178 Diseases of Greenhouse Crops 



ally the first to suffer; soon, however, the entire 

 plant exhibits similar symptoms and ceases to grow. 

 Control. Where this disease is prevalent, the 

 soil should be changed, or sufficient organic matter 

 in the form of well rotted manure incorporated in 

 the beds. Malnutrition as a rule is not prevalent 

 in the greenhouse, for it is rare that a greenhouse soil 

 is lacking in humus. 



Downy Mildew 



Caused by Peronospora efusa Rabenh. 



Symptoms. The trouble is characterized by yel- 

 low spots of conspicuous size on the upper part of 

 the leaves. On the under side of the leaves, and 

 corresponding to the spots above, is seen a mat com* 

 posed of the dirty white or violet gray fruiting 

 bodies of the fimgus. The disease is often prev- 

 alent in the field. 



The Organism. Downy mildew is caused by the 

 fungus Peronospora efusa. The spores of the para- 

 site are borne on branches, which generally emerge 

 through the breathing pores or stomata of the lower 

 part of the leaf and germinate by sending out a 

 slender germ tube. Infection takes place when the 

 germ tube penetrates the upper side of the leaf, gen- 

 erally through the stomata. The winter stage or 

 oospores may be found in the affected leaves. 



Control. All infected material should be de- 

 stroyed. Water should be withheld, and plenty 



