DOWNY MILDEW OF CUCUMBERS 



By William L. Doran, Research Professor of Botany 



INTRODUCTION 



Downy mildew, caused by the fungus Peronoplasmopara cubevsis (B. & C.) 

 Clint., is one of the most common and important diseases of field-grown cucum- 

 bers in Massachusetts. It was present here in seven out of the past eight years. 

 In greenhouses, too, its onset is so sudden and its spread so rapid that it is re- 

 garded by growers with much concern. 



The disease is by no means new, for it was present in Massachusetts as earl,v 

 as 1890 (21).' Its distribution has recently been discussed by Weber (48), and, 

 as he points out, it has been found at some time in all regions of the United 

 States except the more arid parts. Because of its economic importance and gen- 

 eral distribution, this disease and the fungus which causes it have received con- 

 siderable attention from earlier investigators. There is, nevertheless, much about 

 the fungus and the disease which remains obscure. 



Work described in the present paper was undertaken with the object of add- 

 ing to the knowledge of host relationships, source of the primary infection, effect 

 of climatic factors on sporulation and on spore germination and infection, and 

 prevention of infection as by the use of fungicides. 



HOST RELATIONSHIPS 

 Relative Susceptibility of the Species of Cucurbits 



Peronoplasmopara cubensis has been reported (39, 40, 48) as occurring on 28 

 species of plants, all in the family Cucurbitaceae. These include cucumber, 

 muskmelon, watermelon, squashes, pumpkin, and several gourds, ornamentals, 

 and species not cultivated. 



The species named in Table 1 were grown in the field with rows of cucumber 

 plants alternating with rows of the other species. All plants were inoculated 

 three times by the application of a water-suspension of the conidia of P. cubensis 

 obtained from the leaves of cucumber. As was evidenced by the severity of downy 

 mildew on cucumber, the season was very favorable to the disease, and the 

 relative susceptibility of the several species was put to a fair test. The degrees 

 of infection which resulted on each species in the course of the growing season are 

 recorded in Table 1. 



Downy mildew was severe on cucumber, moderate on muskmelon, present 

 as only a trace on watermelon, and absent from squash and pumpkin. There 

 was no infec'tion or only a trace of infection on the other species tested. 



In other fields and in several years, downy mildew has been observed causing 

 the death of cucumber leaves when it was absent from or present as no more 

 than a trace in adjoining plots of watermelon, summer squash, Hubbard squash, 

 and pumpkin. 



The only species of the Cucurbitaceae w^hich grow without cultivation here 

 are Sicyos angulalus and Echinocystis lobata. The former is rare and the latter 

 is conmion. But downy mildew has not been found here on E. lobata, even when 

 growing near infected cucumbers. These observations and those recorded in 

 Table 1, like those of Clinton (10) and Halsted (18) support the conclusion of 



'Reference is made by number to "Literature cited," p. 20. 



