144 DISEASES OF CULTIVATED PLANTS 



Taphrina aurea (Sadeb.) forms large blisters on leaves of 

 the black poplar. The blisters are quite prominent, and 

 become bright golden-yellow on the concave side, due to 

 the presence of innumerable asci containing yellow spores. 



Taphrina Sadebeckii (Johans.) produces slightly elevated 

 blisters of a white or yellowish colour, on the under or, less 

 rarely, the upper, surface of alder leaves. 



Taphrina bullata (Tul.) forms convex, thickish blisters on 

 pear leaves. The blisters are green at first, then become 

 brown and covered with whitish asci on the under surface. 

 Quince leaves are also sometimes attacked. 



Taphrina ulmi (Johans.) forms blisters on leaves of the 

 common elm and the wych elm. The blisters are at first 

 green, then dull brown. Asci appear on the under surface 

 of the leaf. 



Various other species of Exoascus and Taphrina occur in 

 Britain, but are of no economic importance. 



Atkinson, 'Leaf-curl and Plum-pockets,' Cornell Univ. 

 Agric. Exp. Station, Bull. No. 73 (1894). 

 Rostrup, Taphrinaceae Danicae (1890). 

 Sadebeck, Die Parasitischen Exoasceen (1893). 

 Sadebeck, Ber. d. deutsch. Bot. Ges. (1895). 

 Tubeuf and Smith, Diseases of Plants (Engl. ed.) 1897. 

 Ward, Diseases of Plants, p. 107. 



PERISPORIACEAE 



The members of this family come under the popular 

 designation of mildews, the species commonly occurring in 

 this country forming the well-known white patches on leaves 

 of hops, peas, vine, etc. The mycelium is entirely super- 

 ficial, and sends haustoria into the epidermal cells of the 

 host-plant. In the genus Phyllactinia, however, certain 

 hyphae enter through the stomata into the intercellular 

 spaces of the leaf, and send haustoria into the cells. One 

 group, most abundant in tropical countries, has black 

 mycelium, which forms a thick film on the surface of living 

 leaves and fruit. These black species are not such destruc- 

 tive parasites as the white forms are, but depend on the 

 presence of ' honey-dew,' deposited on the leaves by insects. 

 The family is divided into two groups, as follows : 



