ARTIFICIAL INFECTION 



79 



the spread of many fungi under the more or less " forced " condi- 

 tions of the greenhouses. Overhead watering or the general sprin- 

 kling of plants is sometimes alone sufficient to facilitate greatly 

 the spread of disease, as in the case of the rust of chrysanthemums. 

 Unusual succulence in the pear is said to be a favorable condition 

 for infection by the pear blight organism. In general, it is believed 

 that any conditions leading to the suffusion of the tissues of the 

 host with water invite disease, particularly disease accompanied by 

 the general destruction of the tissues, and finally by decay. 



From extended observations Atkinson was able to say that the 

 absence of a sufficient amount of potash in the soil predisposes the 

 cotton plant to the attacks of Macrosporium nigricantium, which 

 fungus is then the cause of a new and graver phase of the disease. 

 Many analogous cases might be cited, all of which suggest the 

 necessity of experimental work from the standpoint of inoculation. 

 Recently Ward has reported that the lack, or poverty, of one or 

 more necessary elements in the nutrition of the brome grasses does 

 not seem to predispose those hosts to the rust fungi parasitic upon 

 them. It may well be inquired if this is a special case, and particu- 

 larly if there may be any difference in this regard between obligate 

 and facultative parasites. In this connection, moreover, the experi- 

 ments made by Salmon with Erysiphe graminis may be cited. He 

 found that a wound sometimes sufficed to break down completely 

 the immunity of certain species of grasses to a particular form, or 

 race, of this fungus. 



The method of penetration of the germ tube of the fungus can 

 only be definitely determined by careful infection experiments. It 

 is just as true for a fungous disease of plants as for a bacterial dis- 

 ease that a thorough study of the conditions has not been made 

 until the possible methods of infection are determined. Not only 

 is it necessary in the general etiology of the disease, but extremely 

 important in the formulation of preventive measures. Fungi gain- 

 ing entrance only through injuries or wounds are, in general, much 

 -more readily suppressed or confined. 



3. Inoculation studies with certain species of Gloeosporium have 

 indicated that many distinctly disease-producing organisms may 

 have a considerable range of host plants. A species of "Rhizoc- 

 tonia" (Corticium -cagum H. & C., var, Solani Hurt) causing a 



