3 



The Poivdery Mildew of the Cucumber is due to the presence of 

 a fuugiis which has been long known, but which has not been hereto- 

 fore reported as occurring in America, so far as I can learn. It has 

 been received during the past season, on leaves of greenhouse cucum- 

 bers, from Dr. Jabez Fisher of Fitchburg and from Prof. L. H. Bailey 

 of Cornell University. The fungus, as has been said, -attacks the 

 leaves, on whose upper surfaces it forms at first rounded spots which 

 appear like blotches of a white powder. These spots gradually en- 

 large and become confluent until the leaf is practically covered. 

 Those parts of the leaf which are attacked soon turn yellow and 

 finally become dead and dry. Under favorable conditions the disease 

 spreads quite rapidly and is very destructive. 



The fungus which causes the trouble is known as Oidium erysiphoides 

 Fries, var. Cucurbitarum, and is the conidial or summer spore stage 

 of one of the fungi known as Powdei-y Mildews. It is impossible to 

 say certainly to which of the perfect or winter spore forms of the 

 group it belongs. 



It has been found by Prof. Bailey and by Dr. Fisher that the 

 fungus may be kept in check by frequent spraying with a solution of 

 sulphide of potassium (liver of sulphur) in water. The proportion 

 usually given is one ounce of the sulphide to two gallons of water, 

 but both Dr. Fisher and the writer have found this solution injurious 

 to the foliage and the young cucumbers. A preparation containing 

 an ounce to three gallons is certainly strong enough and one with an 

 ounce to four gallons is probably so. 



As recommended for the lettuce disease, a house in which this 

 fungus has been troublesome should be thoroughly cleaned and fumi- 

 gated before the next season's crop is started. 



All persons interested are urged to send to this Department speci- 

 mens of any plants attackefl by fungus diseases, in the interest of a 

 better knowledge of these maladies, and that instructions how to 

 avoid or to treat them may be furnished them. Such instructions 

 will be promptly sent on request. 



Jas. Ellis Humphrey, 



Professor of Veg. Physiology- 



