92 MASSACHUSETTS HORTICULTURAL SOCIETY 



Woronichin (9) reports experiments with the fungi causing 

 powdery-mildew of the rose and peach in which negative results 

 were obtained from inoculations on the peach with the fungus 

 causing the disease of rose. He also states that a study of the 

 perithecia, asci, and spores of the fungi from the two hosts show^ed 

 differences in their dimensions. He concludes that the biological 

 and morphological differences noted are sufficient to separate the 

 species into the varieties S. pannosa rosae and »S. pamiosa persicae. 



Morphology. Under the microscope the white patches on the 

 rose plant are seen to consist of a mould-like growth (mycelium) 

 composed of slender white threads with numerous branches which 

 form a net-work over the surface of the leaf. At various points 

 upright branches are developed which bear chains of egg-shaped 

 spores. These spores are easily detached and He in masses giving 

 the older spots a powdery appearance. They are produced 

 throughout the year under glass, but only during the summer on 

 plants growing in the open. 



At various points the mycelial threads are attached to the 

 surface of the host, minute branches called haustoria being sent 

 into the outer cells of leaf or stem from which the fungus obtains 

 its food supply. The cells into which the haustoria are sent may 

 be stimulated at first but are killed sooner or later. 



Somewhat rarely, and probably only out of doors, ascospores 

 are produced in spore-cases called asci which in turn are born 

 singly in dark fruit-bodies (perithecia) embedded in the felt-like 

 growth on stems, thorns and leaves. These ascospores can live 

 over winter out-of-doors and may serve as the inocula the following 

 spring. 



Life history of the parasite. 



Norton (10) states that it is probable that the mycelium of the 

 fungus is able to live over winter out-of-doors in the buds of roses. 

 Others assert that the mycelium is perennial, reappearing in suc- 

 cessive years on the same shoots of infected plants. Salmon notes 

 that in specimens examined the fresh centers of disease which appear 

 in the spring did not occur at the places where the fungus grew in 

 the previous year. The fact that sexual spores are somewhat rare 



