220 



THE FUNGI WHICH CAUSE PLANT DISEASE 



P. ribesia (Pers.) Sacc. is found in Ribes twigs and 



P. virgultorum (Fr.) Sacc. on birch. Both are European. 



P. agaves occurs on the maguey.^' 



Dothidea Fries, distinguished from Plowrightia by its colored 

 spores, contains some twenty-five species which occur on twigs of 

 Sambucus, Rosa, Buxus, Betula, Juniperus, Quercus and many 

 other woody plants. 



D. rosae Fries, is common as the supposed cause of a rose tumor. 



D. nozia Ruhl. causes an oak twig disease in Germany.'^ 



Phyllachora Nitschke (p. 217) 



Stroma sunken, united to the parenchyma and epidermis of 

 the host leaf, rarely erumpent, encrusted, usually jet-black; peri- 

 thecia sunken in the stroma, rather numerous, 

 with more or less distinct ostioles; asci cylindric, 

 8-spored; spores ellipsoid or ovate, 1-ceIled, 

 hyaline or yellowish; paraphyses present. 



More than two hundred species, largely 

 tropical, are known on a wide" range of hosts. 

 All are leaf parasites. 



P. graminis (Pers.) Fcl. Stromata variable in 

 size and form, causing conspicuous black spots 

 on leaves of the host; perithecia immersed, os- 

 tiolate; asci short-pedicillate, cylindric, 70-80 x 

 7-8 n; spores obliquely imiseriate, ovoid, hya- 

 line, 8-12 X 4-5 fi; paraphyses filiform. No 

 conidia are known. 



This fungus occurs on many grasses and 



Fig. 157. — P. gram- 



inia. B, stroma in 



section; C, an ascus j ..i i- i . • • , ., 



and spores. After sedges With shght mjuTy to them. 



^"'*^- P. pomigena (Schw.) Sacc. produces black 



spots, scarcely ever above 5 mm. in diameter, on apples, especially 

 the Newton Pippin, in the eastern United States. Little is 

 known of the species. 



P. trifolii (Pers.) Fcl. causes small black spots 1 mm. or less 

 in diameter on clover leaves; asci cylindric; spores uniseriate, oval, 

 hyaline, 8-10 x 5-6 fi. 



Conidia (=Polythrincium trifolii) precede the asci on the stro- 



