DOWNY MILDEW OF CUCUMBERS 17 



Table 5. — Effects of Sulfur and Copper-Lime Dust on Yields of 

 Cucumbers Infected with P. Cubensis 



Plot treatments 



Check, no fungicide 



Sulfur dust, applications begun after first infection 



Sulfur dust, applications begun before first infection 



Copper-lime dust, applications begun after first infection 



Copper-lime dust, applications begun before first infection 



heavily, Cuba (loc. cit.) concluded that not more than 6 pounds of sulfur dust 

 should be applied in a cucumber greenhouse 280 by 40 feet (for the control of 

 powdery mildew). The same caution is suggested whenever greenhouse cucum- 

 bers are dusted with sulfur for any reason. 



The dusting of greenhouse cucumbers with sulfiu- for the control of powdery 

 mildew may, as discussed above, help to protect them against downy mildew 

 also; but a copper fungicide, a Bordeaux mixture, is preferable if only downy 

 mildew is to be combated. In the experiments of the writer, as in those in Cal- 

 ifornia'" and Florida (48), sulfur did not satisfactorily control downy mildew of 

 cucurbits under conditions favorable to the fungus, and similar results when 

 sulfur was used were secured by Clayton (8) with downy mildew of lima bean, 

 by Boyd (6) with late blight of potato, by Salmon and Ware (37) with downy 

 mildew of hop, and in California (30) with downy mildew of lettuce. We may 

 conclude that although sulfur is, of course, and generally, a valuable fungicide, 

 this is probably not the case when downy mildews are to be combated. 



In the field experiments, the results of which are summarized in Table 5, 

 yields of cucumbers in 1927 were increased 97 per cent by copper-lime dust when 

 the first applications were made before infection by P. cubensis, but only 57 per 

 cent when the tii'st applications were made after infection. Similar results were 

 obtained the previous year, 1926, when yields were increased 68 per cent by 

 copper-lime dust applied fii'st before infection and only 2 per cent, or not signi- 

 ficantly, when this treatment was first applied after infection. These results 

 support the conclusions or recommendations of most earlier investigators (10, 

 42, 32, 48, 22, 7) to the effect that for maximum protection of cucurbits against 

 downy mildew, it is necessary that fungicides be applied before rather than after 

 infection. 



Date of Beginning Applications 



As has been observed by a number of investigators (9) (48, 42, 15), it is, how- 

 ever, quite possible to injure cucumber plants with Bordeaux mixture, and 

 Clayton (9) and Zimmerly and co-workers (54) have found that young plants 

 of cucumber and canteloupe, respectively, are more susceptible to such injury 

 than are older plants. We may conclude that although cucumbers should be 

 sprayed or dusted with a copper fungicide before infection, the first applications 

 should not be made unnecessarily early. It is, therefore, important to know the 

 approximate date at which the disease usually appears in the field in a given 

 locality. 



U. S. Dept. A^r , Bur. Plant Ind. Diseases of field and garden crops in the United States in 

 1919. U. S. Dept. Agr., Bur. Plant Ind., Plant Dis. Rptr., Sup. 10, pp. 180-273. 1920. 

 (Mimeographed . ) 



