BREEDING DISEASE-RESISTANT PLANTS 401 



food in similar fashion. Pathogenic fungi and bacteria are wholly or 

 partially parasitic. Bacteria which cause plant diseases are those capa- 

 ble of establishing themselves and multiplying in number within the 

 living tissue of the host. A few of the important plant diseases caused 

 by bacteria are "fire-blight" of pears and apples, crown gall of many 

 fruit trees, grapes and other plants, and the black rot of the cabbage. 

 Some fungi, such as rusts and smuts, are strictly internal obligate 

 parasites (as distinguished from those obligate parasites which are wholly 

 or partially epiphytic), i. e., they cannot exist outside the body of a 

 particular host plant or plants except in the spore stage. In such cases 

 the relation between parasite and host is symbiotic. The specific re- 

 lations between parasites and their hosts vary from a condition of tol- 

 erance of the parasite without serious injury to the host to one in which 

 the destruction of the host finally ensues. Many fungi, such as the 

 powdery mildews, are epiphytic although they derive their nourishment 

 from the living plant tissue by means of haustoria. Between the epi- 

 phytes on the one hand and the internal parasites on the other are many 

 types of endophytic fungi in which various proportions of the parasite's 

 life cycle are spent within the host plant. 



Thus there are many agencies, some non-living as well as many living 

 things, which threaten the normal development of cultivated plants. 

 Even among the parasitic fungi themselves there are many devices for 

 invading the host plant and many instances of specific physiological 

 relationship between parasite and host. 



The Nature of Disease Resistance in Plants. Disease resistance in a 

 plant may be defined as the ability to develop and function normally 

 under conditions such that other plants of the same species fail to develop 

 or are destroyed. Resistance is always either partial or complete. The 

 avoidance of disease by such means as precocious or delayed maturity is 

 hardly to be considered as true disease resistance. Since there are so 

 many agencies which may cause disease in plants it is evident that the 

 ability to resist disease may depend on any one of many characters or it 

 may involve every function of the plant. In either case this ability is a 

 manifestation of the physiological individuality of the plant and hence it 

 may be inherited. Nowhere is this more strikingly shown than in the 

 disease resistance of certain natural species. 



Disease Resistance in Natural Species. The nature of disease resist- 

 ance in a particular instance is indicated by the nature of the cause of the 

 disease. In the case of non-living causes resistance on the part of certain 

 plants can be explained only as a manifestation of the inherent properties 

 of the protoplasm. Thus the alkali resistance of salt grass, the Australian 

 salt bushes, the common beet and asparagus is a heritable character. If 

 it were not so these species could not perpetuate themselves on soils which 



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