THE FUNGI WHICH CAUSE PLANT DISEASE 175 



Perithecial appendages indeterminate, 

 similar to the mycelium, simple or ir- 

 regularly branched 

 Perithecia containing a single ascus. . . 1. Sphsrotheca, p. 175. 



Perithecia containing several asci 2. Eiysiphe, p. 177. 



Perithecial appendages determinate 

 Appendages hooked or coiled at the 



apex 3. Uncinula, p. 180. 



Appendages dichotomous at the apex 

 Perithecia containing a single ascus. 4. Podosphaera,' p. 182. 

 Perithecia containing several asci. . . 5. Microsphsera, p. 185. 

 Mycelium with special intercellular haus- 

 toria-bearing branches which enter the 

 host by the stomata; perithecial ap- 

 pendages rigid, with a bulbous base. . . II. Phyllactinieae. 

 A single genus 6. Phyllactinia, p. 187. 



Sphaerotheca, Leveill6 



Perithecia subglobose; appendages floccose, brown or hyaline, 

 spreading horizontally and often interwoven with the mycelium, 

 simple or vaguely branched, frequently obsolete; ascus single, 

 8-spored. Five species, according to Salmon;** Engler and 

 Prantl ' give fourteen. 



S. humuU (D. C.) Burr.^*^* 



Amphigenous; mycelium usually evanescent; perithecia usually 

 somewhat gregarious, but varying from scattered to cespitose, 

 58-120 II in diameter; cells small, averaging 15 ii; appendages 

 few or numerous, usually long, 'often exceeding nine times the 

 diameter of the perithecium, more or less straight, septate, dark 

 brown throughout: variations are, short, flexuose, pearly-brown, 

 white or even obsolete. Ascus broadly-elliptic to subglobose, 

 rarely abruptly stalked, 45-90 x 50-72 /*; spores 20-25 x 12-18 /i, 

 rarely larger, averaging 22 x 15 /i. 



Conidia (=Oidium fragarise) ovate, white, membrane smooth. 



Salmon ^ has shown that subjecting the conidia of this variety 

 to low temperature, 0° two hours, increases their germinating power. 

 Sowing ascospores from the hop, on hop, Potentilla and Spirea he 

 secured infection only on the hop.*^ Conidia from hop infected 

 hop but not Spirea. 



