338 POMOLOGY 



losa). While he records that P. betulcefolia is somewhat 

 susceptible to blight in Oregon, it has been remarkably free 

 in South Dakota where it has been grown for twenty years, 

 and in the trial plots at Washington, D, C, 



Pyrus Calleryana is a recently described species of pear 

 native to China which is veiy promising as a stock. Hence, 

 if a variety of high quality can be produced by hybridizing 

 P. communis (from which all the commonly grown pears 

 originated) with one of the hardy, blight-resistant species, 

 the solution to one of the most serious problems in pomology 

 would be at hand. 



Hansen has crossed P. sinensis and P. betulcefolia with 

 several of the best cultivated pears (P. communis) and dis- 

 tributed thirty-nine promising sorts throughout several states 

 for trial. It is hoped that they may prove to be the basis of 

 pear breeding to secure valuable varieties immune to blight.^ 



301. Stock for grapes. — What appears to be a clear case 

 of the Mendelian behavior of disease-resistance is seen in 

 the work of Rasmuson ^ who attempted to secure varieties 

 of V. vinifera which would be immune to the great scourge 

 of phylloxera. He made crosses between certain American 

 species and V. vinifera, and also crosses between different 

 varieties of V. vinifera. In studying the F2 generation of 

 these crosses, he found that the vinifera crosses yielded off- 

 spring susceptible to the disease while crosses between the 

 American species and vinifera yielded varieties part of which 

 were resistant and part susceptible, but the latter were in 

 the minority. The fact that disease-resistance proved to 

 be dominant and susceptibility recessive in the progeny of 

 this latter set of crosses, bodes well for the future. 



1 Reimer, F. C. Proc. Amer. Pom. Soc. 1915. Hansen, N. E. 

 S. D. Agr. Exp. Sta. Bull. 159. 1915. Galloway, B. T. Jour. Heredity, 

 Vol. 9. Jan., 1920. 



2E.S.R. 36:537. 



