HYBRIDS AND DISEASES 



By L. H. Pammel, Botanist, State College, Ames, Iowa. 



In studying the diseases of plants the writer has been interested in ob- 

 serving the comparative immunity of certain plants and the abundance of 

 fungus diseases on others. Not only are there striking differences in plants 

 that apparently belong to the same species, but these differences are noticeable 

 in certain hybrid forms. In the "Life and Letters of Charles Darwin'" is an 

 account of e.xperiments made in England to breed a variety of potatoes that 

 would be comparatively free from the potato rot fungus. It has been shown 

 l)y Dr. Erwin F. Smith and others who have paid particular attention to the 

 plant diseases that it is possible to breed certain varieties of plants that will 

 be more resistant to the attacks of certain fungi. It seems to me that the 

 attention of plant breeders should especially be called to the importance of this 

 subject from an economic standpoint. In order to overcome the difficulties 

 careful studies must first be made of the characteristics of the plants that are 

 used for cross breeding purposes. I will cite a few illustrations of how the 

 diseases have manifested themselves in the progeny by not paying sufficient 

 attention to the question of eliminating bad qualities that exist in the parents. 

 Where the hybrid or cross comes by chance this question cannot be eliminated, 

 but where hybrids are produced as a result of careful hand pollination these 

 undesirable points should be eliminated as far as possible. 



Raspberry Hybrids and Diseases. A few years ago I obtained 

 what was said to be a chance seedling of the red raspberry. I was 

 told that it was a highly desirable plant, much hardier than the red 

 raspberry, and free from diseases. A considerable number were planted in 

 my garden. After a study of the plants I became convinced that this chance 

 seedling was a hybrid between Rubiis strigosus and Rubiis occidcntalis. In 

 fruit it had partially the characteristics of both. The leaf was nearly inter- 

 mediate between the two species, except it partook more of the character of the 

 R. strigosus than the R. occidentalis. The stem was intermediate in color be- 

 tween the two species, and its method of propagation was that of the black cap 

 raspberry. For some years the anthracnose (Gloeosporium venetum) has 

 been common in the vicinity of Ames upon several varieties of the black cap 

 raspberry. Not far from this patch of black cap raspberries the new seedling 

 raspberry was grown. For several years the plants were seemingly very 

 thrifty, bore abundantly, and there was no evidence of disease. But during 

 the last three years this parasite has been nearly as destructive to this hybrid 

 seedling as to the black cap raspberry. I made a careful search for this 

 fungus upon the red raspberries growing in the same vicinity in my garden, 

 but without, however, finding any trace of this fungus, nor have I ever col- 



