374 ROBERT F. PATTON 



INTRODUCTION 



In a program of breeding for resistance, providing a broad base of 

 exposure to the pathogen in the testing phase can be as important as 

 providing a broad base for initial selection of the suscept. The testing 

 stage must include the wide range of virulence exhibited by different 

 strains of the pathogen, as well as providing favorable conditions for 

 infection and disease development to insure as severe and uniform 

 exposure as possible. Here are included such steps as learning how to 

 create a localized epidemic in the experimental field or nursery, locating 

 test plots at such places where the environmental factors are conducive 

 to an epidemic outbreak, and manipulating cultural conditions to increase 

 chances for infection. 



In developing strains of white pines resistant to the blister rust 

 disease incited by Cronartium ribicola J. C. Fischer ex Rabenh., it is 

 perhaps most important, as indicated by Borlaug (1966), to develop 

 screening tests for progenies that will identify the best selections, the 

 best crosses among these, and the most outstanding seedlings within these 

 outstanding crosses. One way is to produce an artificially induced rust 

 epidemic in the nursery. This paper summarizes some experiences with 

 artificial inoculation in programs concerned primarily with resistance in 

 eastern white pine {Pinus strobus L.) and points up some of the major 

 problems associated with such testing. An oversimplified but nevertheless 

 characteristic statement of the theme might be that the methods and 

 techniques are relatively simple, but consistently obtaining severe and 

 uniform infection is often difficult. 



NATURAL VS. ARTIFICIAL INOCULATION 



The use of natural vs. artificial inoculation has arguments on both 

 sides (Patton and Riker, 1966). Probably any large-scale, resistance- 

 breeding program will have to incorporate both procedures. Rarely, 

 however, can natural inoculation alone be relied upon; artificial inocu- 

 lation methods must be used for maximum efficiency and reliability of 

 screening tests. 



The usual pathological sequence in disease development is broken 

 down into three successive series of activities following dispersal of 

 the inoculum: (1) inoculation - the arrival of inoculum at the infection 

 court; (2) incubation - the revival of activity of the inoculum at an 

 infection court, entrance into the suscept, and initiation of disease; 

 (3) infection - the subsequent activities of the pathogen, which cause 

 progressive disease in the suscept and development of a characteristic 

 syndrome. Most considerations of artificial inoculation include the 

 first two of these series of activities, and this paper also deals 

 largely with these aspects. 



Certain limitations are inherent in dependence upon natural inocu- 

 lation. We have relatively little control over natural inoculation 

 except through proper choice and preparation of the test site, and perhaps 

 also through cultural operations leading to an increase in the amount of 

 available inoculum. Such choices and site manipulations are aided by our 

 knowledge of etiology and epidemiology of the disease concerned. Varia- 

 bility in results of natural inoculation tests is constantly to be 

 expected, however, largely because of the effect of unexpected or unknown 

 influences . 



