222 PRACTICAL BOTANY 



212. Grape mildew: sexual reproduction. In the interior of 

 the leaf short hyphal branches develop into obgonia and an- 

 theridia (Fig. 181, (7). Each obgonium develops an egg, which, 

 when fertilized by one of the many sperms from the anther- 

 .idium, becomes an obspore. This obspore has a heavy wall 

 and also is within the tissues of the leaf, so that apparently 

 it is well fitted to endure severe winter conditions. Upon the 

 decay of the leaf the obspores are set free. They probably ger- 

 minate to produce new plants, but " much work needs to be 

 done in the way of determining to what extent the obspores 

 are necessary in the annual propagation of this species." 1 



213. Grape mildew: preventive measures. Usually it is pos- 

 sible to control the growth of this parasite so that serious damage 

 is prevented. In 1881 the Frenchman Millardet began experi- 

 menting with the Bordeaux mixture 2 as a method of treating 

 grape mildew. His experiments resulted in a chemical mix- 

 ture which, when properly used as a spray, will obviate most of 

 the ill effects of grape mildew. The same mixture has been 

 found of great value in treating many other plant diseases, 

 and almost all the state agricultural experiment stations issue 

 special directions concerning local uses of this mixture. 



214. Potato blight (Phytophthora infestans). This parasite is 

 a near relative of grape mildew. Its vegetative characteristics 

 closely resemble those just described. Its asexual reproduc- 

 tion by conidiospores and the consequent z obspores gives it 

 very ready and wide distribution. The parasite may infest 

 leaf, stem, or tuber of the potato, and is one of the several 

 fungous diseases that have proved very destructive of potato 

 crops. It may be held in check by proper spraying (Fig. 349) 

 with the Bordeaux mixture. 3 



1 Duggar, B. M., Fungous Diseases of Plants. Ginn and Company, Bos- 

 ton, 1909. 



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

 the following proportions : copper sulphate, 5 pounds ; stone lime, 6 pounds ; 

 water, 50 gallons. Other proportions are often used. 



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

 "Certain Potato Diseases and their Remedies," Bulletin 7,Vt. Agr. Exp. Sta. 



