350 Minnesota Plant Diseases. 



infect the leaves of thorns or apples, where the cluster-cups are 

 formed, also in a similar manner to the above-mentioned rusts. 

 The disease may be dangerous to both ornamental junipers and 

 to orchard apple trees. 



The preventive measures are similar to those of the leaf- 

 rust of apples produced by the cedar-apples of red cedar. 



The birds'-nest rust of red cedar {Gymnosporangium nidus- 

 avis Thaxt.). This is a rust disease similar to that of the cedar- 

 apple of red cedar. When this fungus attacks the red cedar 

 an enormous number of short branches are formed. They are 

 densely bunched together and look like a miniature tree 

 perched on the limb of the cedar tree. This bush-like growth 

 is known as a witches'-broom. At a distance it is not unlike a 

 very large birds'-nest in appearance. On examining the 

 branches of the broom, one sees that the leaves are larger and 

 stand out at a greater angle from the branch than do the leaves 

 on the normal branches; they are also very sharp-pointed and 

 the general habit of the branch is more similar to that of the 

 common juniper tree. Near the base of the leaves in the dis- 

 eased portions of the cedar are found small, brownish, gelatin- 

 ous cushions of the winter spores. These appear at the end. 

 of April. The cushions, just as do the beak-like processes of 

 the cedar apples, swell up in wet weather and shrivel up again 

 when dry. Under moist conditions the winter spores germi- 

 nate and produce tiny spores (sporidia), which are carried by 

 the wind to june-berry bushes or apple trees. Here the fungus 

 again develops a mycelium and causes a rust disease which is 

 very dififiicult to distinguish from that caused on the same plant 

 by the cedar-apple rust. 



The preventive measures are similar to those recommended 

 for cedar-apples of red cedar and their le.af-rust of apples. 

 (Fig. 26.) 



Plum leaf rust (Puccinia pruni Pers.). On the lieaves of 

 many of our wild cherries and also on those of cultivated plums, 

 cherries, etc., is often produced a rust known as plum-leaf rust. 

 Only summer and winter spores are produced and they occur 

 in groups or sori on the under surface of the leaf. The sum- 

 mer spores are light brown or reddish and the winter spores 

 are darker. The spores arise in small, yellowish spots on the 



