40 WILLIAM Q. LOEGERING 



you see the symptoms in the head. This is similar to what you are 

 referring to. I would like to also comment on your first statement. 

 You said you have an unusual situation in blister rust. I say you have 

 an unusual situation in every combination of culture and variety that 

 you deal with in this area. I only gave a simple illustration of a 

 single combination and I mentioned all the other areas that vary one way 

 or the other. They all fit into the concept and I have never found any 

 of them that don't. 



GERHOLD: You indicated that environment does modify infection type. 

 How much difficulty would this cause in trying to distinguish between 

 the different infection types? Do they overlap? 



LOEGERING: There are a whole series of answers to your question. 

 I will start out with one and I may end up by giving a different one. 

 For instance, we have a gene called Sr 6 and a corresponding gene Psr 6. 

 This combination will vary from zero flecks up to what we call a type four 

 over a temperature range of about seven degrees. It's one of the 

 beautiful illustrations of what you are talking about. It is . repeatable 

 and there is no problem. This very thing can be used in genetic studies 

 because regardless of what the genetic constituion of the host and 

 pathogen are, with respect to this combination, by growing it at a high 

 temperature you eliminate the combination from your system and it thus 

 can be used positively in studying other genes. Now, your question about 

 differentiating infection types. I mentioned when I was talking, that 

 the infection types are characteristic for each combination but may not 

 be unique. Different corresponding gene pairs may give you the same 

 infection type as far as the eye can tell, but it is characteristic of 

 each particular corresponding gene pair. If you are dealing with two 

 corresponding gene pairs giving the same infection type you would get a 

 normal segregation of 16 to 1 in either the host or the pathogen. 

 Whereas, if you had two discreet infection types which are characteristic 

 and unique in that particular system, you would get a 12-3-1 segregation. 



PERSON: I would like to add that there are certain genes for 

 resistance to the stem rust in wheat that I know are temperature, 

 sensitive. Now, there is a great deal of talk about Ts mutants in 

 bacteria and, more recently, Drosophila, but temperature sensitivity 

 for resistance genes has been known for quite some time. Studies have 

 shown that a difference of as few as five degrees, Fahrenheit, will make 

 the difference between resistance and susceptibility. That is with a 

 rise in temperature of five degrees Fahrenheit the plant that would have 

 been resistant now becomes susceptible, so that there are some problems 

 associated with temperature. 



LOEGERING: On the other side there are certain combinations which 

 don't change, regardless of what you do with temperature and environment. 



PERSON: If you had a temperature-sensitivie gene in a wheat plant 

 together with another one that wasn't temperature sensitive, and then 

 you raised the temperature beyond the threshold for sensitivity, would 

 the other gene, the stable one, still express itself? 



LOEGERING: Yes and there is a paper on this. I used two sensitive 

 gene pairs and one non-sensitive, and you could determine the genetics 

 by using only changes in temperature and cultures and not making any 

 crosses. I might mention that one thing to come out of this was the 

 quadractic check. Working with wheat mildew, Slesanski and Ellingbo of 



