Minnesota Plant Diseases. 361 



Powdery mildew of apple [Podosphaera leucotricha {E. and 

 E.) Salmon]. This mildew attacks apples, pears, thorns and 

 juneberries. It affects chiefly the seedling plants by injuring the 

 leaves. It forms a fine, whitish, powdery mycelium on the sur- 

 face of the leaves. The small black capsules appear in late sum- 

 mer. Summer spores are produced in the manner usual for the 

 powdery mildews. The small, black sac-cap.sules are produced 

 in late summer. They are provided with appendages, which 

 form, a crown on the summit. The appendages are branched 

 several times in a forking manner. The capsules, when broken 

 open, are seen to contain each a single spherical sac, enclosing 

 about eight spores. 



Spray seedlings with bordeaux or ammioniacal solution of 

 copper carbonate shortly after the buds have opened and at inter- 

 vals of ten to twelve days for two months. 



Powdery mildew of plums and cherries [Podosphaera tridac- 

 tyla (Wall.) DeBy.]. This mildew attacks leaves of plums and 

 cherries. It is found chiefly on young plants. It forms a fine 

 mycelium on the surface of the leaves^ The small, black fruit- 

 ing-bodies appear in the fall. These sac-spore capsules are pro- 

 vided with appendages which resemble those of the powdery 

 mildew of apples. Each capsule contains a single sac with eight 

 spores. 

 For preventives see Powdery Mildew of Apple. 



Plum pockets (Exoascus pruni Fckl.). Plum pockets are 

 very familiar objects to all raisers of plum trees. Cherries are 

 also affected by a similar disease. In this disease the fruit is 

 peculiarly enlarged to considerably more than its natural size and 

 is at first yellowish, becoming grey as a coat of spores form on 

 the surface. The diseased fruit has no stone, the entire fruit 

 wall being soft. The mycelium, permeates the tissue of the 

 pocket and forms spores in sacs on the surface. The mycelium 

 may live over the winter in the twigs of the plants, so that a plant 

 part once infected may produce pockets yearly. This yearly pro- 

 duction of pockets does not always take place, but they may ap- 

 pear only every other year. The disease does not seem to spread 

 with great ease, for it has been observed that trees neighboring 

 on a diseased one may remain free from pockets for a long time. 

 The spores are borne in elongated sacs which are arranged in 



