324 Diseases of Greenhouse Crops 



drafts favor mildew. These statements seem to be 

 borne out by actual observations. Mildew often 

 starts first on rose plants facing broken panes. From 

 these, the spores are then carried by the draft to 

 other plants until the disease becomes thoroughly 

 established in the house. It is, therefore, impera- 

 tive that attention be directed to broken glass. 

 While an abundance of ventilation is necessary, 

 drafts of all sorts should be avoided. 



Mildew may also be kept in check by boiling 

 sulphur in the greenhouse for two to three hours, 

 twice a week. The house is closed tightly during 

 the operation, and ordinary flowers of sulphur is 

 placed in a kettle over a small kerosene flame, as 

 otherwise a big flame may cause the sulphur to catch 

 fire. Mildew may also be controlled by spraying 

 with potassium sulphide, at the rate of one ounce 

 of the chemical dissolved in two gallons of water. 

 The spray is only effective when used fresh. The 

 chemical should be kept in a tightly closed bottle. 



Black Spot 



Caused by Diplocarpon rosa Wolf. 



Symptoms. Black spot is often very troublesome 

 on greenhouse roses. Attacked plants lose their foli- 

 age and the general effect is a weakening of the 

 plant and the formation of stunted blossoms. The 

 spots are more or less circular, black, with a char- 

 acteristic fringed border (fig. 66, a.). Frequently 

 the leaf tissue adjacent to the spots becomes pale or 



