374 Diseases of Truck Crops 



How to Develop a Resistant Variety 



iThis may be accomplished by selecting, from the 

 sickest piece of land on which the crop is growing, the 

 healthiest individuals, and taking the seed from them. 

 The following year the selected seeds are again 

 planted on the same infected land. The best in- 

 dividual plants from this sowing areselected and their 

 seeds saved. By continuing this method of selection 

 -for a number of years it may be possible to develop 

 a strain which will yield one hundred per cent, of 

 healthy plants in a sick soil. To maintain the purity 

 of the selected strain as well as its resistance, it is 

 necessary to reserve a plot of the sick soil, upon which 

 the selected strain is grown for seed purposes. Care 

 must be taken toward carrying any of the sick soil 

 of this plot to other parts of the field. 



Drawbacks. With some crops and with certain 

 diseases it seems hopeless to try to develop a resist- 

 ant strain. If a variety is resistant to one disease 

 it may be susceptible to several others, which are 

 perhaps more serious. The resistance may often be 

 local, in which case it becomes necessary to develop 

 resistant tj^es for each local condition. Resistant 

 varieties often may not embody the requirements of 

 the market. Nevertheless, the development of re- 

 sistant strains shotdd be tried wherever it gives 

 any promise of success. 



