DISEASE RESISTANCE IN PLANTS. 131 



fruits. Obviously it would take a longer period to obtain results 

 in developing varieties of perennials than annuals and the plant 

 breeder will naturally select for study those problems which give 

 promise of solution in the shortest time. It is, however, a matter 

 of common observation that some varieties of our tree fruits are 

 more resistant to certain of their maladies than others. For ex- 

 ample, in Delaware it is known that among apple varieties the Lily 

 of Kent is above all others most resistant to scab and bitter rot, 

 while the Jonathan is especially susceptible to rot. The whole 

 Winesap group is said to be relatively resistant to rot but susceptible 

 to scab, while the York Imperial is resistant to scab. Among 

 pears the Keiffer is known to be resistant to fire blight, while the 

 Bartlett and others are very susceptible. A host of such observa- 

 tions might be cited to show that among our tree fruits there is 

 varietal variation as regards resistance to disease. 



There are some diseases of tree fruits which are especially trouble- 

 some and impossible to combat satisfactorily, for which it would 

 be highly desirable to have resistant varieties. I refer particularly 

 to the peach yellows. The observations of Professor E. W. Morse, 

 reported in the Bulletin of the Bussey Institution, indicate that 

 individual peach trees from which to propagate may be found 

 which are resistant to yellows. Since no satisfactory method has 

 ever been found to combat this dread disease, the study of the 

 problem of prevention from this standpoint is highly desirable. 



Another disease of great importance is the crown gall of rasp- 

 berry and blackberry. This is a very serious trouble in infested 

 districts. In Sussex County, Delaware, the raspberry industry, 

 once very large, has been practically wiped out by this trouble. 

 Since the organisms of the disease are retained in the soil and attack 

 the roots or crown only, no preventative measures are practicable. 

 It seems highly desirable to attack the problem of the . treatment 

 of this disease by the selection or breeding of disease resistant varie- 

 ties. 



An interesting application of disease resistant varieties illustrating 

 one of the uses to which such varieties may be put is seen in the 

 American grape "sdnes used as stocks on which to graft the Euro- 

 pean wine grapes. Many years ago an American ^ine insect 

 known as the Phylloxera was introduced into Europe where it 



