Circular No. 51. (Revision of No. 40.) 



March, 1915. 



MASSACHUSETTS AGRICULTURAL EXPERIMENT STATION, 



AMHERST. 



DOWNY MILDEW OE CUCUMBERS. 



(Peronoplasmopara cubensis (B & C) CI.) 



By George E. Stone. 



Downy mildew, which is not difificult to distinguish from other leaf 

 fungi, is most likely to occur on greenhouse cucumbers in August and 

 September, and has never been observed by us in greenhouses in the 

 winter. The fungus occurs on the under side of the leaf, causing 

 whitish or yellowish angular spots (see Eig. i,) and these spots are 

 more prominent on greenhouse than on field cucumbers. Ordinarily, 



Fig. I. Showing characteristic spotting of 



cucumber leaf by downy mildew 



( Peronoplasmopara) 



an attack of this mildew lasts but a few weeks, and causes only an 

 insignificant spotting of the leaf. Reproduction is by means of 

 small spores, which retain their vitality longest in a moist atmosphere. 

 Downy mildew was first observed on material received from Cuba 

 in 1868, and was later noted in Wisconsin in 1882. In 1888 it was 

 found on cucumber leaves obtained from Japan, and in 1889 it was 

 found in New Jersey, causing injury under glass, at wliich time it 

 was also common on field cucumbers. During the same year it was 

 reported abundant in Florida and Texas. In 1890 it appeared in 

 Massachusetts and again in 1892, when it was found on greenhouse 

 and field crops. In New Jersey it was common in 189 1, 1892 and 

 1893, and in Connecticut and New York about the same time. 



