1893.] MICROSCOPICAL JOURNAL. 193 



fertilization a thick-walled spore is produced, the oosjiore. The 

 outer part is roughened (Plate I, iig. 8). 



The oospores lie dormant for sometime. They germinate in 

 the presence of water, the contents of the oospore break up into 

 a number of small bodies which when free are provided with 

 two cilia. These are called zoospores. After a short time a 

 wall is formed about them. The zoospore germinates and en- 

 ters the leaf through the stomata v/here an abundant mycelium 

 is developed which in a short time produces the conidia or 

 white-rust stage. Although this disease is not so destructive 

 as some of the downy mildews, seldom causing a total destruc-. 

 tion of the leaf, except in some cases where the stems of flowers 

 become abnormally enlarged as in the white-rust on mustard- 

 radish (Cystopus Candidas), it may cause some damage. 



III. — Spot Disease of Beets. 



This disease (Cercospora beticola), manifests itself on the leaf 

 by producing round spots, at first not larger than a pin head. 

 Gradually these spots increase in size, becoming round, ellipti- 

 cal or irregular in outline (Plate III, fig. 1). Frequently they 

 run together and form one large patch. The spots occur on 

 both upper and lower sides of the leaf. There appears to be no 

 difference in regard to the formation of spots on the upper and 

 lower surface of the leaf, occurring as frequently on one side ab 

 the other. The spots are of a pale brown color when the leaf 

 is green, but later become darker. The sides of the spot where 

 the spores arise are of a lighter color, the margins being pale 

 brown when the spores are forming. Most of the spots are 

 about one-eighth of an inch in diameter, though some of the 

 larger are nearly one-half inch across. The spots frequently 

 run down the petiole,.giving it a black appearance. One small 

 leaf, four and one-half by two and one-half inches, had some- 

 thing over one hundred spots of various sizes, and tliis was a 

 newly affected leaf. In very old specimens the spots are much 

 more numerous. In fact, they were colored black from the ef- 

 fects of the fungus. In badly diseased patches the lower leaves 

 were all black. They were dead or in the process of wilting, 

 while the upper or center leaves were becoming discolored by 

 the fungus. 



