BREEDING DISEASE-RESISTANT PLANTS 411 



In an attempt to breed blight-resistant pears of horticultural value 

 Hansen has produced and distributed for trial thirty-nine first generation 

 hybrids between various commercial varieties and either the Chinese 

 Sand Pear or the Birch-leaved Pear. Should these hybrids prove to be 

 unsuitable as commercial varieties they may be used as foundation stock 

 in further efforts to produce a hardy, blight-resistant variety. Although 

 the Kieffer and the Le Conte are presumably F t hybrids between sinensis 

 and communis, they have not been used by Hansen because they are not 

 hardy in the north. For lower latitudes however these two partially 

 resistant varieties should be utilized not only by raising seedlings from 

 them but also by an extensive series of crosses especially with other 

 partially resistant communis derivatives of high quality such as the 

 Seckel. The work of Reimer and of Hansen indicates that perfectly 

 resistant stocks may be developed which are adapted for each important 

 pear-growing region. If to this achievement may be added the creation 

 of fairly resistant varieties of really excellent quality, the worst diffi- 

 culties in pear production will be removed and the world's supply of 

 this delicious fruit will be practically assured. 



Creating Rust-resistant Commercial Wheat by Crossbreeding. The 

 grain rusts are the most important of all fungous plant diseases. The 

 annual losses they entail for the -grain crops of the world must be estimated 

 in the hundreds of -millions of dollars. Although prevention of wheat 

 rust to some extent is now possible by giving careful attention to the 

 water and soil relations of the wheat plant and by early seeding or the 

 planting of early varieties which sometimes escape attacks by rust, yet 

 these diseases still remain a serious menace to the maximum production 

 of wheat. Hence, the creation of rust-resistant varieties has become a 

 very important problem. The diversity among varieties of wheat as 

 regards resistance and susceptibility to rust fungi was recognized by 

 Knight in 1815 and the desirability of creating new varieties which should 

 be resistant to rust as well as highly productive and of good milling 

 quality was fully realized by such breeders as Pringle, Blount and Farrer. 

 Although they were not familiar with the Mendelian principles of seg- 

 regation and recombination of characters, these breeders of wheat, a 

 self-fertilized annual crop plant, were naturally led to persist in their 

 efforts beyond the FI generation. The work of Farrer especially was 

 thorough and reliable. He found that he could not secure absolute 

 resistance to the black stem-rust, Puccinia graminis Pers., combined with 

 good milling quality in his wheat crosses even when rigorously selected 

 in the F 2 or "wild" generation as he called it. Most of the soft bread 

 wheats are very susceptible to rust and, when crossed with the resistant 

 durums, poulards and spelts, they give rise to strains which are either 

 poor bread wheats or are rust susceptible. Biffin discovered in 1903 



