PERSISTENCE OF PUST RE SI STANCE 569 



which have some intermediate type of resistance. This has happened 

 several times. The danger is, when you work in the laboratory, that you 

 may select for major genes only. We have had breeders of onions who were 

 selecting for Botritis resistance. They complained to me that they never 

 could get any resistance but it just should be there in the material. I 

 asked them about their inoculation technique. Well, they did it in the 

 laboratory. After additional questioning, I concluded that they far 

 overdid the work. They killed everything outright just by too heavy a 

 dosage of inoculum and too good conditions for infection. I think for 

 tree breeding, controlled inoculation is just not necessary if you can 

 increase the average of resistance in a population without it. You have 

 done already a lot of work and I believe the increase in resistance 

 could be done in the field using natural infection. 



HATTEMER: Your first question was on the problem of isolates versus 

 "populations". If you can be sure that the quantity of spores you use 

 for inoculation contains all races of the pathogen in proportions that 

 reflect the future risk of field infection you can of course save money. 

 If this is not the case you can do nothing but investigate the genetic 

 variation of the pathogen, test host resistance against isolates or just 

 various samples of the pathogen population, and make periodical surveys 

 of the race spectrum that occurs in the field. You may also know that 

 Idaho alone is many times larger than Switzerland and this may have an 

 enormous impact on whether the Swiss approach works for white pine. You 

 just have to realize what you are working with, that your final goal is 

 minimum loss at the age of maturity, and the same I think is true with 

 the question of testing after artificial versus natural inoculation. If 

 you can be sure one or the other or maybe both have a close enough 

 correlation to the goal of the resistance breeder, you can make the 

 choice. The third problem is that of killing possibly the most valuable 

 selection material. We have, of course, to be aware of the fact that we 

 should not overkill. There are various breeders, not only of trees, who 

 have been unsuccessful for years and years because they have virtually 

 extinguished all the material that they had in their garden. 



