ORDER ERYSIPHALES (PERISPORIALES OF MOST AUTHORS) 317 



in large plantings. Roses in Europe are frequently seriously injured by 

 S. pannosa (Wallr.) Lev., but according to Salmon the similar injury to 

 roses in America is more often due to another species. In America Micro- 

 sphaera alni (Wallr.) Salmon is exceedingly common as the cause of the 

 powdery mildew on the common lilac {Syringa vulgaris L.) but it almost 

 never occurs on that host in Europe. Erysiphe cichoracearum DC. is 

 probably the most widely distributed species of powdery mildews, attack- 

 ing hosts in the most varied families. It is not in general very destructive. 

 E. graminis DC. on the contrary is often very destructive to various 

 small grains in all parts of the world. 



Family Meliolaceae (Perisporiaceae of most authors; see footnote on 

 p. 307). In this family, too, the mycelium is mostly superficial and spreads 

 in a network from the initial point of infection by conidium or ascospore. 

 The hyphae, in contrast to those of the Erysiphaceae, are usually dark in 

 color. They may form a dense crust or be more or less separate. They may 

 send haustoria into the epidermal cells or even into the next layer of cells 

 beneath or may, without producing haustoria, adhere closely to the epi- 

 dermis whose outer wall becomes more or less corroded with evidence of 

 some injury to the contents of the cell. In Meliola and some other genera 

 the creeping hyphae produce short opposite or alternate two-celled 

 branches, the terminal cell of which is enlarged, and rounded or angular. 

 These are the hyphopodia. They serve the double function of anchoring 

 the hypha in place and of producing a haustorium which penetrates the 

 cuticle to enter an epidermal cell of the host. The swollen cell of the pair 

 is homologous to the structure called the appressorium in many fungi. In 

 some genera of the order the hyphopodium may consist of only one cell. 

 In the genus Pampolysporium the mycelium and perithecia are sub- 

 epidermal, in Alina and Lasiobotrys subcuticular. In Stomatogene and 

 Piline the mycelium forms a sort of foot which enters through the stoma 

 into the substomatal chamber. Conidia produced singly on the superficial 

 mycelium are reported in one or two genera. In several genera ostiolate 

 pycnidia containing numerous conidia are the characteristic mode of 

 asexual reproduction. It has been shown that most of the reported cases 

 of conidia not in pycnidia are based upon fungi parasitic on the mycelium 

 of Meliolaceae, e.g., Arthrohotryum and Helminthosporium of each of 

 which Stevens (1918) has described several species. 



The perithecia are typically without appendages, mostly black or 

 dark brown, without ostioles in the majority of genera. The asci arise in a 

 single layer or tuft in the base of the perithecial cavity. The ascospores 

 are usually four or eight in number. They are one-celled and hyaline in 

 one genus but in most cases are two- to many-celled or even muriform 

 and either hyaline or brown. In general the ascospores do not represent 

 types that would seem to be primitive. (Figs. 100 B, 102 A.) 



