Minnesota Plant Diseases. 363 



Witches'-broom of cherries [Exoascus cerasi (Fckl.) Sad.]. 

 One not infrequently meets with wild cherry trees which have 

 the peculiar disease known as witches'-broom. On account of 

 the abnormally large number of branches developed in the af- 

 fected part of the tree, a bush-like object is produced which looks 

 not unlike some foreign shrub, parasitic on the cherry tree. In 

 this broomed portion the mycelium of the fungus, which is a 

 close relative of the fungi of plum pockets and peach leaf-curl, 

 may be found. This mycelium is perennial. The leaves may 

 be considerably distorted, resembling curl, and over the surface 

 of these leaves the spores are formed in sacs. These sacs are 

 arranged in a palisade on the surface and give to the latter a 

 greyish-white appearance. There are about eight spores in each 

 sac. They germinate directly to an infection tube. The 

 broomed portions should be removed and burned. 



Plum scab (Cladosporium carpophilum Thiim.). This is an 

 imperfect fungus. Many of the species of the same genus are 

 exceedingly common mold-like saprophytes forming black, moldy 

 growths on dead sticks, stems, seeds, etc. The plum scab is 

 found on plums and cherries. Spots arise on the fruits shortly 

 before ripening. These spots are covered with brown or olive 

 growths of fungus threads from which dark spores are pinched 

 off. The spots may increase in size and number until the whole 

 fruit is covered. The latter then shrivels and is rendered unfit 

 for the market. Many varieties of plums are attacked and the 

 wild American plum seems to suffer as much if not more than 

 any other variety. The fungus is probably one of the black fun- 

 gus group of the sac fungi but its winter-spore stage has not yet 

 been discovered. It is possible that the fungus lives over the 

 winter in a sterile thread condition on the branches and bark of 

 trees. The fungus has been observed in Minnesota but the ex- 

 tent of its damage is not yet known. 



Spraying with bordeaux has been recommended. Several 

 treatments should be given, beginning when the flowers are well 

 set. Diseased plums should be destroyed. 



Black rot of apple (Sphaeropsis malorum Peck.). The black 

 rot attacks apples usually in the ripening stages or when the fruit 

 is in storage. It also attacks the leaves, forming reddish brown 

 spots, or the twigs, where blackish spots are produced. The 



