THE FUNGI WHICH CAUSE PLANT DISEASE 



183 



and straight or swollen and knob-shaped; appendages rarely of 

 two kinds, one set apical, brown, rigid, unbranched or rarely 

 1 to 2-times dichotomous at the apex, the other set basal, short, 

 flexuous, frequently obsolete. 



Salmon recognizes four species; Engler and Prantl ' seven. 



P. oxyacanthae (D. C.) De Bary io«' i»» 



Amphigenous; mycelium variable, persistent in thin patches 

 or evanescent; perithecia scattered or more or less gregarious, 

 subglobose, 64-90 yu; cells 10-18 /x; appendages spreading more 



Fig 132 — P owacanthje a penthecium sliowing the appendages with 

 tip&j b, the one large ascut. containing eight spores; c, the summer 

 spore-form; d, a spore germinating in water. After Longyear. 



or less, equatorial, variable in number and length, from 4-30 in 

 number and from J^-6 or even 10-times the diameter of the 

 perithecium, usually unequal in length, dark brown for more than 

 half their length from the base, apex 2 to 4-times dichotomously 

 branched, branches usually short and equal, ultimate branches 

 rounded, swollen, and more or less knob-shaped. Fig. 133; ascus 

 broadly obovate, or subglobose, 58-90 x 45-75 /x; spores 8, rarely 

 6, 18-30 X 10-17 fi. 



Conidia ( = Oidium cratsegi). 



Salmon finds the species very variable but cannot set aside as 

 separate species P. tridactyla and P. myrtillina as is done by some 

 authors. On some hosts perithecia are rare. It is thought that 

 the mycelium remains alive over winter. 



Hosts: Amelanchier, Crataegus, Diospyros, Prunus, Pyrus, 



