DOWNY MILDEW OF CUCUMBERS 3 



Table 1. — Relative Susceptibility of Several Species of Cucurbits 

 TO Infection by P. cuhensis from Cucumber 



Species inoculated Degree of infection 



Cucumis sativus L Cucumber Very severe 



Cucumis sativus L. var. anglicus Bailey English forcing cucumber Very severe 



Cucumis Melo L Muskmelon Moderate 



Cucumis Melo L. var. inodorus Naud Cassaba melon Moderate 



Cucumis Anguria L Bur gherkin None 



Cucurbila Pepo L Field pumpkin None 



Cucurbita Pepo L. var. condensa Bailey Summer squash None 



Cucurbila moschala Duchesne Cushaw squash None 



Cucurbila maxima Duchesne Hubbard squash None 



Citrullus vulgaris Schrad Watermelon Trace 



Sicyos angulalus L One-seeded bur cucumber Trace 



Echinocyslis lobaia Torr. & Gr Wild balsam-apple None 



Luffa cylindrica Roera Rag gourd Trace 



Benincasa hispida Cogn White gourd Trace 



Lagenaria leucanlha Rusby White-flowered gourd Trace 



Trichosanlhes Anguina L Serpent gourd Trace 



Momordica Balsamina L Balsam-apple None 



Melolhria scabra Naud None 



Bryonopsis laciniosa Naud. var. erythrocarpa Naud Moderate 



Wilson (52) that the native species of the Cucurbitaceae are comparatively 

 immune to downy mildew. 



It seems safe to conclude that cucumbers are not in danger of infection by 

 P. cubensis from wild plants in Massachusetts, and that P. cuhensis from cucum- 

 ber does not infect or does not readily infect certain other cultivated species 

 (watermelon, pumpkin, squashes, and certain gourds) which have been reported 

 as hosts. There is, of course, a possibihty of biologic specialization by this fungus. 



Factors Affecting Susceptibility of Cucumbers 



Varietal Differences 



Twentj'-eight varieties of cucumber, including those common in the field and 

 in greenhouses, have been grown, and downy mildew resulting from natural 

 infection has been equally severe on all of them. The results of these observations 

 agree with those of Clayton (9) and Weber (loc. cit.), neither of whom found 

 any differences in the susceptibility of the varieties of cucumber to downy 

 mildew. 



Age of Leaves 



The oldest cucumber leaves are usually the first to be attacked bj^ downy 

 mildew in nature. The disease is almost never found on young leaves of cucum- 

 ber and, as was observed by Halsted (17), this is true also of squash. To deter- 

 mine the relation of age of cucumber leaves to their susceptibihty to infection 

 by P. cubensis, leaves of various ages were inoculated by placing them on drops 

 of a water-suspension of conidia. These leaves were grouped into eight classes 

 on the basis of age. There was no infection of the three youngest leaves of any 

 plant, and there was but little infection of the fourth leaves. Older leaves were 

 all and equally susceptible. In these and other experiments, the youngest leaves 

 of cucumber were entirely resistant to infection by P. cubensis. 



According to Clinton (loc. cit.) infection may take place either through the 

 stomata or directly through the epidermis. Stewart (42) and Selby (38), how- 

 ever, observed infection by the germ tubes of the zoospores of this fungus only 



