BREEDING SNAPDRAGONS 15 



The Nature of Disease Resistance 



The nature of disease resistance in plants still remains one of the most interest- 

 ing phases of biological science. Much has been published on reactions of host 

 plants to disease organisms, but knowledge concerning the fundamental nature 

 of disease resistance in plants is still meager. Walker (12), in his comprehensive 

 review on disease resistance in vegetable crops, considers the subject with respect 

 to such biological factors as disease escape, which pertains to the earliness or 

 lateness of maturity of the crop in relation to infection as affected by environmen- 

 tal influences, by exclusion of an insect vector, by mechanical exclusion where 

 the host develops tissues which are resistant to penetration by the fungus, by 

 chemical exclusion where certain plant cell substances such as phenolic and 

 protocatechuic acids are formed as inhibiting agents, by physiological exclusion 

 which covers many of the phenomena that have not been explained, and lastly 

 by strain reassortment along with physiologic races of the fungous organisms. 



References which deal specificc lly with morphological and physiological aspects 

 of rust resistance in snapdragon are few and not conclusive. Doran (3) presents 

 data on stomat? counts for leaves taken frorr; snapdragon plants which he reports 

 were resistant and susceptible to rust, and concludes that since the stomata 

 count per unit area of leaf surface was greater on susceptible varieties and less on 

 resistant, susceptibility to rust is directly proportional to the number of stomata. 

 However, he does not state what varieties were used for the stomata counts, 

 nor is his comparative scale of determining resistance or susceptibility' of plants 

 to rust, between the varieties, very explicit. Peltier (10) noted all degrees of 

 morphological variations in the varieties of snapdragon he tested, but concludes 

 that all varieties were equally susceptible to rust disease. 



Most of the available data on rust resistance in plants as related to morpholog- 

 ical and physiological relationships between host and fungus are on cereals. 

 Extensive investigations with cereals by Hurd (5), Hursch (6), and Peterson (11) 

 show that specific morphological and physiological conditions could not be readily 

 correlated with the observed degree of resistance of wheat plants to rust disease. 



At Waltham plants of rust-resistant and susceptible strains were not subjected 

 to anatomical studies to determine relationships between fungus and host, but 

 no correlation could be observed between degree of resistance or susceptibility 

 and readily visible external morphological characters of the plants. The resist- 

 ance of snapdragons to rust disease has been demonstrated and accepted as being 

 regulated by hereditary factors. However, the manner in which this resistance 

 is biologically manifest by perceptible, measurable differences in physiological, 

 chemical, or morphological responses is not as yet clearly understood. 



Summary 



Snapdragon rust attracted public attention because of an epidemic of the dis- 

 ease in greenhouses in 1913. Since that time the disease has spread throughout 

 the United States and has become widely disseminated through England and most 

 of Europe. 



The economic aspects of this disease are concerned with its destructiveness 

 to the ornamental value of snapdragon plants, and reduction of seed productive- 

 ness of commercial seed crops on the west coast. 



By interbreeding greenhouse forcing strains of snapdragons with rust-resistant 

 strains of Dr. E. B. Mains, improved strains have been obtained which are highly 

 resistant to rust. 



Resistance to rust in snapdragons has been determined to be inherited as a 

 simple dominant hereditary factor. 



