436 Diseases of Economic Plants 



TULIP 



Gray-mold (Botrytis). — All parts of the host except the 

 roots are affected. The disease was apparently imported 

 from Europe and is now established throughout the United 

 States. 



VERBENA 



Powdery-mildew (Erysiphe cichoracearum DC, Oidium). 

 — Powdery-mildew is quite common upon verbena under 

 glass or in the open. 



The use of potassium sulphid, 1 ounce to 3 gallons, twice 

 a week has proved effective. 



VINCA 



Rust {Puccinia vincm (DC.) Berk.) was apparently im- 

 ported from Europe and does occasional damage. Vinca 

 leaf-spot {Sphceropsis vincce Sacc.) occasionally injures leaves 

 and stems. 



VIOLET, PANSY 



Spot-disease '*'^*"' {Alternaria vioIcb G. & D.). — This is one 

 of the widespread and destructive violet diseases. It attacks 

 the plant at any stage of its growth and upon any green part, 

 but is most destructive upon the foliage. It first appears as 

 small, definite, circular, greenish or yellowish- white spots, 

 varying from mere dots to 1 mm. in diameter. The light cen- 

 ter is surrounded by a narrow ring, usually dark brown to 

 black, but turning lighter with age. As the spot enlarges the 

 freshly diseased tissue is usually watery and translucent, 

 and the center changes to yellow or gray- white and may 

 fall out. Frequently by the combined effects of several 

 spots the whole leaf is destroyed. If the air is damp, a 

 thin coating of rusty-colored spores may be seen in the old 

 spots. 



Damp, warm, cloudy, summer weather, unclean houses, 



