disease resist.\nce in plants. 133 



Difficulties axd Objections. 



in view of present-day knowledge no one will deny that all our 

 common plants vary, and every grower of plants knows that culti- 

 vated varieties are especially given to variation. How many of us, 

 however, have ever given thought to the fact that the organisms, 

 causing the diseases of our cultivated plants may also vary ? I say 

 "fact" advisedly, for there is an abundance of evidence to prove 

 hat fungi and bacteria are capable of as wide variation compara- 

 tively as we are familiar with in the most variable of cultivated 

 plants. 



That fungi vary in form when growing under different conditions, 

 has been repeatedly proven. I wish to point out that fungi vary 

 in their physiological characteristics and in their degree of parasitism. 

 I wish that time would permit me to go into a detailed discussion 

 of this interesting subject, but I will content myself with saying 

 that the breeder of resistant varieties has this difficulty to contend 

 with: The organism which he has been endeavoring to evade by 

 the development of resistant varieties may in time vary in the direc- 

 tion of greater virulence, and his carefully selected variety become 

 as susceptible as was the original stock. In support of this ob- 

 jection, the history of the Keiffer pear may be cited, ^^^len first 

 introduced it was especially resistant to the bacterial disease 

 known as fire blight, which in some sections has prevented the 

 cultivation of any variety except the one under discussion. There 

 seems to be considerable evidence to show that it is much less 

 resistant to the disease than formerly, particularly in southern 

 Delaware. ' A prominent apple grower of Kent County, Delaware, 

 who has practiced spra}ing against apple scab for nearly twenty 

 years, recently expressed the opinion that apple scab was much 

 harder to control now than when he began to spray, and said that 

 he believed it was due to variation on the part of the scab organism 

 in the direction of greater virulence. The history of the coffee 

 leaf disease in Ceylon is also in support of this objection. 



Another serious objection has frequently been raised. Will 

 resistant varieties developed in a given locality and in a particular 

 soil retain their resistance if transferred to another climate and to 

 soil of a verv different character ? Doubtless some of the varieties 



