Chapter 10 



HOST PENETRATION 



The production of disease by pathogenic fungi involves the fol- 

 lowing sequential processes: inoculation, incubation, and infec- 

 tion, "in the first process is included distribution of the pathogen 

 bv any agencies whatsoever that bring into contact inoculum and 

 suscept. The inoculum may take an active part in this process, as 

 occurs among swarm spores of certain Phycomycetes and among 

 ascospores and basidiospores that are forcibly expelled. On the 

 other hand, the inoculum may be entirely passive and therefore 

 be dependent upon water, currents of air, and insects or other 

 bioloo-ical agencies. In incubation are included penetration of 

 the tissues of suscepts by germination and growth of the inoculum 

 to set up the parasitic relationship. Penetration may be accom- 

 plished by entrance through natural openings, such as stomata, 

 lenticels, and hvdathodes, by direct passage through cuticle and 

 epidermal walls, or bv entrance through wounds. Length of the 

 incubation stage is definite for each specific pathogen and termi- 

 nates when symptoms appear. All of those physiologic and 

 morphologic responses (symptoms) that express the interaction 

 of pathogen and suscept are infection phenomena, and they con- 

 stitute a "continuous process. It may be impossible to determine 

 when the incubation stage ends and the infection stage begins, the 

 disturbances being imperceptible when first initiated. 



In the present instance concern centers upon the beginning of 

 the incubation stage, especially upon the phenomena of penetra- 

 tion and the subsequent relationship of the fungus to the invaded 

 tissues. Numerous studies have been made of this complex prob- 

 lem, beginning with those by de Bary (1886) and carried forward 

 by Ward (1888) and later by Blackman and by Brown and his 

 associates. The status of this problem has been summarized by 



Blackman (1924) and Brown (1936). 



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