272 Minnesota Plant Diseases. 



growth the mycelium permeates the wood and then the grow- 

 ing zone of the tree and the bark are destroyed. The fungus 

 continues to live in the dead wood as a saprophyte and under 

 such conditions produces its fruiting bodies. At first, small 

 clusters of soft, bright red, button-like cushions arise. From 

 the surface of these cushions are produced tiny summer spores, 

 which are successively pinched off from upright threads on the 

 surface of the cushion. These spores are capable of immediate 

 germination and may spread the disease very rapidly. After 

 the formation of these spores has gone on for some time, they 

 decrease in number and finally cease to form. There then 

 appear upon the same cushion small, red, pear-shaped to spher- 

 ical protuberances, which contain a central cavity and a pore- 

 like opening to the exterior. These are the sac-spore capsules. 

 At the base of the cavity are found long, cylindrical sacs, each 

 of which contains eight spores. The opening of the capsule is 

 lined internally with hairs which clothe the whole canal, leading 

 from the cavity to the exterior. The sac-spores and the sacs 

 are extruded through this opening. The spores will germinate 

 under favorable conditions and will again produce a mycelium 

 in the wood. 



The treatment of this disease is similar to that of wound par- 

 asites in general, i. e., burning of the infected twigs and wood 

 and clearing up of felled wood to prevent the growth of wild 

 spores. 



Leaf blister of oak (Species of Taphrina). This fungus is 

 a relative of the fungi of plum pockets and leaf curls and of the 

 witches'-broom fungus of cherry and birch. The spores are 

 produced in sacs which are arranged in a dense palisade on the 

 tinder surface of the leaf. The leaf is usually distorted in a blis- 

 ter-like fashion, whence the common name of the disease. The 

 red oak has been found in this state attacked by this fungus, 

 though not to any serious extent. The removal and burning 

 of the affected parts would be advisable to prevent a severe re- 

 currence of the disease. 



Willow blight or powdery mildew [Uncinula salicis (D. C.) 

 Wint.]. The blight of willows is an exceedingly common dis- 

 ease not only of the willows, but also of the poplars, cotton- 

 woods and birches. The mycelium is usually very abundant and 



