184 



THE FUNGI WHICH CAUSE PLANT DISEASE 



Fig. 133.— p. oxycan- 

 t h a e, appendage 

 tips. After Sal- 

 mon. 



Spirea and Vaccinium. Especially damaging to cherry and apple. 

 Throughout the northern hemisphere. 

 P. tridactyla (Wal.) De Bary is considered by Salmon ^'^ as a 

 variety of the last species. Hosts: Plum and 

 other species of Pnmus and of Spirea. 



Similar to the preceding in habit and general 

 character but differing in more critical charac- 

 ters. Perithecia 70-105 fi; cells 10-15 /i; ap- 

 pendages 2-8 usually 4, 1 to 8-times the 

 diameter of the perithecium, apical in origin, 

 more or less erect, apically 3-5 or 6-times 

 dichotomously branched, primary branches usually more or less 

 elongate, sometimes slightly recurved; asci globose or subglobose, 

 60-78 X 60-70 n; spores 8, 20-30 x 13-15 /*• 

 Chiefly European but found also in Asia and America. 

 P. leucotricha (E. & E.) Salm. 



Mycelium amphigenous, persistent, thin, effused; perithecia 

 densely gregarious, rarely more or less scattered, 75-96 /t, sub- 

 globose, cells 10-16 n; appendages of two kinds, one set apical 

 the other basal; apical appendages 3-11 in number, more or less 

 widely spreading, or erect-fasciculate, 4 to 7-times the diameter 

 of the perithecium, apex undivided and blunt or rarely once or 

 twice dichotomously branched, brown basally; basal appendages 

 nearly obsolete or well developed, short, tor- 

 tuous, pale brown, simple or irregularly branched; 

 ascus oblong to subglobose, 55-70 x 44-50 ft, 

 spores 22-26 x 12-14 fi, crowded in the ascus. 



Conidia (=Oidium farinosum): ellipsoid, trun- 

 cate, hyaUne, 28-30 x 12 fi. 



Primarily American but occurring in Europe 

 and Japan. A most serious pest of the apple. 

 This and P. oxyacanthae, the apple mildews of 

 America, have been variously treated by writers ^°- P*"7^- ^™ 



,-, , t m, cotncna, 



so that the literature presents an almost inex- 

 tricable tangle as has been pointed out by Pam- 

 mel and by Stewart,^'^ Podosphsera oxyacanthse being fre- 

 quently reported instead of P. leucotricha. Sphserotheca mali 

 and Podosphaera oxyacanthae have also been much confusedj due 



pendage tips. 

 After Salmon. 



