COMPARATIVE RESISTANCE OF PRUNUS TO CROWN 

 GALL^ 



Professor CLAYTON 0. SMITH 

 University op Caupornia 



In making a study of the plant disease popularly known as 

 erown gall, plant tumor, or plant cancer, it seemed desirable to 

 ascertain the relative resistance of the different species of Prunus 

 to this disease. It was soon evident that the usual methods em- 

 ployed in discovering disease resistance would be of little value. 

 The cause of the disease, life history and pathogenic nature of 

 the organism had already been studied by Dr. Erwin F. Smith^ 

 and his assistants of the United States Department of Agricul- 

 ture. They showed, by artificial inoculations, the wide range of 

 plants susceptible to infection and also found that some were 

 apparently resistant. Their experiments encouraged the writer 

 to follow with slight modifications the method of artificial inocu- 

 lations on a number of species and varieties of the genus Frunus. 

 It was hoped that suitable resistant stock might be discovered 

 that would be adapted to the propagation of the stone fruits. 



Before considering in detail the methods employed, the general 

 characteristics of the disease will be briefly given. The affected 

 part of the tree, shrub or plant is generally found a little dis- 

 tance beneath the surface of the soil at the crown or point where 

 the roots are given off from the trunk. The disease is characterized 

 by an enlargement or gall more or less spherical in shape and 

 consisting of tissue that is usually much softer in texture than 

 normal. The surface may or may not be covered with a normal 

 bark. This enlargement is now known to be caused by a stimulus 

 that comes from the presence of a definite motile bacterial organ- 

 ism known as Bacterium {Fseudomonas) tumefaciens, which lives 

 within certain of the plant cells in relatively small numbers. 



Considerable attention was given to perfecting methods for 



1 Paper No. 28, Citrus Experiment Station, CoUege of Agriculture, Uni- 

 versity of California, Riverside, California. 



2 United States Department of Agriculture, Bureau of Plant Industry, 



