ELM DISEASES 



153 



every season on the leaves in late autumn just before the 

 leaves fall. 



The first indications of the spots are seen on the upper surface 

 of the leaf. The dead leaf-tissue is grayish, and either scat- 

 tered over the spot or 

 grouped in the center are 

 one or more black pustules 

 (Fig. 21). The under sur- 

 face of the leaf shows no 

 evidence of the spot until 

 later in the season, when 

 brown dead areas appear 

 with a few raised pustules. 



After the leaves fall to 

 the ground, fruiting-bodies 

 containing ascospores de- 

 velop which are the source 

 of primary infection the 

 following spring. The 

 pycnidia of many species 

 of fungi are found on elm 

 leaves and it is not known 

 which one of these is con- 

 nected with the Gnomonia. 

 For a further discussion of 

 leaf-spots and their con- 

 trol, see page 27. Fig. 22. — Powdery mildew of elm. 



Powdery Mildews 



Caused by Uncinula macrospora Peck, Microsphoera alni (Wallr.) 

 Salmon and Phyllactinia corylea (Pers.) Karst. 



Three species of the powdery mildew fungi attack the leaves 

 of elms in the United States. In the case of all three species 

 the characteristics, so far as visible to the unaided eye, are 



