568 HANS H. HATTEMER 



GERHOLD: It would be interesting to know whether the balanced 

 systems that have been mentioned started from a catastrophe, or whether 

 they developed gradually with both host and pathogen evolving at similar 

 rates . 



BORLAUG: I might make a comment about a case of an equilibrium 

 system that I think is more valid than any that has been proposed here 

 and one that is truly in equilibrium. In maize rust in a center of 

 origin of -maize such as Mexico, it's no problem so long as you don't act 

 foolishly and use too narrow a genetic base in trying to make other 

 improvements. You never see an epidemic of maize rust in the open 

 pollinated maize varieties, and it takes practically no toll as long as 

 you don't move these varieties around or start inbreeding in them. 

 I want to tell you this, don't believe there isn't variability just 

 because you don't see it when you have this kind of system. You inbreed 

 and a large percentage of these populations will be killed outright, in 

 the same location where they are perfectly in balance. It will be more 

 so if you bring a tropical maize race into a highland and start inbreeding 

 it. These will fall apart even worse. When an organism, a virus in the 

 case that I wish to cite, was introduced, it swept across Mexico from 

 the North. It's now in Bolivia 20 years later. It had swept the maize 

 varieties before and there was apparently a varying level of genes for 

 resistance in these populations. But the disease was of such intensity 

 that there was ruinous harvest or lack of it for about 2 to 3 years. 

 Long before anyone ever figured out just what was happening, the peasant 

 farmers, reselecting in these populations, solved the problem in Mexico. 

 But it is still a problem in Bolivia as it advances. The virus is insect 

 transmitted and it apparently came off a grass host growing in the 

 Southern U.S.A. So, you have both extremes through a long period of 

 time. As in the case of the two species of rust that are part and parcel 

 with the maize system in the open populated varieties of Latin America 

 in the mountainous country where they live together here, they cause 

 practically no loss. The virus that came in was ruinous for two or three 

 years and before the scientists could find out what was going on, the 

 peasant farmers solved the problem. So let's not get oversized heads 

 about what we can do and what we can't do. 



ZADOKS: We were talking here about inoculation infection techniques. 

 If I may make a few remarks. There are several choices to be made. One 

 is the choice whether we should work with selected isolates which can be 

 identified, or whether we work with populations of the pathogen. This is 

 largely a matter of taste and of money. All sophisticated rust work in 

 cereal breeding is with selected isolates, and a result of it was the 

 well-known race between rust and breeder. The breeder just keeping in 

 advance of the rust to get new varieties. I know of one country, 

 Switzerland, which has kept its head cool and always worked with popula- 

 tions and has decided to continue to work with rust populations. Their 

 argument was very simple. They said we had too little money to do all 

 this work and to spend on this expensive equipment; so, they collected 

 isolates from all over the country, mixed them to produce one big popula- 

 tion, and infected their fields with this mixed population. They seem 

 to go on quite nicely. Then another question about inoculation techniques 

 is whether you should inoculate in inoculation chambers under controlled 

 conditions to get rather severe inoculations, or whether you should leave 

 the inoculations to the field. Here again it's a matter of taste and a 

 matter of money. There are advantages in the laboratory techniques. 

 You can select those strains or plants which are really resistant. The 

 disadvantage is that you overdo the work, and kill most of the plants 



