774 



SCIENCE 



[N. S. Vol. XLI.- No. 1065 



out thoroughly and before the parasite has 

 reached too advanced a stage of develop- 

 ment this method may be crowned with suc- 

 cess. A striking example of this is the con- 

 trol of pear blight in the Rogue River 

 Valley, Oregon. In this region the disease 

 has been kept within bounds, while in the 

 Eastern States it was permitted to gain a 

 foothold and is now a calamity. 



The spraying of potatoes against Phy- 

 topJitJiora infestans, by which the fungus is 

 destroyed before it is able to penetrate the 

 tissues of the host, may be included in this 

 class of control measures. Another example 

 is the destruction of the smut spores, which 

 cling to the outer covering of the grain, in 

 the case of stinking smut, for instance. 



It is a more difficult task to cure a plant 

 already diseased than to prevent the dis- 

 ease, and only in rare cases is the method 

 of cure known, the reason for this being 

 that plants are not organized like animals, 

 and in most cases it is impossible to influ- 

 ence a central system. The cure of fungus 

 diseases of different trees by giving the 

 roots an abundant water supply is an ex- 

 ample of treatment based on the principle 

 that many fungi are unable to grow in 

 tissues which show a high water pressure. 

 In dry soils the water content is kept on a 

 low basis and this favors the attacks of the 

 fungus. 



Another example of the curing of the 

 plant is the hot-water method of seed treat- 

 ment for loose smut of wheat and barley, 

 this treatment being founded on the de- 

 struction of the fungus germs within the 

 seed. 



We now come to the third method of dis- 

 ease control, that is the use of disease- 

 resistant plants. The importance of this 

 method is well understood by both scientists 

 and growers, but the application of the 

 principle, it must be confessed, is in its 

 infancy. 



Utilization of the factor of immunity in 

 disease control may be divided into two 

 parts, that is the breeding of resistant 

 plants and the artificial immunization of 

 plants. From a scientific point of view, 

 however, both of these rest on the same 

 basis. 



Before a disease-resistant race can be 

 bred resistant individual plants must be 

 found. It is a well-known fact that in the 

 vegetable kingdom closely related species 

 suffer in different degrees from attacks of 

 the same parasite. The difference in resist- 

 ance of the various species of one of our 

 most important cultivated crops, wheat, is 

 unusually prominent, as shown by the re- 

 searches of Wawelow. Of the eight botan- 

 ical species which are generally thought to 

 have produced the cultural varieties of our 

 wheat, Triticum vulgare, T. compactum 

 and T. spelta are attacked by red rust; 

 T. durum, T. polonicum, T. turgidum and 

 T. monococcum are resistant; the western 

 European varieties of T. dicoccum are 

 resistant, and the eastern varieties of 

 Turkestan are liable to rust. T. dicoccum 

 dicoccoides, which was found in Palestine 

 some years ago and which has sometimes 

 been regarded as the ancestor of our com- 

 mon cultivated wheat, T. sativum, is also a 

 non-resistant species. 



The varying susceptibility of species of 

 the same genus makes it possible to substi- 

 tute for highly susceptible species others of 

 nearly equal cultural value which are less 

 susceptible or resistant. In the ease of the 

 coffee plant very good results have been ob- 

 tained by this means. It is well known that 

 Coffea arahica was completely destroyed 

 throughout the Asiatic tropics by the rust 

 fungus Hemileia vastatrix. The related 

 African species C. liherica appeared to be 

 resistant to the disease and was brought 

 under cultivation in the entire territory in 

 which C. arahica had been grown and in 



