CHAPTER 17 

 THE PHYSIOLOGY OF PARASITISM AND RESISTANCE 



A discussion of parasitism occupies an important position in any 

 treatise on the physiology of fungi, particularly for those students who are 

 interested in plant diseases or the fungi which cause them. This phase 

 of study offers many challenging unsolved problems. Parasitism involves 

 primarily two living organisms, the parasite, whose actions are offensive, 

 and the host, whose reactions are defensive. If the defenses of the host 

 plant, either before or after penetration by the parasite, are successful, 

 the plant is resistant; if not, it is susceptible. To be successful, a parasite 

 must find the nutritional and environmental conditions favorable for its 

 development. If even a single important factor is unfavorable to the 

 parasite, the fungus may fail to establish a parasitic relationship with its 

 proposed host. Such factors may exert their influence either before or 

 after penetration by the fungus. Environmental factors acting before 

 penetration may in reality bring about an escape from a disease rather 

 than true resistance to it. 



The present discussion is divided into three main parts: (1) penetra- 

 tion; (2) parasitism, the action of the parasite in becoming established and 

 obtaining its food ; (3) resistance of the host to penetration or against the 

 parasite after penetration. The comprehensive reviews of the physiology 

 of the host-parasite relationship given by Brown (1936, 19-48) should be 

 read by all students. Similar reference is made to Arthur et al. (1929), 

 who give an excellent discussion of the parasitic relations of the rusts, 

 and to the treatise of Gaumann (1946, 1950) on the principles of plant 



infection, 



PENETRATION 



A parasite may gain entrance into the host (1) through the natural 

 openings, such as stomata or lenticels, (2) by direct penetration through 

 the uninjured epidermis, or (3) through wounds. 



Through stomata. Viable spores may fall upon a host plant and pro- 

 duce germ tubes, which by chance grow over or near stomata. The 

 outer walls of the epidermal cells of aerial plant parts are covered with 

 cutin, which is somewhat resistant to penetration by some fungi. The 

 germ tube which enters through a stoma may then be favored by the moist 

 atmosphere in the substomatal cavity. In some cases, the unspecialized 

 hyphae may penetrate the host cells; in other fungi, haustoria, which 



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