202 Parasitic Moulds and Mildews. [Sess. 



of the Mucedinese, or forms which have the hyphae and 

 conidia pale or lightly-coloured, but never brown or black ; 

 and by 5 genera of the Dematiese, or species in which the 

 hyphae or conidia, or both, are of a brown or blackish hue. 



The genus O'idium includes most of the fungoid pests 

 known as " mildews," and consists mostly of the conidial 

 forms of certain species of Perisporiacese, a group of Asco- 

 mycetes. They usually form a white mycelium which creeps 

 over the surface of leaves of trees and shrubs, and bears erect 

 conidiophores, each terminated by a chain of one - celled 

 hyaline conidia. At a later period of the season the 

 perithecia are developed from the same mycelium. These 

 are of a rounded form, pale yellow in colour at first but 

 becoming brown or blackish at maturity, and are often fur- 

 nished with simple or branched appendages. In various 

 species of O'idium, however, the perfect or ascigerous con- 

 dition is as yet unknown. The following are the more 

 notable species : — 



0. erysiphoides Fr. — Common on the leaves of many species 

 of herbaceous plants, which it covers with a thin mealy or 

 powdery white coating. 



0. leucoconium, Desm. — The common mildew of Eoses, and 

 conidial stage of Sphcm^otheca pannosa. 



0. Tiickeri Berk. — The Yine mildew. 



0. farinos^im Cooke. — On leaves and green twigs of Apple- 

 trees in gardens, which it covers with a white powdery coat- 

 ing. It has been observed in Perthshire and Ayrshire, and is 

 probably not uncommon throughout the country. 



0, chrysanthemi Eab. — The Chrysanthemum mildew. 



0. aceris Eabach. — Common on leaves of the Sycamore- 

 Maple. This is the conidial stage of Uncinula aceris. 



0. monilioides Link. — The mildew of Grass, and conidial 

 stage of Erysiphe graminis. 



Another form of O'idium represents the conidial stage of 

 Podos'phcera oxyacanthce. It occurs on leaves and young twigs 

 of hawthorn, and is sometimes so abundant as to impart to 

 the hedgerows an appearance of having been sprinkled with 

 whitewash. The perfect perithecia or ascophores are not 

 usually developed very abundantly, and should be looked 

 for in late autumn. 



