POWDERY MILDEWS (PERISPORIALES) 373 



tree is killed. The deeply buried mycelium is not reached by 

 sprays, and the total destruction of the infected trees is the only 

 available method of checking the disease. 



Powdery Mildews (Perisporiales) . This group includes 

 many Fungi, but they are all very similar in their habits. The 

 mycelium commonly occurs on the surface of leaves, but some- 

 times on the stems and fruits of the higher plants. The myce- 



FIG. 324. Powdery Mildew of the Hop. Below, diagrammatic "draw- 

 ing of a section of a Hop leaf, showing the superficial mycelium which has 

 grown haustoria into the epidermal cells, and produced erect conidiophores 

 bearing chains of conidia (X about 50). Above, epidermal cell, hypha, and 

 invading haustorium more highly magnified. From Bulletin 328, Cornell 

 University Agr. Exp. Sta. 



Hum forms quite noticeable powdery patches. The asci are 

 produced in closed ascocarps called cleistothecia. In Figure 323 

 is shown the mildew of the Apple. 



The Lilac Mildew (Microsphaera) is the one most commonly 

 observed of the Mildews. Often in late summer and autumn ; 

 the leaves of the Lilac are so generally covered with the whitish 

 dusty-looking patches, that the entire bush appears covered with 

 street dust. But there are also Mildews that occur on fruit 

 trees, Roses, Gooseberries, Peas, and other cultivated plants, 

 which do considerable damage. From the superficial hyphae 



