S76 Diseases of Greenhouse Crops 



leaves of the plants should be kept dry; all the water 

 should be applied with the hose on the ground, a 

 method that also avoids the splashing of soil par- 

 ticles. 



Twig Blight 



Caused by Fhytophthora syringa Kleb. 



This disease was found by Klebahn * to be very 

 serious in propagating beds in Germany. The causal 

 organism attacks and kills the twigs at a distance 

 of several intemodes above ground. The flower 

 buds from the affected shoots fail to develop alto- 

 gether. However, new shoots may appear below the 

 affected area. The disease is of no economic sig- 

 nificance in the United States. 



Powdery Mildew 



Cimstdhy Microsphara alni (Wal.) Salm. 



Symptoms. Powdery mildew is perhaps one of 

 the commonest troubles of forced lilacs. The dis- 

 ease is characterized by white powdery patches on 

 the surface of the leaves and stems of the plant. The 

 causal fungus attacks a large number of outdoor 

 plants besides the lilac, as chief of which Lonicera, 

 Alnus, Betula, Quercus, Carya, Castanea, Juglans, 

 and Platanus may be mentioned. Outdoor lilac 

 often suffers greatly from this mildew. 



The Organism. The perithecia are either scat- 



* Klebahn, H., Krankheiten des Fielders (Berlin) : 75, 1909. 



