PROBLEMS IN PREVENTING PLANT-DISEASE EPIDEMICS 73 



Nevertheless it is pertinent to ask whether the large investment in time, 

 effort, and monies spent in trying to control one epidemic disease has paid 

 dividends. The answer is a categorical and emphatic yes. The breeding of 

 wheats primarily to control stem rust was not restricted to that one objec- 

 tive. There were many very valuable by-products. Earliness, potential pro- 

 ductivity, quality, and resistance to other diseases have also been attained 

 for considerable periods of time for some characters, and permanently for 

 others. The necessity of breeding against stem rust was a powerful stimulus 

 to breeding generally, with the result that the wheats of today, although far 

 from perfect, are far better than those of fifty years ago. Finally, the value 

 of an effort can sometimes be measured by assuming that it had not been 

 made. If the approximately half a billion barberry bushes had not been 

 destroyed during the past four decades but had been permitted to multiply 

 and escape from cultivation, there would now be a local source of early rust 

 near almost every wheat field north of central Kansas and the physiologic 

 race problem would have been far more complex. Barberry eradication was 

 started just in time; had it been delayed until the present it probably would 

 have been impossible and probably unnecessary, because it is doubtful if 

 wheat growing could have continued. It is almost certain that it would now 

 be impossible to grow wheat successfully in the Upper Mississippi Valley 

 if breeding for resistance and barberry eradication had not been undertaken. 



Stem rust of wheat has been taken as an example because it is the most 

 complex plant-disease problem in many of the wheat-growing areas of the 

 world. Nevertheless there are similar problems with respect to the prevention 

 of epidemics of other rusts, such as the orange leaf rust of wheat, the yellow 

 stripe rust of wheat, the stem rust and crown rusts of oats, and fllax rust. 

 All these rusts can become widely and destructively epidemic in a short time. 

 Although their life histories differ from that of stem rust of wheat, all of them 

 are alike in their prolificacy, spreading power, and potential destructiveness. 

 All of them are disseminated by the wind, and all of them comprise physio- 

 logic races, thus increasing the difficulty of producing and maintaining re- 

 sistant varieties. 



Some diseases tend to be perennially epidemic because the causal organisms 

 accumulate and persist in the soil. As one example, flax was long a migratory 

 crop in the United States, moving continually westward to new lands, because 

 it could not long be grown successfully in the same soil or even in the same 

 locality. Prior to 1900, the explanation was that the soil became "flax-sick," 

 but nobody knew why until 1900, when it was shown that the trouble was 

 due to Fusarium, a fungus that accumulated and persisted in the soil after 

 having been introduced with flax seed. Under appropriate conditions, the 

 fungus then killed the plants and often ruined the crop. Fortunately, how- 

 ever, a few plants usually survived. Seed from the survivors was then sown 

 back on "sick soil," and the survivors again propagated. In this way several 



