ARTIFICIAL INFECTION 8 1 



experimentation. Finally, in this it is again to be urged that in a 

 study of the relationship of stages to one another and to the host 

 plant, by artificial infection, experiments should be made upon 

 properly isolated hosts. 



5. In some regions the production of the oospores of such fungi 

 as Cyst opus candidus and Cystopiis Bliti are practically unknown ; 

 yet in other regions, and during certain seasons, the oospores are 

 produced in great abundance. A somewhat similar fact is the con- 

 tinuous production of conidia by some species of Erysiphaceae in 

 certain habitats. Together with infection experiments under differ- 

 ent conditions, and upon host plants of various ages, physiological 

 studies of the host will be required. 



6. It is well known that certain Uredinales and Exoascaceae are 

 the immediate causes of the witches' brooms of the hosts in con- 

 nection with which these fungi are found. On the other hand, 

 SpJicerotheca phytoptophila grows only upon branches deformed by 

 phytoptids. Fungi are associated also with many abnormalities 

 commonly referred to as knots, cankers, etc., and in nearly all such 

 cases infection experiments should be called into service to deter- 

 mine not merely if the fungus is parasitic, but also to determine 

 if it is the primary cause of the abnormal development. Even if 

 the fungus is known to be parasitic, from the point of view of 

 pathology, the fungus becomes a matter of secondary importance 

 when it is parasitic merely in consequence of some other injury or 

 excitation. 



In general, infection experiments may be carried out either in 

 the open or in the greenhouse. Frequently it is possible to study 

 natural infections. Nevertheless, adequate opportunities for plant 

 pathological work have not been secured until a good greenhouse 

 is constantly available. 



The methods of making inoculations are necessarily various but 

 always simple. With such fungi as the rusts, mildews, and many 

 species producing leaf spots, the germ tubes usually gain entrance 

 by boring through the epidermis or by passing in at the stomata. 

 No injuries or abrasions of the organs inoculated being necessary, 

 the plant may be moistened, preferably by vigorous spraying, with 

 distilled water. Bell glasses may often be employed if ventilation 

 is provided (Fig. 16). In some cases perfect precautions must be 



