72 STAKMAN 



mately 275 that are now recognized. The problem is to iind wheat varieties 

 that reveal the differences. As new combinations of genes are made in varieties 

 of wheat, additional differences become apparent between and within rust 

 races. 



It is of course impossible to predict the outcome of the long fight against 

 wheat stem rust until we find out the maximum genie potentiality for viru- 

 lence in the rust and the maximum genie potential for resistance in wheat. 

 This obviously requires extensive and basic studies of the genetics of the 

 rust, of the genes for resistance in wheat, and of the nature and variability of 

 resistance. It is known, of course, that resistance may be due to physiologic 

 and to structural characters. Nothing is known about the real nature of 

 physiologic resistance, but there are several morphological characters, such 

 as thick epidermis and woody stems, that contribute to resistance. Physio- 

 logic resistance varies widely with temperature and light in some rust-race- 

 wheat-variety combinations but not in others. Morphologic resistance seems 

 more constant. Can a universally resistant variety be built by combining all 

 known characters for resistance in a single variety? The answer may be 

 available in the centenary year of the Botanical Society, but it is hoped 

 that it may be sooner. A world-wide search must be made among the score 

 of thousands of wheat varieties, those that appear to have desirable characters 

 must be thoroughly studied and tested under an adequate sample of environ- 

 mental conditions, and the problem still remains of combining all needed 

 characters. If the desired combinations of characters cannot be found or 

 made, there is still the possibility that gene changes can be induced by 

 chemicals or by irradiation. And this possibility should be investigated 

 thoroughly. 



Researches on the virulence of the rust fungus, on the resistance of wheat, 

 and on the interaction between the parasite and its host will require very 

 intensive studies on the physiology of parasitic relations and on the effect of 

 environmental factors on the processes involved. There must be far more 

 extensive ecological investigations on a world-wide scale and much more 

 intensive investigation of the intimate relationships between host and para- 

 site. We now know something about what happens, but too little about how 

 and why it happens. 



The title of this paper imposes the obligation to discuss problems in the 

 control of epidemics; it may seem that problems have been magnified and 

 progress minimized. In reality, of course, great progress has been made. 

 Considering the nature of the problem, a record of protecting spring wheat 

 twenty years in fifty is not bad. Moreover, the spring bread wheats now 

 grown are 25 to 50 per cent more resistant than those of fifty years ago. 

 It is true that they can be ruined by rust, but fewer races can ruin them, and 

 it takes a longer time and more favorable conditions for even the most viru- 

 lent North American race to ruin them. 



