PRACTICAL TREE SURGERY 325 



pathology. The presence of tannin and other protective chemical 

 substances in the plant may explain immunity or non-immunity.' 



Disease resistance and disease susceptibility are understood imper- 

 fectly. The determination of the cause of the inherent differences in 

 the tendency of this or that variety to suffer from disease is a matter 

 of great importance. Breeding for disease resistance is a promising 

 field of research.'' Something has been accomplished along this line, 

 but the amount which we do not know vastly exceeds the knowledge 

 which we now possess. Rustproof varieties of wheat have been ob- 

 tained. At the Ohio Experiment Station by selection of hills of 

 potatoes that withstood attacks of the early blight fungus and planting 

 tubers therefrom with subsequent repetition of this line of work, early 

 blight resistant strains were secured. Progress has been made with 

 cotton resistant to wilt and with musk melons resistant to leaf blight. 



Recently Jones and Oilman', Wisconsin, have undertaken to con- 

 trol the disease known as yellows caused by the parasitic soil fungus, 

 Fusarium conglulinans, by breeding cabbage plants that show disease 

 resistance. By repeated selection of the occasional sound heads in 

 fields of diseased cabbages, strains of winter cabbage of the Hollander 

 t3rpe have been secured which have proved in a high degree resistant 

 against the attacks of Fusarium. The chances for research along these 

 lines are practically unlimited and full of promise for the future of 

 agriculture and horticulture. 



* Cook, Mel T. and Taubenhaus, J. J. : The Relation of Parasitic Fungi to 

 the Contents of the Cells of the Host Plants, (i. The Toxicity of the Tannins) 

 Bull. 91, Del. Agric. Exper. Stat., February, 191 1. 



* Orton, W. a. : The Development of Farm Crops resistant to Disease. Year- 

 book of the United States Department of Agriculture, 1908 : 453-464. 



' Jones, L. R. and Oilman, J. C. : The Control of Cabbage Yellows through 

 Disease Resistance. Research Bull. 38, Agric. Exper. Stat. Univ. Wis., December, 

 1915; Norton, J. B.: Methods used in Breeding Asparagus for Rust Resistance, 

 U. S. Bureau of Plant Industry, Bull. 263, 1913. 



