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- 
Fic. 324. — Powdery Mildew of the Hop. Below, diagrammatic draw- 
ing of a scction 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. 
lium 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 
