BREEDING DISEASE-RESISTANT PLANTS 41] 
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’; 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 F; 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. 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 
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