304 



Minnesota Plant Diseases, 



protective covering to the central spores, which have retained 

 their germinating power. Thus the functional spores obtain an 

 additional protection by means of the surrounding layer of sterile 

 spores. There are usually two or three functional spores in 



each spore mass. Upon ger- 

 mination the spores produce 

 a tube from which secondary 

 spores are formed in the man- 

 ner usual for smuts. Jensen's 

 hot water method has been 

 recommended when the fun- 

 gus appears in abundance. 

 Formalin would probably 

 prove useful. 



Powdery mildew of grasses 

 {Erysiphe graminis DC). The 

 cereal grasses are sometimes 

 seriously damaged by the at- 

 tacks of this disease. A fine 

 whitish mycelium is formed 

 on the leaves in the summer 

 time. The mycelium threads 

 derive their nourishment from 

 the skin cells of the host by 

 short sucker branches sent in- 

 to these cells. These sucker 

 branches are known as haus- 

 toria. Summer spores are 

 produced in large numbers 

 and rapidly carry the disease 

 from leaf to leaf and plant to 

 plant. These spores are 

 spherical or egg-shaped cells 

 microscopically small ; they 

 are formed in chains which stand upright, often over the 

 whole upper surface of the leaf. Toward fall the sacs with their 

 spores are formed in sac-capsules. As is usual in powdery mil- 

 dews, the capsules appear as small black spheres about the size 

 of a pin point. In the earlier stages these capsules are whitish 



Fig. 152.— Powdery mildew of grasses (Ery- 

 siphe graminis) , on wild grass-plant 

 leaves. The white coat of the fungus 

 mycelium is very conspicuous. Original. 



