PEAR DISEASES 



343 



by brown depressed scars. In the late spring dark-brown, 

 wedge-shaped horns, which later become yellowish, are devel- 

 oped. 



Cause. 



The eastern pear rust is caused by the fungus Gymnospo- 

 rangium globosum. Like other rust fungi it has two distinct 



host plants, the i 



pear and the red 

 cedar. If the spots 

 of an affected pear 

 leaf be examined, 

 the upper surface 

 will show small 

 black fruiting 

 bodies known as 

 pycnia. These do 

 not function in the 

 life-history of the 

 pathogene so far as 

 is known, but are 

 mentioned for their 

 diagnostic value. 

 They are found in 

 the early summer. 

 Shortly after their 

 appearance another 

 type of fruiting body develops on the lower surface opposite 

 the pycnia. These structures are known as aecia. Occasion- 

 ally they are found on the upper side of a leaf and even on the 

 petioles. Ordinarily they are grouped irregularly over the 

 affected area, although sometimes they are arranged in two 

 rows, one on either side of the midrib. JScia may also develop 

 on the apple, hawthorn and mountain-ash. These bodies are 

 deep cup-shaped, with their walls split part way to the base, 



FIG. 94. Eastern rust on pear-fruits. 



