DISEASE RESISTANCE IN ANNUAL CROPS 63 



FLOOR DISCUSSION 



BINGHAM: I think one of the most helpful suggestions you may have 

 made to us was the possibility of selection for long-lasting "uniform" 

 resistance, usually expressed quantitatively. In respect to selecting 

 for uniform resistance to C. vibioola, among other selection criteria 

 perhaps useful might be things you suggested like low infection ratio, or 

 prolonged latent period. It is easy to foresee how we might select for 

 low infection ratio--merely by selecting plants with fewer foliar or bark 

 lesions per unit area of host infection court. But how we might benefit 

 from selecting for a prolonged latent period, with the rust perhaps 

 fruiting a year later (as actually noted by Van Vloten in the Netherlands 

 and much earlier by Eriksson in Sweden), escapes me. Aecial sporulation 

 in certain P. strobus provenances was delayed, I believe up to a full 

 year, but thereafter bark lesion development was normal and the trees all 

 died. Would you care to comment on how selection for lengthened latent 

 period might be useful in selection for resistance to cronartium rusts? 



ZADOKS : Of course your situation with cronartium rusts is quite 

 different from that encountered in rusts of annual crops. In annual 

 crops one rust lesion does not kill the plant; with pines, one stem 

 lesion usually does. So latent periods may not be a very helpful selec- 

 tion criterion, unless linked with other more valuable criteria. I don't 

 know whether there is a real, genetic linkage. More often than not, 

 however, a prolonged latent period reduces the rust population build up 

 rate in annual plants. Even though a prolonged latent period may not 

 seem to have much value for you, it's a fairly easy criterion to select 

 for, and it may be linked with more valuable criteria. 



