BREEDING DISEASE-RESISTANT PLANTS 407 



chestnuts have already been under way some 20 years. To these we 

 shall refer again. 



A bacterial organism which finds similar wide diversity in the resis- 

 tance of possible hosts is the fire-blight pathogene, Bacillus amylovorus. 

 Being indigenous in eastern North America, this organism must have 

 maintained itself on the native species of apples and related genera 

 previous to the introduction of European apples, pears and quinces since 

 it cannot survive long even in the dead tissues of the host. The dis- 

 ease is spread naturally by insects that visit infected plants; it may 

 also be carried on pruning tools. Fire-blight is the most widely de- 

 structive of all pomaceous fruit diseases; but the pathogene manifests 

 different degrees of virulence in different species. Its most susceptible 

 hosts are the commercial varieties of the pear, which are all derivatives 

 of the European species, Pyrus communis. In several regions naturally 

 well adapted for pear growing the culture of this fruit has been abandoned 

 on account of the destructiveness of pear-blight. Even the more resistant 

 varieties of communis as well as certain hybrids between communis and 

 other species, such as the Kieffer, a supposedly resistant pear, have all 

 proven to be susceptible to the disease when grown in the humid climate 

 of the southern States. Therefore in discussing the problem of blight- 

 resistance in pears it must be remembered that the pathogene itself is 

 very susceptible to environmental conditions and that a particular host 

 which is known to be resistant under one set of conditions will not neces- 

 sarily prove to be generally resistant. Hence the breeding of blight- 

 resistant pears should be carried on in a region ideal for pear culture in 

 every respect except that it is ideal for the fire-blight organism also. 

 Such conditions exist in southern Oregon where Reimer has made a 

 complete collection of the known species of pears and has conducted 

 scientific tests of their resistance to blight by means of inoculations with 

 pure cultures of the bacillus. The results to date indicate that the fol- 

 lowing species are highly resistant: Pyrus sinensis, P. ovoidea and P. 

 variolosa. Under P. sinensis he finds there are several distinct species 

 which will be classified after they have fruited, but they are all resistant. 

 The birch-leaved pear, P. betulifolia, which is used as a stock in China, 

 proved susceptible when the inoculations were made on 1- and 2-year 

 old trees. But it is probable that older trees will show greater resistance 

 and the same may be said of the 16 other species in which inoculation 

 established the disease and which might be considered as susceptible 

 varieties. However, varying degrees of susceptibility were exhibited by 

 these species. Hansen reports the birch-leaved pear as quite resistant 

 to blight in South Dakota where it has grown for over 20 years. 



We have now considered one plant disease which may be considered 

 typical of each of the three great classes of disease-causing organisms and 



