312 



GENERAL PLANT PATHOLOGY 



Infection by natural 

 growth of the fungus 



A. By means of spores, or h>i)luc, into stoinata and 

 water stomata. 



B. By rerment action of a fungus on the epidermis of 

 the host. 



f By developing from a dormant state in the seed into 

 [ an active state in the seedling. 



[ Beasts 

 I. Mechanic injuries ) Man 



Infection through 



induced by 



II. Meteorologic in- 

 juries induced by 



III. Chemic injuries 

 induced by 



Fall of fruit 



Combined weight action of fruit 



Wind 



Snow 



Ice 



Hail 



Lightning 



Sun 



Frost 



Factory gases 



Sewer gases 



Locomotive gases 



Chemicals at roots. 



Alkali soils 



Gases and chemicals in geysers, etc. 



IV. Non-classifiable 

 injuries induced by 



Natural grafting and budding 



Incubation.^ — The period of incubation is the time between ex- 

 posure to the cause of the disease and the first appearance of the symp- 

 toms, or physical signs of the disease. This period in plants is quite as 

 variable as in animals, and it is dependent on the nature of the organ- 

 ism, whether it is virulent, or its virulency attenuated, on its food re- 

 quirements, on its temperature requirements, the volume of infectious 

 material, the stage of development, or age of the host plant, the amount 

 of water and air in the invaded tissues, and individual or varietal re- 

 sistance. The period of incubation may be as short as a few hours, 

 or as long as three to four weeks. Presumably on seedling tissues the 

 period of incubation of the damping-off fungus, Pythium de Baryanum, 

 is only a few hours. Experiments performed by Erwin F. Smith^ 

 1 Smith, Erwin F.: Bacteria in Relation to Plant Diseases, ii: 66. 



