Ill] ERYSIPHALES 79 



Aerial mycelium dark-coloured or rarely absent. Peri- 

 thecia globose or ovoid, without appendages. 



Conidia not of oidium type. Perisporiaceae. 



Aerial mycelium dark-coloured or absent. Perithecia 

 flattened or shield-shaped, with an ostiole at the 

 apex, without appendages. 



Conidia absent. Microthyriaceae. 



A further and probably important distinction which separates the 

 Erysiphaceae from the other two families is the character of the ascus and 

 ascospores. In Erysiphe and its allies the ascus is more or less globose, the 

 spores are always continuous and colourless, and the number of spores in 

 the ascus is frequently reduced. In the Perisporiaceae, on the other hand, 

 the ascus is relatively elongated and sometimes cylindrical, and the spores 

 are commonly two or more celled and often dark-coloured. Most species 

 agree, however, with the Erysiphaceae in the absence of paraphyses. The 

 Microthyriaceae approach the Perisporiaceae rather than the Erysiphaceae 

 in the characters of the ascus and spore, but, as already stated, they are 

 clearly distinguished by the curious flattened perithecium. 



It remains for future research to determine whether these families should 

 be grouped together, or whether the Perisporiaceae and Microthyriaceae are 

 true Pyrenomycetes and should be placed in that alliance. In our present 

 extensive ignorance of the initiation and development of their perithecia, it 

 seems unwise to remove them from their traditional position in the neigh- 

 bourhood of the Erysiphaceae. 



Erysiphaceae 



The members of the Erysiphaceae are popularly known as white or 

 powdery mildew or blight. They have a practically worldwide distribution, 

 but have been recorded especially in Europe and the United States. 



They are obligate parasites on the leaves or young shoots and inflores- 

 cences of flowering plants. The germinating conidium or ascospore gives 

 rise to an abundant, superficial, septate mycelium of uninucleate cells, which 

 ramifies over the leaf in every direction, forming a white web-like coating, 

 and sends haustoria into the epidermal cells of the host. In the simplest 

 cases the haustorium is a slender tube which swells up inside the host cell; 

 in other species it is branched, sometimes forming finger-like processes, and 

 frequently there is an external disc or swelling (adpressorium), from which, 

 or from the mycelium in the neighbourhood of which, the haustorium proper 

 arises and pushes into the epidermal cell. As a rule the fungus does not 

 penetrate further, but in Erysiphe Graminis Salmon was able to induce 

 •endophytic growth and nutrition by allowing the conidia to germinate on 

 wounded leaves ; in Phyllactinia Corylea the branches of the aerial mycelium 



