116 THE ASCOMYCETES 



world-wide in range. Moreover Salmon (1900) lists it as having 

 140 hosts belonging to 36 families. Other widely dispersed 

 species are Erysiphe polygoni^ having 355 hosts belonging to 42 

 families, and E. cichoraceariim, having 280 hosts belonging to 

 27 families [Reed (1913)]. On the other hand, Fodosphaera bi- 

 iincmata is limited to \\'itch hazel, Hamamelis virgijiiana; and Un- 

 cinula gemcidata, to red mulberry. Moms rubra. Uncimda cir- 

 chiata is confined to maples. Infrequently, several species of 

 mildews occur on a single host; on Ribes grossularia, for example, 

 Sphaerotheca mors-uvae is most common, but Microsphaera 

 grossidariae and Fhyllactinia corylea may also occur. 



liJiportant species and their activities. Since these fungi are 

 all obhgate parasites, they can be expected to cause a great deal 

 of damage if they appear on plants of economic importance. 

 Among destructive pathogens are Uncimda necator on grape, 

 known in Europe since 1847 as Oidium tuckeri. Its cleistothecial 

 stage was not found there for over 40 years, hibernation being 

 accomplished by means of special resistant mycehal cells. Cleis- 

 tothecia are commonly developed, however, in California. 

 Sphaerotheca mors-uvae is very destructive to gooseberries, S. 

 humidi to hops [Blodgett (1913)], and 5. pannosa to roses, 

 especially climbing varieties. Erysiphe graminis is widespread 

 on oats, wheat, barley, rye, and bluegrass; E. polygoni is very 

 prevalent on clovers but seldom, if ever, forms cleistothecia in 

 the southeastern United States. Erysiphe cichoracearinn seriously 

 attacks cucumbers and squashes, especially those grown in green- 

 houses. Fodosphaera oxyacanthae is very damaging to cherries 

 and may also attack apples and peaches, especially when the trees 

 are grown in nursery io\ys. 



Meliolaceae. The Meliolaceae, or sooty molds, resemble the 

 Erysiphaceae closely in structural features, but, as the common 

 name implies, the mycelium is dark in color instead of being 

 white. The hyphae may be so profuse as to form crusts. They 

 may adhere to the epidermal cells so intimately as to absorb their 

 food, may attach themselves by means of holdfasts, hyphopodia, 

 or may send haustoria into the host tissues. Lasiobotrys is sub- 

 cuticular. Stomatogene and Pihne form foot-like structures 

 within the substomatal cavities. 



Sooty molds occur most abundantly in the tropics. Conidia 

 are lacking in most species. Conidia of the Helminthosporium 



