274 GENERAL PLANT PATHOLOGY 



the inner epidermis of the pod. It has been found that infection of the 

 seeds with Ascochyta pisi is facihtated by the presence of the hairs, for 

 the fungus grows, as in a culture medium, and infects every seed, while 

 in the hairless forms infection takes place only where the seed actually 

 touches the infected spot of the pod. 



The presence of certain chemic substances may explain immunity, for 

 the disease resistance of Vaccinium vitis idcea is supposed to be due to the 

 presence of benzoic acid. So, too, the presence of tannins may increase 

 the power of resistance to fungus and insect diseases, as indicated by 

 Cook and Taubenhaus.' Enzymes also play an important role in the 

 production of chemic substances, which increase disease resistance. 

 Such hereditary disease resistance may be made to play an important 

 part by breeding and growing the varieties which have been proved to 

 be disease resistant. 



Immunity may be acquired by growing the susceptible form at a 

 different season of the year from its accustomed one. Grafting has 

 been used with success. The method is to graft a non-resistant variety 

 on a resistant one, as in the case of the European vine on the American 

 vine, which resists the attack of the Phylloxera insect, which devastated 

 the European vineyards until this method was adopted. Crossing has 

 been resorted to as a second means of increasing disease resistance. The 

 weak variety is crossed with a disease resistant form to increase its 

 immunity. The third way to obtain immune forms is to select resistant 

 individuals and from them breed pure strains. This has been accom- 

 plished with some degree of success by Orton with cotton, by BoUey 

 with flax, by L. R. Jones with cabbage. It should be emphasized that 

 the inheritance of the unit characters and their behavior in the next 

 generation is one of the fundamentals of breeding resistant races. 



Determining Causes. — Having considered the general reasons for the 

 predisposition of plants to diseases and the immunity of others, it is 

 important to describe next the causes which determine disease. These 

 may be divided into those of external origin and those ofinternal. The 

 external factors of disease are the chemical conditions of the soil, as a 

 determining cause, also the physical character of the soil. The influ- 

 ence of a superabundance of water, or its absence, is important. Cli- 



^ 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 Tannin, BuU. 91, 

 Delaware College Agric. Exper. Stat., Feb. i, 1911. 



