392 ROBERT F. PATTON 



standardizing inoculations. Successful storage is relatively easy and 

 is feasible for some pathogens, such as Melampsora on aspen leaves, or 

 the aeciospores of P. pini, but a good storage method for telia of 

 Cronartium species would be very helpful. 



Successful inoculation presumes a certain amount of knowledge of the 

 infection process. The basic objective must be kept in mind. Screening 

 methods that bypass critical steps in the infection process or in resistance 

 reactions may be all right as long as we are aware, but inoculation 

 results may be misinterpreted if such bypassing is ignored or uninten- 

 tionally overlooked. It appears that one hindrance to a successful 

 inoculation technique for P. pini may be lack of information about the 

 infection process. 



We have heard about several major types of inoculations with tree 

 rust fungi. The basic method for large-scale inoculations with C. 

 ribicola 3 C. fusi forme s and M. pinitorqua is to induce oasts of basidio- 

 spores from teliospores directly onto test material. This has been a 

 simple and efficient method, but no standardization or quantitative 

 evaluations have been possible. Monitored spore casts in an air stream 

 in chambers as described by Snow and by Dwinell are attempts at standardi- 

 zation for laboratory trials. Perhaps a modification of the method could 

 be adapted for large-scale progeny testing. Dusting dry spores onto 

 plant surfaces is a commonly used technique with aeciospores or uredio- 

 spores of Cronartiton rusts on the alternate hosts, but was not very 

 successful in inoculation of pine with aeciospores of P. pini. Spore 

 suspensions have been used largely for experimental inoculations. Spray 

 applications of spore suspensions are appealing but have given little 

 success so far, whereas their injection into tissues by hypodermic syringe 

 has proved useful as an experimental technique with P. pini and C. 

 fusiforme but is hardly adaptable to a large-scale progeny screening 

 method. Tissue grafts both with infected needle tissue and bark tissue 

 may prove most useful in experiments on the differentiation of resistance 

 or on races of rust. At present the lack of standardization of inoculum 

 for a natural type of infection is one of the greatest gaps in screening 

 in rust-resistance programs. 



Many factors will influence the outcome of our inoculations and the 

 interpretations to be made from them. Host age effects have been noted 

 with C. ribicola and P. pini. Host vigor may be of particular importance 

 in making comparisons of progeny or selection tests in different areas. 

 The rigor of the test, as exemplified by inoculum amount, has always 

 been a point of controversy. Most workers believe it is safer to err on 

 the conservative side by screening large numbers of plants with heavy 

 spore dosages, even at the risk of losing some valuable materials, rather 

 than to err on the other side by screening too lightly. 



In conclusion, it seems that although we do have some relatively 

 successful inoculation techniques, if infection is a measure of success, 

 still we have no reason to be smug about our progress. Our methods so 

 far are not very sophisticated and often result in more problems than 

 efficiency. We are still handicapped by many gaps in our knowledge of 

 the basic infection biology of probably all the rusts in which we are 

 interested. In particular, we need means for standardizing amounts of 

 inoculum applied and for quantifying the host responses, including good 

 indices of infection and resistance reactions. 



