32 INSECT AND FUNGOUS ENEMIES OF THE APPLE. 
every year, but as a rule destructive outbreaks occur at rather wide 
intervals, depending upon weather and other conditions. Since the 
fungus causing the disease passes the winter on cedar trees and infec- 
tion can take place only with spores from that source, the disease is 
confined to localities where the red cedar occurs. 
CHARACTER OF THE INJURY. 
On the leaves the cedar rust disease manifests itself at first as 
minute, pale-yellow spots, which slowly enlarge, finally attaining 
Fic. 17.—FWoliage of York Imperial apple affected with cedar rust. 
one-eighth to one-fourth inch in diameter and becoming orange- 
colored, with small black dots in the center. (See fig. 17.) Some 
time later, on the under side of the leaf beneath each spot the tissues 
become swollen, forming a blister or cushion on which cup-shaped 
spore receptacles are produced. These cluster cups are small tubular 
projections with fringed margins. 
When only a small percentage of leaves is affected, as is frequently 
the case, no noticeable damage results, but susceptible varieties adja- 
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