Minnesota Plant Diseases. 



273 



conspicuous in late summer and forms a thick, whitish covering 

 over the leaf. On this arise the spore-sac capsules as tiny, 

 black, pinhead-like bodies, which often occur in great numbers. 

 The mycelium may cover both sides of the leaf and may some- 

 times be found on almost every leaf of the affected tree, though 

 it is usually most abundant or entirely confined to the leaves 

 of the lower branches. All species of willow are attacked. 



FIG. 134. Powdery mildew of willow leaf (Uncinula salicis). The minute black spots are 

 the spore-sac-capsules and under these can be seen the whitish mycelial coat of threads. 

 Original. 



The sac-capsules are large compared with most other Min- 

 nesota powdery mildews. They are black in color and have a 

 ring of numerous, colorless, thread-appendages, each of which 

 terminates in a single-pointed hook, in a manner similar to 

 that of the vine powdery mildew. Each capsule contains at 

 least several sacs, in each of which are found eight spores. The 

 summer spores occur in the manner usual for the powdery mil- 

 dews, producing a powdery, starchy, dust-like coating. 



18 



