DOWNY MILDEW OF CUCUMBERS 19 



second application of fungicides. Every leaf of check plants without fungicide 

 became infected. There was no infection of any of the plants sprayed with any 

 of the Bordeaux mixtures, including the 1:1:50 formula. In these and other 

 experiments of the writer, infection of greenhouse cucumbers by P. ciihensis was 

 prevented by Bordeaux mixture 1:1:50, and in these experiments, as in those of 

 Guba (15), Bordeaux mixture 1:1:50 did not have the injurious effect on leaves of 

 greenhouse cucumbers which sometimes follows the use of Bordeaux mixtures 

 containing more copper. It is concluded that a Bordeaux mixture of the 1:1:50 

 formula is to be preferred to a more concentrated mixture when a fungicide is 

 needed for the control of downy mildew of cucumber in greenhouses. 



SUMMARY 



Downy mildew caused by Peronoplasmopara cubensis is common and often 

 severe on all varieties of cucumbers in both fields and greenhouses here. The 

 fungus from cucumber was found to infect muskmelon, l)ut not squashes, pump- 

 kin, or watermelon. Cucumber is in no danger of infection by this fungus from 

 weed hosts in this State. Youngest leaves of cucumber were found to be resist- 

 ant to infection. The susceptibility of cucumber was not affected by applica- 

 tions of potash or lime to the soil. 



No oospores or other wintering-over stages of the fungus have been found 

 here, and inoculation of plants with material wintered-over in the soil has not 

 resulted in infection. There is no relation between previous cropping systems 

 and the occurrence of the disease here. The fungus enters greenhouses from 

 fields in summer and fall, but it very rarely lives through the winter in green- 

 houses in Massachusetts. The fungus lives through the winter on living hosts 

 in the South. All available evidence indicates that the fungus probably reaches 

 Massachusetts from states farther south after a gradual northward movement 

 l)y the aid of wind. 



Conidia germinate in water, including dew and water of guttation. They 

 were found to germinate at temperatures from 9° to 30° C. and to germinate 

 best at 16° to 19° C. Infection took place at temperatures from 10° to 28° C. 

 with most complete infection at 16° to 22° C. which was also about the optimum 

 temperature for sporulation. Two to three hours on a wet leaf is long enough 

 for germination. Conidia may emit zoospores and these may initiate infection, 

 all within 5 hours. Length of life of conidia depends on moisture conditions. 

 Some conidia remained viable for 50 hours in moist air. 



Leaves which remained wet only 2)^2 hours were not infected, but there was 

 some infection of leaves which remained wet 5 hours after inoculation. 



Infection usually ends in sporulation within 6 to 9 days after inoculaton. 

 Dew may provide enough moisture for sporulation. In dry air, the fungus may 

 live in infected leaves for several weeks without sporulating. 



Sulfur was more effective than copper-lime dust in preventing sporulation 

 of the fungus. Conidia were killed by contact with dry sulfur for 5 hours at 22° 

 C. Sulfur was, however, not toxic to these conidia in water. Infection was not 

 prevented when the conidia in water fell upon leaves dusted with sulfur. Not 

 only did sulfur fail to provide protection in the field, but it was also definitely 

 injurious to cucumber plants. In both field and greenhouse copper fungicides 

 are to be preferred to sulfur for the control of downy mildew. 



