DOWNY MILDEW OF CUCUMBERS 5 



of the Phycomycetes have occasionally, although rarely, been found by the 

 writer in the remains of cucumber leaves, especially petioles which, after having 

 been killed by downy mildew, had passed through one winter in the soil in the 

 field. Efforts to germinate them or to infect cucumbers with them have, however, 

 been unsuccessful. Thus no proof was obtained that they are oospores of P. 

 cubensis or that this fungus produces oospores. 



Clinton (loc. cit.) suggested the possibility that the oospores of this fungus, 

 if there be any, may develop only after the union of distinct and sexually different 

 mycelial strains. With this possibility in mind, the writer has inoculated the 

 same leaves of cucumber plants with conidia of P. cubensis obtained from several 

 sources. After leaves so inoculated had become infected and killed by downy 

 mildew, they too were buried in soil over winter. The decomposed residue of 

 such leaves was examined for oospores one and two years later, but none were 

 found. 



It is conceivably possible that this fungus might live through the winter in 

 the soil in some other stage than as oospores, e.g., as saprophytic mycelium or 

 specialized and long-lived conidia. None have been found, however, and no 

 infection resulted when Clinton (loc. cit.) and Adams (2) inoculated cucurbits 

 with soil in which cucurbits had died of downy nuldew the previous season or 

 with the remains of the leaves of cucurbits which had been killed by downy 

 mildew. 



On numerous occasions in the course of this investigation, cucumbers have 

 been inoculated with soil in which cucumbers had grown and died of downy 

 mildew one and two years before, or with inoculum consisting of cucumber leaves 

 killed by downy mildew the previous fall and wintered-over in the soil outdoors. 

 In very few cases has infection followed, and these few were always at a time of 

 year when the conidial stage was present in the field here. It is, therefore, at 

 least possible that such little infection as did occur was accidental rather than 

 from inoculation with over-wintered material, and the only safe conclusion is 

 that the results of such inoculations were negative. Thus, as in the work of 

 other investigators mentioned above, no evidence was secured that downy 

 mildew is associated with the over-wintering of P. cubensis in dead host plants 

 here. 



As supporting evidence, it should be added that there is no apparent relation 

 between previous cropping systems and the time of appearance or severity of 

 cucumber downy mildew. The disease often reappears in fields where it has 

 occurred before, but the writer, like Stewart (loc. cit.), has often found it in 

 fields in which no cucurbits had been grown for several or many j^ears before. 

 Adams (1) did not find any evidence that the previous crop had any relation 

 to the prevalence of the disease; and in Florida, too, Weber (loc. cit.) has noted 

 that crop rotation does not protect cucumbers against dowTiy mildew. 



It has been observed that the disease when it appears at all in Massachusetts, 

 and it usually does, is to be foxmd in every or almost every field of cucumbers, 

 and at approximately the same time, whether or not cucumbers were previously 

 grown in these fields. It seems evident, therefore, that the source of the inoculum 

 may be distant and that the inoculum is probably air-borne. 



In this connection, it should be noted that Adams (1) found no evidence that 

 P. cubensis is seed-borne. The fungus has not been found in or on the fruit of 

 the cucumber, and inoculations of cucumber fruits by the writer have never 

 resulted in infection. 



