352 ROBERT A. SCHMIDT 



to gall formation. As mentioned previously, stem injection techniques 

 could be useful in studies of bark resistance factors and differential 

 resistance types. 



INOCULATION OF OAK 



Artificial inoculation of oak with aeciospores of C. fusiforrne is 

 less demanding than the sporidial inoculation of pine and is easily 

 accomplished in the laboratory or field. Usually fresh or stored aecio- 

 spores are dusted onto the wetted, lower surface of susceptible oak leaves 

 Optimum temperature for germination of aeciospores is approximately 20°C 

 (Siggers, 1947; Roncadori and Matthews, 1966). Most critical, however, 

 is the age of the oak leaves; maximum uredial and telial formation occur 

 on leaves inoculated at 4-6 and 8-12 days of age, respectively (Snow 

 and Roncadori, 1965). 



DISCUSSION AND CONCLUSIONS 



Problems associated with inoculation techniques are of fundamental 

 importance to disease resistance programs because they influence the 

 judgment of relative susceptibility of test plants and of the mode of 

 resistance. It is generally agreed that both artificial and natural 

 inoculations are required(Patton and Riker, 1966; Borlaug, 1966) and that 

 both have inherent advantages and disadvantages (Schreiner, 1966) . There- 

 fore, it is appropriate to consider how available techniques can be used 

 best in light of present knowledge. 



In regard to the development of rust-resistant pines, inoculation 

 techniques must minimize errors of omission (a failure to identify a 

 resistant tree) and commission , (the identification of a susceptible tree 

 as resistant) . Certainly, the latter is requisite with a crop such as 

 pine which has a rotation age of at least 20-25 years. Artificial 

 inoculations should approximate natural inoculations and minimize chance 

 deviations in inoculum density, availability of susceptible tissues, 

 temperature, atmospheric moisture, and all such factors which contribute 

 to disease escape of susceptible trees under natural conditions. Like- 

 wise, natural inoculations should aid in the interpretation of geographic 

 interaction due to climate, pathogen, or host variation, effects of 

 cultural treatments, stand manipulation, and such factors which are 

 difficult to include in artificial inoculations. 



With respect to fusiform rust, a feasible inoculation routine which 

 accomplishes many of the above objectives would include: (1) mass 

 artificial inoculation of 1-3 month-old seedlings in screening sheds using 

 mixed inoculum, i.e., inoculum which samples possible existing racial 

 variation in the fungus, (2) reinoculation of these seedlings at age 

 1-3 years, using mixed inoculum in tent-chambers over nursery beds, and 

 (3) field testing of promising individuals for 3-5 years in rust nurseries 

 strategically located to test geographic interaction. The latter should 

 be a joint regional effort wherein cooperators would accept test material 

 from other areas. A similar plan has already been outlined for white 

 pine blister rust (Borlaug, 1966) . If pines are to be used only locally 

 more intensive tests with only local inoculum might be appropriate. 



