134 MASSACHUSETTS HORTICULTURAL SOCIETY. 



of certain crops so far developed will retain their resistance through 

 a fairly wide range of conditions, but some may be found wanting. 

 The person who introduces a disease resistant variety should test 

 his production for several seasons, and under a variety of condi- 

 tions. 



The objection has been raised that resistant sorts are inferior. 

 This is frequently the case. In the development of immune 

 varieties the market requirements must be kept in mind. It is, 

 however, something gained to have developed a variety showing 

 high resistance to some malady, even if the variety is worthless 

 commercially for this sort may then be used as one parent in cross- 

 ing with commercially desirable varieties. 



Another difficulty in this work which presents itself is that most 

 of our cultivated plants have more than one parasitic organism 

 attacking them. Some plants have a score or more such diseases. 

 A plant breeder usually develops his immune variety in some 

 particular section to the most serious disease in that locality. 

 When the variety is transferred to another section where another 

 disease is the prevalent one, he may find that the plant is not at all 

 resistant to the second trouble. 



Ward, Salmon, and others have shown that there exist what 

 have been termed biological forms of certain species of fungi, that 

 is a species of fungus may include certain forms, one of which is 

 able to infect only a certain species or group of species of host plants, 

 while another may infest other species not infected by the first 

 form, etc. These forms are indistinguishable from each other 

 morphologically. In developing a resistant variety of plant in 

 one locality we may be developing resistance to only one biological 

 form of the fungus. "VN^en we transfer the variety we may encounter 

 another form of the same fungus to which our plant will be sus- 

 ceptible. 



Salmon also points out the possibility of there being strains of 

 the same fungus having great virulence. 



Cause of Immunity. 



So far I have omitted any reference to the cause of disease re- 

 sistance or immunity. It is perhaps beyond the scope of this talk 



