182 



THE FUNGI WHICH CAUSE PLANT DISEASE 



perithecia which had been exposed to the weather until spring 

 and were then placed in a hanging drop culture afforded spores, 

 some of which grew though many of them burst as they emerged 

 from the perithecium. Ascospores are known to have remained 

 viable for at least eighteen months.'"* No successful infections 

 were made from ascospores. 



Though perithecia are frequently found in America they were 

 not found in Europe imtil 1892 '"' and are now found there but 

 rarely. It appears that in their absence the fungus hibernates in 



.1 B 



Fia. 131. — U. necator. Photomicrographs of perithecia oq surface of leaf. 



A, Magnified 8 times. B, Magnified 35 times. After Bioletti. 



specially resistant cells of the mycelium which develop within 

 knotty swellings near the haustoria.''" 



U. salicis (D. C.) Wint. on willow and poplar in Europe, Asia, 

 and America, U. aceris (D. C.) Sacc. and U. circinata C. &. P. 

 on maple are common species. U. flexuosa Pk. occurs on Ji^sculus. 

 and elm, U. clandestina (Biv.) Schr. on elm, U. pmnastri (D. C.) 

 Sacc. on species of Prunus, especially P. spinosa in Europe. tJ. 

 mori Miy. is on Morus in Japan.'"' Several other species of small 

 importance affect numerous hosts. 



Podosphaera Kunze (p. 175) 



Perithecia globose or globose-depressed; ascus solitary, sub- 

 globose, 8-spored; appendages equatorial or apical, dark-brown 

 or colorless, dichotomously branched at the apex, branches simple 



