408 GENETICS IN RELATION TO AGRICULTURE 



in each we find the same diversity among natural species as regards 

 disease resistance. It is unnecessary to multiply instances further. In 

 all likelihood the resistance of the Chinese chestnut to Endothia parasitica 

 and of the Chinese Sand Pear to the fire-blight bacillus is due to some 

 specific quality of the protoplasm probably something in the nature of 

 an antitoxin. That this quality is heritable will be seen from the results 

 of hybridization experiments. 



Breeding Disease-resistant Varieties by Hybridization. Allusion 

 was made in Chapter XX to the fact that first generation maize hybrids 

 are often more drouth resistant than either parent. Presumably this 

 is merely one manifestation of heterosis. Hybridization is a very im- 

 portant means, however, for the production of improved varieties which 

 are better adapted to specific adverse elements of the environment. 

 Witness the important results already secured in the production of cold- 

 resistant varieties of fruits, grains and forage plants, by Hansen, Patten 

 and Saunders and at the U. S. Agricultural Experiment Stations in 

 Alaska. 



At one stage in the anti-phylloxera campaign in France and California 

 viticulturists held definitely to the ideal of securing through hybridi- 

 zation "a vine that, while resisting the phylloxera, the two mildews, 

 the black rot, etc. (all of which diseases are natives, and which the 

 American vines resist more or less well), will give without grafting a grape 

 that has size and the quantity and quality of the Vitis vinifera." With 

 this object in mind many crosses were made but they have produced no 

 hybrids between vinifera and American species that can be substituted 

 for the choice vinifera varieties. It, therefore, became necessary to util- 

 ize resistant species and hybrids as stocks on which to graft the pro- 

 ducing varieties. However, it is still possible that, by growing large 

 numbers of F 2 and F s seedlings from some of the most promising F\ 

 hybrids, the dream of the viticulturist might be realized. It seems that 

 no grape breeders have carried out extensive tests of hybrids beyond the 

 first generation from the cross. This is not strange inasmuch as grape 

 breeding for phylloxera resistance was at its height during the latter part 

 of the 19th century and before the importance of testing for several 

 generations after a cross was generally appreciated. That phylloxera re- 

 sistance and susceptibility are conditioned by specific genotypic elements 

 is evidenced by the results of Rasmuson who tested F z seedlings from 

 several crosses between certain American species and between American 

 species and V. vinifera, as well as crosses between different varieties of 

 vinifera. The latter, he reports, yielded only susceptible offspring while 

 the crosses between different American species gave both resistant 

 and susceptible offspring, the latter being in the minority. Resistance 

 appeared to be dominant and susceptibility recessive in the progeny of 



