1893.] PUBLIC DOCUMENT — No. 33. 225 



2. The Powdery Mildew. — Erysiphe Cichoracearum DC. (Plate 



TIL) 



In the last report of this department* an account was 

 given of this disease, wliicli was then described for the 

 first time in America. No new facts concerning it liave 

 since been learned ; but as drawings intended to accompany 

 that account were unfortunately lost in the mail, new ones 

 have been prepared for publication in the present report, 

 and a brief recapitulation of some facts concerning it may 

 serve to explain these figures to those who have not the 

 previous report at hand. 



Like all the powdery mildews, this plant is a surface 

 parasite, its vegetative threads running over the exterior 

 of the host (fig. 14) . The epidermal cells of the latter are 

 pierced at intervals by short, thick branches, sent down- 

 ward from the vegetative threads, whose ofiice is the absorp- 

 tion of nourishment for the fungus from the invaded host 

 cells (fig. 14, h). From the vegetative threads are also 

 produced erect spore threads (fig. 14, sp.), which bear the 

 summer spores of the fungus. These are cut off in basipetal 

 succession by cross partitions, the apical spore being thus 

 always the oldest and falling from the chain as soon as it is 

 fully ripe (fig. 15). These ripe summer spores (fig. 16) 

 germinate quickly, and serve to spread the fungus rapidly. 

 The fact was mentioned in our previous report that the 

 spores on diseased cucumber leaves, received from Fitch- 

 burg, Mass., and from Ithaca, N. Y., respectively, did not 

 fully correspond in size and form, and may possibly repre- 

 sent diflTerent species of fungi. Their differences may be 

 seen by comparing the figures given. While all the other 

 figures are taken from the Massachusetts form, fig. 17 repre- 

 sents the larger summer spores of the New York one. 



Before the publication of the previous account of this 

 fungus, only its summer spore form was known on the 

 cucumber, and its specific identity, which depends upon the 

 winter stage, was therefore undetermined. The appearance 

 of the winter form in our greenhouse has made this identifi- 

 cation possible, and has shown that the Massachusetts form, 



* Ninth Report Massachusetts Experiment Station, p. 222. 



