212 



The Ohio Journal of Science [Vol. XXI, No. 6, 



The most common powdery mildew in Ohio is Erysiphe 

 dehor aceanim DC. This species grows on a large number of 

 hosts and may be collected from mid-summer until late autumn. 

 We get this fungus about Oxford, in June, for study in the lab- 

 oratory at Miami University. From this time until the host 

 plants are killed by frost, the number of hosts increases, and 

 material for study may be had directly from the field. 



Nearly as abundant and much better known is Micro- 

 sphcsra alni (Wallr.) Salm, which comes to maturity a little 

 later and continues in condition for study from some of its 



Fig. 1 



Fig. 2 



Pig. 1. A perinthecium of Sphaerotheca humuli (DC.) Burr, on leaves of Rosa 

 blanda — a form with short appendages. X 150. 



Fig. 2. Mycelium, cohidiophores and conidia of Erysiphe polygoni, (DC.) on 

 leaves of garden peas — largely diagramatical. X 150. 



hosts quite as late in the season. This species is a beautiful 

 object for microscopic study and is the form of powdery mildew 

 most commonly used for laboratory study. However, the 

 appendages, which are wonders of natural beauty, defy the 

 best powers of the artist to reproduce. No drawings that I have 

 seen do them justice, and the sketches ordinarily made by 

 students are far from satisfactory. Some Americans who have 

 known this species from the studies of Dr. T. J. Burrill are 

 doubtless reluctant to follow the revision of Dr. E. S. Salmon. 

 Yet the studies of this worker bear the stamp thoroughness, 

 and it seems best to part company with our American pioneer 

 at points of divergence between him and the later student of 

 the group, with respect to this and other species of the 

 Erysiphacece. 



