404 GENETICS IN RELATION TO AGRICULTURE 



European vineyardists gradually adopted the only other practicable 

 method of grape growing, viz., the grafting of vinifera varieties upon 

 resistant roots. The problem of determining which species of Vitis were 

 both highly resistant to phylloxera and well adapted to the soil and 

 climatic conditions of various European localities required extensive 

 investigations. Eighteen native American grapes have been tested as 

 well as several Asiatic species, but the latter were all less resistant than 

 the most susceptible American species. The American vines which 

 have come into most prominence on account of their proven value in 

 the reconstitution of phylloxera devastated vineyards may be listed 

 according to relative resistance about as follows, if the maximum or absolute 

 immunity be taken as 20. 



18-19. F. rupestris. 



18. V. riparia and cordifolia. 



17. V. berlandieri. 



16. V. cinerea. 



14-15. F. (Bstivalis, linsecomii and candicans. 



All of the above species belong to the sub-genus or section, Euvitis. 

 Two of these, rupestris and riparia, together with certain hybrids 

 between these and between these and vinifera, are now considered the 

 most valuable resistant stocks. Another American species belonging to 

 the section Muscadima, viz., rotundifolia, has been found to have a 

 resistance of 19 or higher inasmuch as the insect has never been observed 

 on its roots. It is also free from the common fungous diseases of the 

 vine, but the difficulty of propagating it from cuttings and its slight 

 affinity for grafts of other species make it a valueless species for the 

 reconstitution of vineyards. On the other hand, the American species, 

 labrusca, has become of great economic importance since it is the parent 

 of the Concord, Isabella, Niagara and many other cultivated varieties. 

 Yet its resistance to phylloxera is ranked at 5, and when grown in Cali- 

 fornia it is no more resistant than is californica when used as a rootstock 

 for producing vines, and the resistance of the latter is ranked at 4. Yet 

 the labrusca derivatives are extensively grown in the northeastern states 

 and in other northern temperate regions. This is explained by the fact 

 that the phylloxera itself does not thrive below a certain minimum 

 temperature. Thus we find that resistance to phylloxera in the species 

 of Vitis varies all the way from zero in vinifera to practically absolute 

 resistance in rupestris, rotundifolia and certain hybrids and that the 

 existence of highly resistant forms which are also suitable for vineyard 

 culture has made possible the preservation of an important agricultural 

 industry. 



The question of the nature of the cause of resistance to phylloxera 

 has received rather wide attention among investigators, but it has not 



