber against Downy Mildew by Sulfur and by Copper 



Fungicides 



Sulfur is often used in the greenhouses of Massachusetts to control powdery 

 mildew of cucumber, as recommended by Guba (15). The extent to which sulfur 

 may also be effective, especially as compared with copper fungicides, in protect- 

 ing cucumbers against downy mildew was, therefore, further investigated. 



Infected cucumber leaves on which the fungus was sporulating were dusted 

 with sulfur. These leaves and similar leaves not sulfured were placed in moist 

 chambers at 25 ° C. for 4 J^ hours. Conidia were then washed from them and used 

 to inoculate cucumber plants. Thirty drops of a suspension of sulfured conidia 

 were placed on each of 20 plants, and conidia which had not been sulfured were 

 applied in the same way to 20 other plants. Seven days later, there v.-as an 



'Guba, E. F. 1928. Mildew of lettuce. Jour. Market Garden Field Sta. of Mass. Agr. College, 

 No. 95:4-5. (Mimeographed.) 



