1893.J 311CROSC0PICAL JOURNAL. 193 



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

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



The oospores lie dormant for sometime. They germinate in 

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

 a numherof small bodies which when free are provided witli 

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

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

 ters the leaf through the stomata where an abundant mycelium 

 is developed which in a short lime produces the conidia or 

 white-rust stage. Altiiough tiiis disease is not so destructi^'c 

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

 tion of the leaf, excej)t in some cases where the stems of Mowers 

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

 radish (Cystopiis candidus), it may cause some damage. 



III. — Spot Disease of Beets. 



This disease (Cercospom heticola), 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 as 

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

 is green, but later become darker. The sides of *:he spot where 

 the spores arise ure 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 this was a 

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

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

 fects of the fungus. In badly diseased patchesthe 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. 



