Minnesota Plant Diseases. 97 



the difficulties appreciated. If they are not, the plant grower is 

 working in the dark. The wheat rusts furnish a good illustra- 

 tion of the complexity of the problem which is presented. 

 Wheat rust may be caused by one or more of several kinds of 

 fungi which are very closely related but are nevertheless dis- 

 tinct kinds. A wheat variety which might resist one of these 

 would be unable to resist others. Now an intelligent solution 

 of the problem must include a knowledge of the particular rust 

 fungus attacking wheat in certain localities and then varieties 

 must be selected which will resist this particular fungus. If 

 more than one fungus is prevalent in this locality the selection 

 of a rust-proof variety becomes more difficult. In other words, 

 rust-proof varieties may have to be selected for certain kinds of 

 ru^ts and not for rusts in general, although, on the other hand, 

 it is not conceivable that certain varieties may possibly be de- 

 veloped which will offer general resistances to the whole group 

 of rusts, e. g., where they are able to withstand the predisposing 

 effects of moisture, or in their early or late sprouting habits 

 might dodge, as it were, the time of year when rusts' spores are 

 most abundant. Early sowing of wheats was an attempt to 

 evade this period of the year but not as yet with very conspicu- 

 ous results. In a word, the greatest success in selection of vari- 

 eties is still to be obtained, but can only be won by more knowl- 

 edge of the habits and forms both of the hosts and of the fungus 

 causing the disease. More knowledge and the hearty coopera- 

 tion of the practical plant grower with the plant disease special- 

 ist are the requirements of the solution of these complex prob- 

 lems. 



Infection of host. We have already seen that fungi use vari- 

 ous methods and agencies for the distribution of spores and 

 there are consequently various methods of inoculation of host 

 plants. In the first place it must be pointed out that inocula- 

 tion and infection are two different things, e. g., a plant may be 

 inoculated with fungus spores which may even start to grow, 

 but a successful continuance of growth does not always follow. 

 When the fungus does succeed in living with the host, infection 

 in the true sense is accomplished. Infection is successful inocu- 

 lation. 



