Nebraska I'orcst I'ni.yi. II 85 



that produces the conidiospores the perithecia are forme:!, later 

 in the summer, usually in more or less restricted areas or some- 

 times scattered well over the whole surface of the leaf. The 

 perithecia are at first tiny white specks which gradually deepen 

 in color through light yellow, orange, brown, and finally become 

 black at about the time that the leaves begin to fall, or sooner. 

 When they occur in colonies the youngest individuals are at the 

 periphery of the colony and the oldest ones near the center of 

 the radiate colony. This character is often shown very distinctly 

 in the Mini .Mildew. The mature perithecia are usually dark 

 brown or black and in shape are spherical or lens-shape;!. In 

 many of the species certain cells of the outer wall of the perithe- 

 cium are extended into peculiar appendages the tips of which are 

 characteristically modified for the various genera and species. 

 The number of asci per perithecium varies from ore to many. 

 The asci are usually oval or ovoid cells and contain from two to 

 eight simple, hyaline spores. The species are mostly leaf para- 

 sites. 



Microsphaera 



The perithecia in this genus are globose or globose-flattened. 

 T! ere are several, 2-8 spored asci. The appendages are free 

 from the mycelium, branched at the tip in a definite form, usually 

 several times dichotomously divide:! and ornate. 



y.icrosphaera aim (Wallr.) Wint. 



The mycelium of this species covers both sides of the in- 

 fected leaves as thin effused areas that are either evanescent or 

 persistent. The perithecia are scattered or densely gregarious, 

 66-110 microns in diameter. The appendages are very variable 

 in number and length with tins more or less closely 3-6 times 

 dichotomously divided with the tips of the ultimate branches 

 regularly and distinctly recurved. The asci are more or less 

 oval and vary in number from 3-8. On: Quercus macrocarpa, 

 O. rulra, and Syringa rulgaris. 



Microsphaera alni fciccinii (Schwein.) Salmon. 



This variety is very much like the species but is usually con- 

 fined to the upper surface of the leaves, and the perithecia are, 

 on the average, smaller. The branching of the appendages is also 

 irregular, being sometimes loose and widely forked. The variety 

 is quite common on Catalpa catalpa, and C. speeiosa. 



Micros phacra grossitlariae (Wallr.) Lev. 



This species may be restricted to the upper side of the leaves 



or it may occur on both sides. The perithecia are scattered or 



