6 MASS. EXPERIMENT STATION BULLETIN No. 283 



Relation of the Disease in Greenhouses to the Disease in the Field 



A fall crop of cucumbers is often started in greenhouses in Massachusetts 

 while field-grown cucumbers infected with downy mildew are still living. Under 

 these conditions, greenhouse cucumbers frequently become infected by P. 

 cubensis in the fall. The observations here recorded lead to the conclusion that 

 such infection comes from the field rather than from the previous greenhouse 

 crop of cucumbers. Downy mildew is not uncommon in the fall in greenhouses 

 in which the soil has been disinfected by steaming and the air by sulfur fumigation 

 since the last crop of cucumbers was removed. The conclusion that the fungus 

 enters greenhouses in the fall from field-grown plants is further supported by the 

 observation, first recorded by Stone and Smith (45), that greenhouse cucumbers 

 started in August and September are more likely to become infected with downy 

 mildew than are greenhouse cucumbers started later, that is, in the fall after most 

 or all field-grown cucumbers have been killed by frost. It should also be noted 

 that in every year from 1925 to 1930, the disease was present in Massachusetts 

 in fields ea(!h summer and in greenhouses each fall; but there was no downy 

 mildew in the fields here in 1931 and the disease did not appear in greenhouses 

 here the following fall and winter. 



It has been suggested (52) that P. cubensis lives through the winter on 

 greenhouse cucumbers and from them is disseminated to field-grown cucumbers 

 the next spring and summer. Observations in numerous greenhouses and in 

 each of the past seven years lead to the conclusion that this does not often occur 

 in Massachusetts, however. Downy mildew has usually been found here on 

 greenhouse cucumbers in the fall, but not in the winter later than January. 

 The observations of Stone and Monahan (44) were similar. They often found 

 downy mildew on greenhouse cucumbers in the fall, but for seven consecutive 

 years they learned of no case in which it was present all winter in a INIassachusetts 

 greenhouse, and in only one year did Stone (43) find the disease in greenhouses 

 here earlier in the summer than it appeared in the field. Guba too, although he 

 has made frequent observations over a period of seven years, has not seen downy 

 mildew on greenhouse cucumbers in Massachusetts earlier in the year than Sep- 

 tember, which is of course no earlier, in fact usually a little later, than it was 

 also present in the field. It should be noted, however, that in one year, 1915, 

 Osmun (33) found the disease in greenhouses in Massachusetts in May, but this 

 was regarded as very exceptional. 



The usual absence of downy mildew from greenhouse cucumbers in Massa- 

 chusetts in winter and spring is explainable by the fact that in very few green- 

 houses in this State is that crop grown uninterruptedly throughout the winter, 

 and by the fact that the drying effect of the heating pipes in winter is ordinarily 

 such as to lower the humidity of the air below that necessary for the fungus. 

 The writer has kept P. cubensis alive and growing on greenhouse cucumbers 

 for 23 consecutive months, but to do this all winter it was necessary to inoculate 

 plants at frequent intervals and to increase and maintain the hiunidity of the 

 air above that which is common in winter in ordinary greenhouse practice. 

 Whitcomb and CJuba (51) have described the prevention of powdery mildew of 

 cucumber by greenhouse management, that is, by keeping down the relative 

 humidity of the greenhouse air. This method is no less effective against downy 

 mildew, and the disease is a menace in greenhouses only in the fall before much 

 artificial heat is used. 



'Letter of January 29, 1931 from E. F. Guba of Mass. .Vgr. Expt. Sta. 



