FUNGI AND FUNGOUS DISEASES OF PLANTS 241 



fifteen or twenty minutes, and then lose its cilia and begin to 

 grow into a new hypha of the mildew. These new hyphae 

 may grow through the stomata of leaves and start a new 

 growth of the parasite. 



Within the host leaf sexual reproduction of the parasite 

 occurs, and oospores (fig. 186, (7) are formed. These oospores 

 are thought to reproduce the parasite in the following spring, 

 when the decay of the host leaf releases them. Our knowledge 

 of oospore formation in grape mildew and potato blight is still 

 incomplete. It is thought, however, by some special students 

 of fungi, that potato blight has largely lost its power of sexual 

 reproduction, and that oospores are not often formed. 1 



229. Prevention of grape mildew and potato blight. Potato 

 blight is a common and extremely destructive potato disease 

 caused by a fungus that resembles grape mildew in its structure 

 and habits of growth. It is readily and widely distributed and 

 often causes serious losses by partially or almost completely 

 preventing the growth of a potato crop. Both potato blight 

 and grape mildew are prevented from reaching their normal 

 growth, and are sometimes completely destroyed, by spraying 

 with Bordeaux mixture. 2 It was in connection with a study of 

 grape mildew that the Frenchman Millardet, in 1881, began 

 experimenting with Bordeaux mixtures. He demonstrated the 

 fact that this mixture, when properly used as a spray, will pre- 

 vent most of the ill effects of grape mildew. The effect of 

 using this spray in treating potatoes is well shown in figure 187. 

 Many other destructive parasitic diseases of plants, though by 

 no means all of them, may be prevented in the same way. 3 



1 G. P. Clinton, "Oospores of Potato Blight," Science, 744-747, 1911. 



2 The preparation as most commonly used consists of materials mixed in 

 the following proportions : copper sulphate, 5 pounds ; stone lime, 5 pounds ; 

 water, 50 gallons. Other proportions are often used, as indicated in agri- 

 cultural experiment station reports. 



3 Duggar, B. M., Fungous Diseases of Plants. Ginn and Company, 

 Boston, 1909. 



" Potato Spraying Experiments in 1906," Bulletin 279, N. Y. Agr. Exp. Sta. 

 "Certain Potato Diseases and their Kemedies," Bulletin 72, Vt. Agr. 

 Exp. Sta. 



