418 GENETICS IN RELATION TO AGRICULTURE 



which were discovered in the 1913 nursery, the result presumably of a 

 mutation. Johnson reports the results of 2 years' experiments on 

 the relative resistance of selected strains of tobacco to the root rot, 

 caused by Thielavia basicola (B & Br.) Zopf. A strain of White Burley 

 tobacco has been developed which possesses a high degree of resistance. 

 The Wisconsin Station has successfully combatted the "yellows" 

 and the black rot of the cabbage by selection. Both are very destructive 

 diseases, the first being caused by a fungus and the second by a bacterium. 

 Starting with a commercial variety that is resistant or immune to the 

 "yellows" a strain which is also highly resistant to black rot has been 

 developed. "A stand of from 95 to 99 per cent, of the rot-proof type 

 was obtained on some fields where both the imported Danish and Puget 

 Sound seed failed to give more than 15 to 20 per cent, of a crop." The 

 practical value of such simple selection as the elimination or roguing of 

 all diseased or weakly plants, an ancient practice, must not be over- 

 looked. As was pointed out in Chapter XXIII, much of the value of hill 

 selection in potatoes doubtless lies in the elimination of diseased tubers. 

 Finally it must be remembered that some diseases or "off-type" 

 states in plants are caused by environmental conditions to overcome 

 which it would probably be impossible to select resistant strains. A case 

 in point is Yellow-berry in wheat which is described as the appearance of 

 yellow or white, mealy or half-mealy, or spotted grains, otherwise without 

 apparent blemish. Its occurrence is believed by Headden to indicate 

 "that potassium is present in excess of what is necessary to form a ratio 

 to the available nitrogen present advantageous to the formation of a 

 hard, flinty grain." He concludes that it is entirely within the control 

 of the grower through employment of proper cultural methods. How- 

 ever, the universal occurrence of Yellow-berry on the Pacific Coast 

 points to something more profound than the potash-nitrogen ratio as the 

 determining cause. If this general factor is climatic, wheat breeding 

 projects on the Pacific Coast should be organized with reference to it in 

 the fundamental tests of commercial varieties and of the various species 

 and sub-species of wheat for the purpose of planning more promising 

 hybridization experiments. Attempts at improvement of existing varie- 

 ties for resistance to Yellow-berry by selection would appear unwarranted. 

 Especially is this the case if the disease can be counteracted by cultural 

 methods. The need for resistant varieties is also less imperative in the 

 case of many destructive diseases for which methods of control have 

 been successfully devised. Yet it is obvious that an enormous saving 

 to agriculture would come from the production of such varieties. There 

 is a vast field here for the combined efforts of the pathologist and the 

 geneticist. 



