BREEDING DISEASE-RESISTANT PLANTS 



403 



present. The principal difference between resistant and susceptible vines 

 as regards reaction to phylloxera attacks is found in the number, size and 

 penetration of the lesions on the larger roots. This phylloxeran is a 

 native of eastern North America and the species of Vitis which are native 

 to this region all exhibit some resistance to its attacks. This resistance 

 of species native to the habitat of a disease-causing parasite is a general 

 fact of great significance to agriculture on account of its potential value 

 in both plant and animal breeding. 



Fia. 164. Effects of phylloxera on vine roots. On left affected root tips or nodosities; 

 in same figure incipient tuberosities are shown at a. Center, non-penetrating tuberosities 

 on an American vine. Right, penetrating and confluent tuberosities on V. vinifera, the 

 most serious form of the disease. (After Viala and Ravaz.) 



The phylloxera was introduced into France through the importation 

 of American vines and it soon became a most serious obstacle to the 

 culture of the choice wine, table and raisin grapes of the Mediterranean 

 region, all of which varieties belong to a single species, Vitis vinifera. 

 In fact, every member of this large and valuable plant group has been 

 found to be susceptible to phylloxera thus making impossible its culture 

 as a direct producer, i.e., on its own roots, in a phylloxera infested region. 

 After striving in vain to exterminate the insect in all infested areas, 



