STOCKS AND RESISTANT VINES 63 



and varieties are, as has been said, more resistant to the root- 

 louse than others. All degrees of resistance exist, as would 

 be suspected, from immunity to great susceptibility. It is 

 obvious that the foundation of the art of growing resistant 

 vineyards is exact knowledge of the immunities and suscepti- 

 bilities of the many varieties and species of grapes. From the 

 first use of resistant vines, experimenters everywhere have set 

 themselves at work to determine not only what the most resist- 

 ant vines are, but what the causes and conditions of immunity. 

 In spite of a wealth of empirical discoveries as to what grapes 

 can best resist the root-louse, causes and most of the conditions 

 of immunity are still little understood. Definite, useful knowl- 

 edge, so far, goes little further than the establishment of lists 

 of species and varieties, the latter subject to change, that are 

 most useful in setting resistant vineyards. 



Phylloxera does little damage to species of Vitis native to 

 the same general region in which the pest has its habitat, but 

 nevertheless there are some differences in resistance in American 

 grapes. Munson, one of the best American authorities on 

 the resistance of species to phylloxera, says : 1 " Rotundif olia 

 is entirely immune, then Rupestris, Vulpina, Cinerea, Ber- 

 landieri, Champini, Candicans, Doaniana, ^Estivalis and Lince- 

 cumii are so high in resistance as to be practically uninjured, 

 though they may be attacked, while Labrusca is low in resist- 

 ance and is much weakened in clay soils, if infested, and Vinif- 

 era is entirely non-resistant." Some of these species are hard 

 to propagate and difficult to suit in soil and climate so that but 

 two of them are much used for resistant stocks. The two most 

 used are Rupestris and Vulpina (Riparia), of both of which there 

 are varieties which give satisfaction. Bioletti, a leading author- 

 ity on resistant stocks in California, says : 2 



1 Munson, T. V. Foundations of American Grape Culture, 217. 

 1909. 



2 Bioletti, Frederic T. Calif. Exp. Sta., Bui. 180: 96-97. 1906. 



